AIDA International Workshop 2014 Abstracts

AIDA International Workshop 2014 Abstracts

 AIDA International Workshop 2014 Submitted Abstracts October 15-­‐17, 2014 JHU Applied Physics Laboratory Building 200, E100 AIDA: ASTEROID IMPACT & DEFLECTION ASSESSMENT
A. F. Cheng1, A. S. Rivkin1, C. Reed1 , O. Barnouin1, A. Stickle1, A. Galvez2, I. Carnelli2, P. Michel3, S. Ulamec4
1
The Johns Hopkins University Applied Physics Laboratory, Laurel, Maryland, ([email protected]). 2 ESA
Headquarters, Paris. 3Lagrange Laboratory, Univ. Nice, CNRS, Côte d’Azur Observatory. 4 DLR, Koeln, Germany
Introduction: The Asteroid Impact & Deflection
Assessment (AIDA) mission will be the first space
experiment to demonstrate asteroid impact hazard mitigation by using a kinetic impactor to deflect an asteroid. AIDA is a joint NASA-ESA mission in pre-Phase
A study, which includes the NASA Double Asteroid
Redirection Test (DART) mission and the ESA Asteroid Impact Monitor (AIM) rendezvous mission.
The primary goals of AIDA are (i) to test our ability to perform a spacecraft impact on a potentially hazardous near-Earth asteroid and (ii) to measure and
characterize the deflection caused by the impact. The
AIDA target will be the binary asteroid Didymos, with
the deflection experiment to occur in October, 2022.
The DART impact on the secondary member of the
binary at ~6 km/s will alter the binary orbit period,
which can be measured by Earth-based observatories.
The AIM spacecraft will monitor results of the impact
in situ at Didymos.
AIDA will return fundamental new information on
the mechanical response and impact cratering process
at real asteroid scales, and consequently on the collisional evolution of asteroids with implications for
planetary defense, human spaceflight, and near-Earth
object science and resource utilization. AIDA will return unique information on an asteroid's strength, surface physical properties and internal structure. Supporting Earth-based optical and radar observations,
numerical simulation studies and laboratory experiments will be an integral part of the AIDA mission.
Motivation: Roughly 54 tons of material falls on
the Earth every year, with impacts of meter-sized bodies a nearly-annual event. However, much larger objects lurk nearby, astronomically speaking: nearly 1000
objects 1 km or larger are classified as near-Earth objects (NEOs), with perihelia of 1.3 astronomical units
or less. Impacts of 1 km objects, which would result in
civilization-threatening effects, are thought to occur on
roughly million-year timescales. The population of
NEOs 50 m or larger is modelled to number in the
hundreds of thousands. While impacts by 50-m asteroids may “only” devastate a region (like the Tunguska
Event of 1908), they also occur much more frequently,
occuring on century-to-millenium timescales.
Uniquely for natural disasters, destructive impacts
can be not only predicted but also potentially avoided
via human action. The United States Congress directed
NASA to find and characterize 90% of potentially haz-
ardous asteroids (PHAs) 140 m and larger, following
up on an earlier charge to find 90% of all km-scale
NEOs. Surveys to meet this Congressional mandate are
underway via ground-based and space-based telescopes, and programs are in place to characterize the
sizes, shapes, rotation periods, compositions, and other
properties of NEOs.
However, there are still a great many unknowns
with respect to how an incoming NEO might best be
deflected. Some elegant techniques like the “gravity
tractor” or changing a target’s albedo and allowing the
Yarkovsky Force to change the target’s orbit require
decades or more for a deflection to be achieved, leading many to support so-called “impulsive” techniques
with immediate effect. The use of nuclear weapons for
asteroid deflection is a possibility (and perhaps the
possibility most ingrained in popular culture), but
brings a host of political and other complications. As a
result, the non-nuclear “kinetic impactor” technique of
impacting an incoming object to alter its trajectory has
gained favor in the NEO science community.
While we may be technically capable of carrying
out a kinetic impactor mission on a threatening asteroid, we are far from understanding its effectiveness or
optimizing such a mission for success. This serves as
impetus for flying a practice mission or missions to
understand how asteroids and asteroidal material behave in a kinetic impact. The Don Quijote mission
study, performed by ESA in 2005-2007, had the objective of demonstrating the ability to modify the trajectory of an asteroid using a kinetic impactor and to measure the trajectory change. The Don Quijote mission
was judged to be unaffordable, but a demonstration of
asteroid deflection by spacecraft impact remains of
interest, as the magnitude of the deflection resulting
from a spacecraft impact is highly uncertain, owing to
the poorly understood contribution of recoil momentum from impact ejecta.
The AIDA mission concept follows Don Quijote as
a cheaper, more flexible alternative. AIDA consists of
two independent but mutually supporting mission concepts, one of which is the asteroid kinetic impactor and
the other is the characterization spacecraft. These two
missions are, respectively, the Double Asteroid Redirection Test (DART) study undertaken by the Johns
Hopkins Applied Physics Laboratory with support
from members of NASA centers including Goddard
Space Flight Center, Johnson Space Center, and the Jet
Propulsion Laboratory, and the European Space Agency’s Asteroid Impact Monitoring (AIM) mission study.
DART will be the first ever space mission to
demonstrate asteroid deflection. This will be done using a binary asteroid target. AIM is a rendezvous mission which focuses on the monitoring aspects i.e., the
capability to determine in-situ the key physical properties of a binary asteroid playing a role in the system’s
dynamic behavior.
Objectives and Requirements: The target of the
AIDA mission will be a binary asteroid, in which
DART will target the secondary, smaller member in
order to deflect the binary orbit. The resulting period
change can be measured to within 10% by groundbased observations. The asteroid deflection will be
measured to higher accuracy, and additional results of
the DART impact, like the impact crater, will be studied in great detail by the AIM mission. AIDA will return vital data to determine the momentum transfer
efficiency of the kinetic impact and key physical properties of the target asteroid. The two mission components of AIDA, DART and AIM, are each independently valuable, but when combined they provide a
greatly increased knowledge return.
The main objectives of the AIDA rendezvous
spacecraft, AIM, are to:
•
•
•
Determine binary asteroid orbital and rotation
state
Analyse size, mass, and shape of both components
of the binary asteroid
Analyse the geology and surface/subsurface properties of the binary system
When AIM is operated together with DART the mission would also cover a supplementary objective:
•
Observe the impact outcome (crater and ejecta
properties) and derive the impact response of the
object as a function of its physical properties.
The strawman payload for AIM could consist of a
Narrow Angle Camera, a Micro laser Altimeter, a
Thermal IR Imager, a NIR spectrometer. In addition to
this, a camera for observation of the impact, a seismometer and the seismic sources for a surface interaction
experiment would also be carried and deployed by the
spacecraft.
DART is a self-standing element in the AIDA mission and will impact the smaller secondary component
of the binary system [65803] Didymos, which is well
observed by radar and optical instruments. The impact
of the >300 kg DART spacecraft at 6.25 km/s will
change the mutual orbit of these two objects. By targeting the smaller, 150 m diameter member of the binary system, DART will produce a larger orbital de-
flection than if it targeted a more massive near-Earth
asteroid.
The DART impact will be observable by groundbased radar and optical telescopes around the world,
providing exciting opportunities for international participation in this first asteroid deflection experiment.
The DART mission will use ground-based observations to make the required measurements of the orbital
deflection, by measuring the orbital period change of
the binary asteroid. The DART impact is expected to
change the period by 0.5% – 1%, and this change can
be determined to 10% accuracy within months of observations. The DART target is specifically chosen
because it is an eclipsing binary, which enables accurate determination of small period changes by groundbased optical light curve measurements. In an eclipsing
binary, the two objects pass in front of each other (occultations), or one object creates solar eclipses seen by
the other, so there are sharp features in the lightcurves
which can be timed accurately.
The DART payload is a long range imager to support autonomous navigation to impact the target body
through its center, to determine the impact point within
1% of the target diameter, and to characterize the preimpact surface morphology and geology of the target
asteroid and the primary to <20 cm/px.
Conclusion: The AIDA mission will combine US
and European space experience and expertise to address an international problem, the asteroid impact
hazard. AIDA will also be a valuable precursor to human spaceflight to an asteroid, as it would return
unique information on an asteroid's strength and internal structure and would be particularly relevant to a
human mission for asteroid mitigation. AIDA will furthermore return fundamental new science data on impact cratering, surface properties and interior structure.
AIDA will target the binary Near-Earth asteroid Didymos with two independently launched spacecraft,
with the deflection experiment to occur in October,
2022.:
THE ASTEROID IMPACT AND DEFLECTION ASSESSMENT MISSION AND ITS POTENTIAL
CONTRIBUTIONS TO HUMAN EXPLORATION OF ASTEROIDS. P. A. Abell1 and A. S. Rivkin2,
1
Exploration Integration and Science Directorate, NASA Johnson Space Center, 2101 NASA Parkway, Houston, TX
77058, [email protected], 2The Johns Hopkins University Applied Physics Laboratory, 11100 Johns Hopkins
Road, Laurel, MD 20723, [email protected].
Introduction: The joint ESA and NASA Asteroid
Impact and Deflection Assessment (AIDA) mission
will directly address aspects of NASA’s Asteroid Initiative and will contribute to future human exploration.
The NASA Asteroid Initiative is comprised of two
major components: the Grand Challenge and the Asteroid Mission. The first component, the Grand Challenge, focuses on protecting Earth’s population from
asteroid impacts by detecting potentially hazardous
objects with enough warning time to either prevent
them from impacting the planet, or to implement civil
defense procedures. The Asteroid Mission, involves
sending astronauts to study and sample a near-Earth
asteroid (NEA) prior to conducting exploration missions of the Martian system, which includes Phobos
and Deimos. AIDA’s primary objective is to demonstrate a kinetic impact deflection and characterize the
binary NEA Didymos. The science and technical data
obtained from AIDA will aid in the planning of future
human exploration missions to NEAs and other small
bodies.
The dual robotic missions of AIDA, ESA’s Asteroid Impact Monitor (AIM) and NASA’s Double Asteroid Redirection Test (DART), will provide a great deal
of technical and engineering data on spacecraft operations for future human space exploration while conducting in-depth scientific examinations of the binary
target Didymos [1, 2] both prior to and after the kinetic
impact demonstration. The knowledge gained from
this mission will help identify asteroidal physical properties in order to maximize operational efficiency and
reduce mission risk for future small body missions.
The AIDA data will help fill crucial strategic
knowledge gaps concerning asteroid physical characteristics that are relevant for human exploration considerations at similar small body destinations.
Small Body Strategic Knowledge Gaps: For the
past several years NASA has been interested in identifying the key strategic knowledge gaps (SKGs) related
to future human destinations. These SKGs highlight
the various unknowns and/or data gaps of targets that
the science and engineering communities would like to
have filled in prior to committing crews to explore the
Solar System. An action team from the Small Bodies
Assessment Group (SBAG) was formed specifically to
identify the small body SKGs under the direction of
the Human Exploration and Operations Missions Directorate (HEOMD), given NASA’s recent interest in
NEAs and the Martian moons as potential human des-
tinations [3]. The action team organized the SKGs into
four broad themes:
1) Identify human mission targets;
2) Understand how to work on and interact with the
small body surface;
3) Understand the small body environment and its
potential risk/benefit to crew, systems, and operational assets; and
4) Understand the small body resource potential.
Each of these themes were then further subdivided into
categories to address specific SKG issues.
Potential AIDA Contributions: The AIDA mission should be able to address specific aspects related
to SKG themes 2 and 3, and possibly some aspects
related to themes 1 and 4. Theme 1 deals with the
identification of human mission targets within the
NEA population. The current guideline indicates that
human missions to binary asteroids may be too risky to
conduct successfully from an operational perspective.
However, no spacecraft mission has been to a binary
NEA before. Hence the information that AIDA will
gather on the Didymos system could be used to reassess the current restriction concerning binary asteroids
as potential human destinations.
SKGs from themes 2 and 3 are undoubtedly where
the data from the AIDA mission will return the most
value. Theme 2 addresses the concerns about interacting on the small body surface under microgravity conditions, and how the surface and/or sub-surface properties affect or restrict the interaction. The AIM spacecraft has a suite of remote sensing instruments (e.g.,
visible and thermal imagers, radar transmitter, radio
science receiver, etc.) that can characterize the surface
of Didymos to a high degree. These instruments can
also be used to infer some of the interior properties of
the binary asteroid system. In addition, AIM has two
payload packages that are planned for deployment for
in situ characterization of the surface. These element
packages will be able to gather vital information on the
geotechnical and compositional properties of Didymos.
The combination of the remote sensing instrument
suite and the in situ payloads with their local perspective, will provide AIDA with good insight into the asteroid’s surface and subsurface properties.
This
knowledge will be useful in planning and designing the
systems required for astronauts to explore and work on
the surface of an NEA.
SKG theme 3 deals with the environment in and
around the small body that may present a nuisance or
hazard to any assets operating in close proximity.
Both the DART and AIM spacecraft contribute to the
SKGs related to this particular theme. AIM will be
able to image the Didymos system and constrain the
size of any particulates that may be present prior to the
arrival of DART. AIM will also be able to image the
impact of DART and monitor the response of the binary system to the kinetic impact. The information
gained on the crater formation processes, the amount
of ejecta released during the impact, and the ejecta’s
response over time will help address issues related to
particle longevity, internal structure, and the nearsurface mechanical stability of the asteroid. Understanding or constraining these physical characteristics
are also important for future human mission planning.
Although Didymos has not been identified as an
organic or volatile-rich asteroid [4], the AIDA mission
data may be used to constrain whether or not potential
resources could exist on similar targets on the surface
or at depth. This would address the SKG theme 4 and
help identify the protocols necessary for understanding
the resource potential of small bodies.
Conclusions: The AIDA mission comprised of
ESA’s AIM spacecraft and NASA’s DART impactor
can provide a wealth of information relevant to the
science and planetary defense of NEAs. However, this
mission to investigate the binary asteroid Didymos can
also provide key insights into small body strategic
knowledge gaps and contribute to the overall success
for human exploration missions to asteroids.
References: [1] P. Pravec et al. (2006) Icarus, 181,
63 – 93. [2] L. A. M. Benner et al. (2010) 42nd DPS,
abstract #13.17. [3] A. S. Rivkin et al. (2013) Small
Bodies Assessment Group Special Action Team report
on small body strategic knowledge gaps.
http://www.lpi.usra.edu/sbag/meetings/jan2013/present
ations/sbag8_presentations/MON_1330_Sykes_SBAG
_SKG_SAT_report.pdf [4] J. de Leon et al. (2010)
Astronomy and Astrophysics, 517, A23, 1 – 25.
The advantage of Synthetic Tracking in the Detection and Astrometry of Small NEOs M. Shao1, C.
Zhai1, J. Sandhu1, T. Werne1, S. Turyshev1, G. Hallinan2, L. Harding2,
Calif. [email protected]
Introduction: Synthetic tracking is a technique that
uses multiple short exposure images of a NEO along
with low cost teraflop computers to detect faint fast
moving objects that would otherwise be undetectable in
a single exposure CCD image. New low noise, high
speed detectors such as SCMOS and EMCCD cameras
can take 120 images in 30sec without adding noise
above the zodi background. These images can be shifted at the velocity of the moving NEO to stack all the
photons into one pixel. Because the velocity of the
object is not known prior to detection many different
velocities must be tried. The algorithm sometimes
called a multihypothesis matched velocity filter has a
high computational cost which has become affordable
with modern graphic processing units. For a NEO that
would streak 30 arcsec during a 30sec exposure synthetic tracking improves the SNR by a factor of ~30.
Streaked images also significantly degrade astrometry
of the NEOs. Synthetic tracking can avoid the astrometric error from streaked images and offers the possibility of getting accurate orbits of NEOs. We have
measured precision of 40 nrad for a 1 min observation.
Over 1000 new NEOs are discovered each year and
almost all of them are subsequently lost. Synthetic
tracking offers the possibility of measuring their orbits
with accuracies almost as good as Radar + traditional
optical astrometry, but do this for all 1000+ new NEOs
discovered each year instead of the ~100 that can be
observed by Radar per year.
1
J.P.L. Pasadena Calif, 2Caltech Pasadena
and fig 3 shows them coadded at the velocity of the
Near Earth asteroid.
The shift/add process is illustrated in fig 1 below.
The 23 mag NEA was detected with a SNR ~15 in ~30sec on
the 5m telescope. It was detected twice ~ 1hr apart, having
moved slightly more than 1000 arcsec between the two
observations. If the object was moving 10km/s, and had an
albedo of ~15%, this would a ~8m asteroid detected at a
distance of ~8 million km, H~29 mag.
A test of the synthetic tracking technique is illustrated
below in fig 2,3. Data was recorded with the Palomar
5m telescope using a high speed camera at ~15
frames/sec while the telescope tracking was set at 5
arcsec/sec below sidereal rate. ~450 frames of data
were coadded with ~10,000 different shift velocities.
Figure 2 shows the images coadded at the sidereal rate
Shao, M. et. al, ApJ, 782 1S.
Zhai, et. al., ApJ 792, 60Z.
DIDYMOS AS A BINARY ASTEROID. D. C. Richardson1 and K. J. Walsh2, 1Department of Astronomy, University of Maryland, College Park MD 20742, [email protected], 2Southwest Research Institute, 1050 Walnut St,
Ste 300, Boulder CO 80302, [email protected].
Introduction: As of January 2014 there were 158
binary or multiple small bodies known in the solar
system [1—source of most of the data below]. Of these, 48 are near-Earth asteroid systems (including 2
triples), 19 are Mars-crossers (1 triple), 93 are Main
Belt asteroids (5 triples), 4 are Jupiter Trojans, and 78
are trans-Neptunian objects (2 triples, plus Pluto with 5
satellites). The discovery rate has generally been much
greater in the past decade than in the first decade [2]
after the discovery of the Ida-Dactyl system by the
Galileo spacecraft in 1993 [3]. Didymos is a nearEarth asteroid with a binary companion [4].
Detection: Binaries and multiples have been detected by direct imaging of components, mutual events
in lightcurves, and radar reflectance, with lightcurves
being responsible for the most discoveries to date [1].
Lightcurves, in which the amplitude of the combined
amount of reflected light from the targeted system is
seen to vary with time according to the rotation and
mutual eclipsing of the components, favors large secondaries with small separations. Radar, for which a
transmitted beam at radio wavelengths is reflected
from the target and provides information on range and
radial motion, favors large, nearby targets, with synchronous secondary rotation. Direct imaging, such as
by the Hubble Space Telescope or ground-based adaptive optics systems, is best for distant targets with slow
relative motion with respect to the observer, and is
limited by the light-gathering power and angular resolution of the instruments. Direct imaging by spacecraft
is limited only by the on-board instruments. The Didymos binary was detected by the Ondřejov Observatory and other assisting observatories using the
lightcurve technique with subsequent confirmation via
radar observations [4].
Binary Properties: Binary near-Earth asteroids
(NEAs) and Mars-crossers (MCs) have mean size ratios of 4.3:1 (median 3.7), mean separations of 4.9 primary radii (median 3.9), and have primaries than tend
to be small, spherical/oblate, and fast-rotating. About
15% of NEAs are binaries/multiples [5]. Among binary Main Belt asteroids (MBAs) and Jupiter Trojans, the
mean size ratio is 7.4:1 (median 3.7)—but can be as
large as ~90:1—and the mean separation is 12 primary
radii (median 4.7). About 2–3% of MBAs are binaries/multiples, but the fraction is higher among small
MBAs [6]. Finally, trans-Neptunian objects (TNOs),
including Centaurs, have mean size ratios of 2.1:1
(median 1.3) and mean separations of 140 primary
radii (median 44). Roughly 10–30% of TNOs are binaries/multiples, depending on population [7]. Figure 1
shows the size ratio of nearby binaries as a function of
their normalized component separation. In this figure
it is evident that Didymos is fairly typical of binary
NEAs.
Binary origins: Due to systematic differences in
physical and orbital properties, it is surmised that different mechanisms dominate the formation of binaries
among NEA, MBA, and TNO populations [2]. For
NEAs and MBAs, the lifetime of a typical binary
against collision, ejection, or disruption is short compared to the age of the solar system, implying recent
formation, particularly for the NEAs and small MBAs.
It should be noted that the inner Main Belt population
might be quite similar to the NEAs: the smallest binary
MBAs detected are now approaching the size of the
largest binary NEA. Formation models fall into 3
broad categories: capture of satellite during close approach, capture of ejecta following impact, and capture
of ejecta following rotational disruption. For closeapproach capture, energy must be removed from the
system to reduce the encounter speed; this can be provided by a third body or a swarm of bodies, requiring
higher population densities than are seen today—this
may be at the origin of binary TNOs [7]. For collisional ejecta capture, binaries could result from the
retention of material in the vicinity of the largest remnant, or mutual capture of escaping ejecta along similar
trajectories—this is the favored mechanism for most
large binary MBAs [8]. For rotational ejecta capture,
the needed torque could be applied impulsively
through tidal encounters with a planet, or gradually
through the YORP thermal spin-up mechanism, but
requires that the affected body have little to no tensile
strength (cohesion), i.e., it would need to be a “rubble
pile” or gravitational aggregate, possibly as a result of
reaccumulation of fragments from a prior disruptive
impact [9]. Tidal encounters tend to lead to elongated
primaries and the likelihood of a subsequent disruptive
tidal encounter, so this mechanism is not likely responsible for the majority of binary NEAs (and cannot account for small, inner MBAs) [10]. YORP spin-up
depends on body size and heliocentric distance, but in
principle can torque a km-sized NEA or inner MBA to
the point of mass loss on a timescale of ~1 Myr (about
10% of the median NEA lifetime) [11]. Supporting
evidence for YORP being at the origin of binary NEAs
is that the primaries of these binaries tend to be oblate
Figure 1: Ratio of secondary physical radius Rs to primary physical radius Rp of nearby binary asteroids as a function of their semimajor axis a normalized by Rp. Symbols distinguish NEA, MC, and MBA populations. Didymos
is indicated by an arrow. Data as of March 2014, from [1]. Triples show largest satellite only.
and rotating near the so-called cohesionless spin barrier for typical asteroid bulk densities (the radar-imaged
binary 1999 KW4 is one such system). There is also
plenty of evidence than many small solar system bodies are rubble piles, with Itokawa being a prime example. Whether YORP-formed binaries result from reaccumulation in orbit of ejecta lost gradually from the
primary equator and leading to the top-shapes that
seem prevalent among radar images of small asteroids
[12]—or from direct fission of a fully-formed secondary from the primary with subsequent fallback of debris invoked to explain the top-shape equatorial ridges
[13]—is a matter of continued debate. The gradual
spin-up mechanism predicts regolith motion from the
pole to the equator, which may have an observable
signature in future direct spacecraft imaging or spectroscopy. Note that the prevalence of asteroid pairs
(detached binaries) suggests that direct fission may
operate in at least some circumstances as well. Due to
its fast primary rotation and small lightcurve amplitude, the Didymos system likely formed through
YORP-induced spinup.
Didymos: The primary of the Didymos system has
a rotation period of 2.26 h, implying it must have cohesion to maintain material at the equator [14,15]. By
contrast, the 1999 KW4 primary has a rotation period
of 2.76 h, close to but still below the spin limit. The
lightcurve amplitude of Didymos is 0.1 compared to
0.13 for KW4 (0 indicates a sphere, 1 indicates extreme
elongation). The secondary-to-primary size ratio is
0.22 for Didymos and 0.342 for KW4. The semimajor
axis of the secondary orbit is 2.8 primary radii for Didymos and 3.87 for KW4. Due to Didymos’ fast rotation, low lightcurve amplitude, and other similar characteristics to KW4, it is likely that it has a top-shape.
Whether it has a pronounced equatorial ridge, and
whether the secondary formed from reaccumulation or
direct fission, may be questions that a space mission
could help answer.
References: [1] Johnston W. R. (2014) NASA PDS,
EAR-A-COMPIL-5-BINMP-V7.0. [2] Richardson D. C.
and Walsh K. J. (2006) AREPS, 34, 47–81. [3] Belton
M. J. S. (1994) Science, 265, 1543–1547. [4] Pravec P.
et al. (2003) IAUC 8244. [5] Pravec P. et al. (2006)
Icarus, 181, 63–93. [6] Warner B. D. et al. (2009) Icarus, 202, 134–146. [7] Noll K. S. et al. (2008) “Binaries in the Kuiper Belt” in The Solar System Beyond
Neptune, pp. 345–363. [8] Durda D. D. et al. (2004)
Icarus, 170, 243–257. [9] Richardson D. C. et al.
(2002) “Gravitational Aggregates: Evidence and Evolution” in Asteroids III, pp. 501–515. [10] Walsh K. J.
and Richardson D. C. (2008) Icarus, 193, 553–566.
[11] Rubincam D. P. (2000) Icarus, 148, 2–11. [12]
Walsh K. J. et al. (2008) Nature, 454, 188–191. [13]
Jacobson S. A. and Scheeres D. J. (2011) Icarus, 214,
161–178. [14] Holsapple K. A. (2007) Icarus, 187,
500–509. [15] Scheeres D. J. et al. (2010) Icarus, 210,
968–984.
THE AIDA TARGET 65803 DIDYMOS: OBSERVATIONAL OPPORTUNITIES BEFORE, DURING, AND
AFTER THE DART COLLISION A. S. Rivkin1 and the DART team, 1JHU/APL
Introduction: The Asteroid Impact and Deflection
Assessment (AIDA) mission consists of two parts: the
Asteroid Impact Monitor (AIM) observer spacecraft,
and the Double Asteroid Redirection Test (DART).
The target of the AIDA mission is the binary 65803
Didymos system, with DART targeting the satellite
specifically.
We know much about Didymos already, which has
fed into the decision to target it: It is has an S-class
spectrum, like Eros and Itokawa. The primary is
roughly 750 m in size with a rotation period under 2.5
hours, the secondary is about 170 m in diameter with
an orbital period of roughly 12 hours around the primary. The
However, there is much that is still unknown about
Didymos, particularly the pole position of the system.
In addition, in order to obtain the maximum results
from this mission, we must measure the lightcurve and
orbital period of the Didymos system to the highest
quality we can muster. Other information, such as the
closest meteorite analog to Didymos will also be of
use.
Beyond observations in the near-term, it is also not
too soon to outline the observing campaign that will
support the DART impact.
Near-term Observing: The Didymos system next
reaches opposition on 10 April 2015, at a visible magnitude of 20.5. The most critical measurements to be
made are of its lightcurve, particularly during the period March-August 2015, when one of the pole solutions
predicts mutual events and the other one doesn’t. Observations in this period should allow us to distinguish
which pole solution is correct, and determine the latitude of DART’s impact, though by early May Didymos reaches V = 21 and dropping so observations become more difficult.
A drop of ~0.1 magnitudes is expected from mutual
events, so a S/N of roughly 20 or better will be needed
for these observations. Eclipse durations are of roughly
100 minutes.
Additional useful observations during this apparition would include higher-quality visible-IR spectra.
At V = 20.5 and fainter this will be challenging, and
likely require telescopes like Magellan, or even Keck.
After the 2015 apparition, The next opposition is in
March 2017 at V ~ 20.3, followed by March 2019 at V
~ 19.8.
Impact observations: The DART impact in 2022
provides the opportunity for a supporting ground campaign. The impact is scheduled during a period of ex-
cellent visibility from the ground, ideally during a period of observability from the Arecibo and/or Goldstone radars. We anticipate an active campaign including visible, near-IR, and mid-IR imaging, with the goal
of extracting information about the impact from direct
observation of the system and modeling of any visible
impact debris.
We will discuss these topics and the facilities we
hope to use.
AIDA Mission Support Capabilities of the Magdalena Ridge 2.4-meter Telescope. W. H. Ryan and E. V. Ryan,
New Mexico Institute of Mining and Technology, Magdalena Ridge Observatory, 101 East Road, Socorro, NM
87801; [email protected]; [email protected].
Introduction: The Magdalena Ridge Observatory
(MRO) 2.4-meter telescope is located in Central New
Mexico at 10,650 feet (3246 meters) and achieved full
operational status in 2008. The observing site has excellent environmental conditions with sub-arc-second
seeing not uncommon below 20º elevation. It is a dualuse facility that has been tasked primarily to study both
natural and artificial near-Earth targets. The Space Situational Awareness (SSA) mission of the observatory is
an inherent progression of the historical development
of the MRO project. The Near-Earth Object (NEO)
focus is primarily motivated by the staff expertise and
interest in the study of solar system collisional events:
both theoretical modeling and the simulation of collisional events in the laboratory [1, 2], and now examination from the observational perspective. At present,
MRO is one of the top-ten astrometric follow-up telescopes in terms of productivity and regularly tackles
the fainter or other challenging newly discovered NEO
targets.
The following describes the capabilities of the
MRO facility with the intent to highlight features useful for ground-based characterization of the binary system 65803 Didymos (1996 GT) in preparation for the
potential AIDA spacecraft mission.
General Capabilities: Due to its SSA mission objectives, the telescope is capable of slewing and tracking at 15 degrees per second with an acceleration of 3
degrees per second squared. This makes even the fastest
NEOs well within reach. Moreover, this tracking is not
limited to perturbations from sidereal tracking, but rather the telescope is able to follow any tabulated track
(typically available from the Minor Planet Center or
JPL). The telescope can also point two degrees below
horizontal, which allows it to take advantage of the
surprisingly good seeing that can manifest itself near
horizon allowing for the imaging of very low elongation targets. In particular, in January 2011, images of
the Stardust-NExT target Comet Tempel 1 that were
acquired in the morning twilight at ~5º elevation generated reliable astrometry to confirm that the spacecraft
was on track for its February encounter that year.
The primary NEO mission of MRO to date has been
astrometric follow-up and spin-rate characterization of
recently discovered targets. For slow moving objects
moving at essentially 'sidereal' rates, V>24 targets have
been captured using a VR-filter in a single 4-minute
exposure under good seeing conditions. An example
image is the potentially hazardous asteroid (PHA) 2012
QK45 shown in Figure 1.
Figure 1. Single 240-second image of 2012 QK45 (purple circle) at V~24.1 taken with the MRO 2.4-meter.
However, in general, astrometric capabilities are a
strong function of target speed and atmospheric conditions. In addition, the current imager has a short
readout time such that NEOs with spin states less than
one minute are easily detected.
Recently, low-resolution visible-wavelength spectroscopic capability has been added to the facility
through the use of the Magdalena Optical Spectroscopy
System (MOSS). Its R~250 resolution and quick access (it is permanently mounted on the telescope’s second Nasmyth port) makes it ideal for characterizing
recently discovered, target-of-opportunity NEOs. Figure 2 shows a sample spectrum taken of NEO 2014 EC
shortly after discovery.
Figure 2. Visible spectra of 2014 EC (dark blue symbols) obtained using MOSS on March 6, 2014 indicates a Sq-Q-type composition (silicate/metallic). The
mean SMASS Q and Sq values (green/light blue symbols) are overplotted to illustrate how the compositional determination is estimated.
Mission Specific Capabilities: A feature of the
MRO 2.4-meter telescope beneficial to the AIDA mission is its ability to detect small-amplitude mutual
events given the telescope’s faint limiting magnitude.
This will be a factor when Didymos is observable at
V~20-21 in the Spring 2015. The current understanding of the 65803 Didymos system allows for two pole
solutions: one that predicts mutual events in Spring
2015 and one that does not. Therefore, observations
taken with a 2-meter class telescope will allow for distinguishing between the two possibilities and help in
identifying a unique solution.
Researchers at MRO have experience studying attenuations in binary systems and are familiar with the
techniques required to detect mutual events from photometric data. In particular, the existence of the binary
nature of Vesta family asteroids 3782 Celle and 3703
Volkonskaya was determined using this method [3].
This is demonstrated in Figure 3 for the Celle system.
Figure 3. Primary and secondary attenuation events in
the binary system 3782 Celle. From [3].
The predicted signal-to-noise (S/N) of the MRO 2.4meter telescope/camera system is sufficient to extend
this technique to V~20-21 in order to confirm or rule
out the existence mutual events of this nature for
Didymos in 2015. In particular, models predict a S/N
ratio of about 50 in two-minute exposures at V~20.5.
This capability is validated by examining the
lightcurves in Figure 4 of 1999 RQ36 (101955 Bennu).
The data for this faint target were taken in September
2011 (V~21) and again in April 2012 (V~20) in
crowded sky conditions.
Figure 4. Lightcurve for 1999 RQ36 acquired in September 2011 (left) and April 2012 (right) using the
MRO 2.4-meter telescope. The spin period is 4.295
hours. The magnitudes for the asteroid at the times the
observations were collected were V~21 and V~20, respectively.
Individual points have photon uncertainties on the order of 0.05-0.08 magnitudes in the 2011 data and ~0.03
magnitudes in the 2012 data, although the scatter in the
composite curves suggests an uncertainty that is slightly larger. This scatter is most likely due to field star
contamination given the crowded sky. Despite the
noisy curves, these graphs indicate that features on the
order of a tenth of a magnitude are marginally resolvable with a V~21 target and clearly evident at V~20.
Didymos will fall between these two cases for the predicted Spring 2015 mutual events. Therefore, given that
mutual attenuation events are predicted to last on the
order of 100 minutes, sufficient data of the required
precision should be attainable to confidently confirm or
rule out the existence of these events and hence allow
for a unique determination of the pole solution.
Conclusions: Researchers at the MRO 2.4-meter
telescope facility are well positioned to meet the
ground support requirements of the AIDA mission.
Aside from the ability to perform and assist in the characterization of the binarity of the Didymos system
through the study of mutual events, it can perform any
mission-required faint object astrometry or visible
spectroscopy.
References:
[1] Ryan, E.V. (2000). Ann. Rev. Earth Planet. Sci.,
28, 367-389.
[2] Ryan, E.V., and H.J. Melosh (1998). Icarus
133, 1-24.
[3] Ryan,W.H., E. Ryan, and C. Martinez (2004).
Planetary and Space Science, 52, 1093 -1101.
Radar Imaging and a Physical Model of Binary Near-­‐Earth Asteroid 65803 Didymos L. A. M. Benner1, J. L. Margot2, M. C. Nolan3, J. D. Giorgini1, M. Brozovic1, C. Magri5, S. J. Ostro1 1Jet Propulsion Laboratory, California Institute of Technology, 2University of California, Los Angeles, 3Arecibo Observatory, 4University of Maine at Farmington We report Arecibo S-­‐band (2380 MHz, 13 cm) and Goldstone X-­‐band (8560 MHz, 3.5 cm) delay-­‐Doppler radar observations of binary near-­‐Earth asteroid 65803 Didymos obtained on five dates between 2003 November 14-­‐26 during the asteroid’s approach within 0.048 AU. The images achieve resolutions as fine as 15 m/pixel, spatially resolve and provide thorough rotational coverage of the primary, and show obvious orbital motion by the secondary. The images and shape modeling indicate that the primary is about 800 m in diameter, that it has a slightly asymmetric and oblate shape resembling a top, and that it has a prominent facet about 300 m in extent along the equator. The S-­‐band circular polarization ratio of the primary averages 0.22+-­‐0.02 and indicates that its near-­‐surface is somewhat less rugged at decimeter spatial scales than the surfaces of 433 Eros, 4179 Toutatis, or 25143 Itokawa, objects that have all been imaged at high resolution by spacecraft. The images do not provide sufficient signal-­‐to-­‐noise, spatial resolution, or rotational coverage to estimate a 3D model of the secondary, but they place up to five range pixels on the secondary and establish a lower bound on its long axis of ~75 m. If the full range extent of the secondary were double the visible extent, which would be true for a sphere, then the secondary would be roughly 150 m in diameter. The Doppler broadening of the secondary is narrow and consistent with rotation that is synchronous with its orbital period. Orbital fits give a semimajor axis of about 1.2 km and an orbital period of 11.9 h in agreement with the period estimated from photometry by Pravec et al. (2006, Icarus 181, 63-­‐93); together they provide a first-­‐order estimate of ~2 g/cm3 for the bulk density of the system. DYNAMICS OF LOFT REGOLITH ON THE PRIMARY OF THE NEA BINARY SYSTEM DIDYMOS,
TARGET OF THE AIDA MISSION.
A. Campo Bagatin 1, F. Moreno2, A. Molina2,3. 1 DFISTS-IUFACyT,
Universidad de Alicante, Spain ([email protected]. P.O. Box 99, 03080 Alicante, Spain). 2Instituto de Astrofísica de
Andalucía, Granada, Spain. 3Universidad de Granada. Spain.
Abstract
An increasing number of Near Earth Asteroids
(NEAs) in the range of a few hundred meters to a few
kilometers in size are found to have relatively high
spin rates (less than 4 hr, down to ~2.3 hr, depending
on taxonomic type). Due to their high spin rate local
acceleration near their equator may in some case be
directed outwards so that lift off of near-equatorial
material is possible. In particular, this is the case of
the primary of the Didymos binary system, target of
the AIDA mission. What are the effects of that phenomenon on surface material at any asteroid latitude?
1. Motivation
Both coherent bodies and gravitational aggregates
(GA) (often called “rubble piles”) may stand spin rates
higher than the critical ones for fluids found by Chandrasekhar [1]. In the case of coherent structures that
is due to internal solid state forces while in the case of
gravitational aggregates shear strength may easily
appear as a consequence of friction among GA components [2] increasing structural yield. Near Earth
Asteroids (NEAs) coming from the asteroid belt are
believed to be mostly GA in the range ~0.5-1 km to
~50 km [3] due to their collisional history. Once in the
inner Solar System, NEAs may undergo spin up evolution through the non-gravitational YORP effect [4]
causing their components to disperse, to shed mass or
to fission and eventually form binary, multiple systems
and asteroid pairs [5, 6]. The end state of those events
often is an object spinning above any Chandrasekhar
stability limit, kept together by friction and characterized in some case by the presence of an equatorial
“bulge”, as shown by radar images [7, 8]. This seems
to be the case of the primary bodies of binary systems
1996 FG3 and 1996 KW4, and the single body 2008
EV5, among others. The Didymos primary has been
spotted by radar in 2003, even if not at a precision
level to evidence any such feature.
In the case of some NEAs, the centrifugal force acting
on surface particles and boulders at near-equatorial
latitudes may slightly overcome the gravitational pull
of the asteroid itself, having the opportunity to leave
its surface.
As centrifugal is a contact force, leaving the surface
does not mean that particles are lost form the asteroid,
in fact, particles leave the surface at negligible velocity and as soon as they lift off they move only under the
gravitational field of the asteroid, the non-inertial
apparent forces due to rotation, the Sun’s gravity and
–in the case of binary systems- the secondary’s gravity. Therefore, particles may levitate for some time,
land on the surface and lift off again, repeating this
cycle over and over. Alternatively they may enter
orbiting states or even transfer to the secondary. Centrifugal and gravitational forces have the same dependence on a given particle mass, their action is then
independent on mass itself: small dust particles may
leave the surface as well as large boulders. Other forces may act as well on small particles, like electrostatic
forces or molecular forces (cohesion), with the likely
result to stick them together and still undergo the
same effects as dusty clumps.
Moreover, small particles may be lost as they undergo
solar pressure force able to subtract them to the asteroid’s gravity while they are levitating. [9] have studied
some of the features of this problem, relevant to binary
dynamics.
2. Methodology
We have collected available data on binary asteroid
systems with very good accuracy in spin rate determination and acceptable uncertainties in mass.
This study follows and develops researches made by
the working group on mechanical properties of the
“MarcoPolo-R” mission proposed to ESA in its past
M3 call. In that case, the goal was to study regolith lift
off features on both 1996 FG3, the former target of the
mission, and 2008 EV5, its nominal target.
We have now focussed on two binary systems: Didymos and 1996 FG3. In order to study the dynamics of
particles in those systems, we use a numerical code
that integrates, by a fourth-order Runge-Kutta method,
the equations of motion of individual particles that are
ejected from the asteroid surface when centrifugal
acceleration is strong enough to overcome local gravity. The equation of motion is written in a non-inertial
asteroid-centered reference frame, taking into account
the asteroid (and the secondary, in the case of binaries) and solar gravity, radiation pressure, and inertial
terms. A version of this code has been successfully
tested and applied by Molina et al. [10] to the study of
particles in comet environments.
We study the motion of particles in the 1 μm to 10 cm
range in the non-inertial reference frame of the rotating primary, accounting for centrifugal and Coriolis
apparent forces as well as the gravitational fields of
the primary, the secondary, the Sun and the radiation
forces by the Sun itself. The eccentricity of theheliocentric orbit of the system is taken into account.
The dynamics of particles of a wide mass range is
calculated during many orbital cycles as a function of
their initial position on the asteroid surface for each
system under study. A relative mass density of levitating particles is calculated as a function of distance to
surface, latitude, and longitude. In the very case of
Didymos, the study is being extended to discuss the
range of size and mass of the primary.
3. Results
We find that fine particles are easily swept away the
parent body by radiation pressure, while larger particles may undergo landing and lift off cycles that form
a dusty environment above the surface at nearequatorial latitudes. We present the results of our
ongoing study in the case of Didymos. The consequences in the planning of missions to NEAs with
similar characteristics are also discussed.
Acknowledgements
ACB acknowledges funding by the Spanish Ministerio
de Ciencia e Innovación (now extinct) research program AYA2011-30106-C02-02. FM and AM
acknowledge funding by contracts AYA2012-39691C02-01 and FQM-4555 (Junta de Andalucía).
References
[1] Chandrasekhar, S. (1969) Ellipsoidal Figures of Equilibrium, Yale Univ. Press.
[2] Holsapple, K.A. (1997) Icarus, 187, 500-509.
[3] Campo Bagatin, A. et al. (2001) Icarus, 149, 198-209.
[4] Rubincam, D.P. (2000) Icarus, 148, 2-11.
[5] Walsh K. et al. (2008) Nat., 454, 188-191.
[6] Jacobson, S.A. and Scheeres, D.J. (2011) Icarus, 214,
161-178.
[7] Ostro, S.L. et al. (2006) Sci., 314, 1276-80.
[8] Bush, M.W. et al. (2011). Icarus, 212, 649-660.
[9] Fahnestock, E. G. and Scheeres, D. J. (2009) Icarus,
201, 135-152.
[10] Molina, A., et al., (2009), in Lecture Notes and Essays
in Astrophysics, A. Ulla and M. Manteiga eds., vol. 4, pp.
75-80.
USING KINETIC PROBES FOR EXPLORATION AND DEFLECTION: LESSONS FROM THE DEEP
IMPACT AND LCROSS MISSIONS. P. H. Schultz, Dept. Earth, Environmental, and Planetary Sciences, Box
1846, Brown University, Providence, RI 02912, [email protected]
Introduction: The Deep Impact and LCROSS missions were the first artificial kinetic probes purposely
designed to explore the surface and subsurface of two
very different planetary bodies: the former unknown,
the latter known.
While not purposed for defending against an approaching comet or asteroid, the Deep Impact mission
to 9P/Tempel-1 (hereafter, 9P/T-1) does underscore the
need to anticipate the response of an unknown target
body to a hypervelocity impact prior to the encounter.
Such efforts not only set constraints to meet mission
objectives but also optimize targeting [1]. This mission
established fundamental properties of a cometary nucleus including composition of the surface/subsurface,
its density (bulk and surface), and results of an experiment at a much larger (~363kg) and faster
(10.2km/s) than possible at laboratory scales.
In contrast, the LCROSS impact experiment impacted the shadowed surfaces near the lunar South
Pole. While both DI and LCROSS sought to characterize subsurface materials, they used dramatically
different approaches. The Deep Impact mission released a well-designed autonomous probe for targeting
with observations made from flyby spacecraft. The
collision by Deep Impact challenged our understanding
of empirically derived scaling relations (crater size,
cratering efficiency, and the amount of ejecta) due to
its much higher speed, very low gravity of the object,
and high projectile-to-density ratio [e.g., 2,3].
The LCROSS mission, however, used the upper
stage of the Centaur rocket (~2300kg) as the impactor.
A detached and trailing spacecraft relayed measurements of the impact event but collided with the lunar
surface soon afterwards. The LCROSS impactor also
challenged our understanding of scaling relations at the
other extreme: large size, higher gravity of the Moon,
and low projectile-to-density ratio [4,5].
The one thing in common with both kinetic-probes
(both before and after impact) was the need for carefully designed experiments in order to predict the outcome and to understand the observations. While hydrocode models remain a critical tool, such codes
could not fully replicate the wide range of conditions.
The following is my personal view of lessons learned.
Deep Impact Strategy: The nature of the 9P/T-1
Nucleus (shape, density, mass, structure, topography)
was unknown prior to the encounter. Such unknowns
challenged mission designs for both targeting strategies and the projectile (composition, density, shape,
instrumentation). For targeting strategies, numerical
“target practice” was used: location of the brightest
region on a wide range of shapes for the body (spheri-
cal, oblong, dumbbell, etc.) in order to minimize the
likelihood for passing through a dip in the shape while
optimizing the impact angle. Because the composition
of the subsurface was a prime mission objective, the
projectile had to be primarily composed of material
(copper) that would not create ambiguity for spectral
analyses. The high density of copper, however, created a serious issue for the impact coupling process.
Interactions between the science and engineering
teams selected a design using a layered copper hemisphere compose of holes in order to reduce the effective density. The layered design allowed adjustments
to the projectile mass as weight requirements changed.
Pre-Encounter: The large number of unknowns
about the nucleus could significantly affect the outcome of the collision, such as the amount of ejecta
produced. As a result, a series of experiments were
performed in order to constrain expectations prior to
the encounter [2]. First, the unknown shape of 9P/T-1
Nucleus could affect the final approach angle, whereas
the local topography (local slopes or roughness relative
to the projectile diameter) could affect crater size. Preimpact experiments revealed that relief on the order of
the projectile diameter significantly changed crater
scaling for oblique impacts. As a result, autonavigation imaging on the probe was not only used to
guide the collision to a general location on the nucleus,
but also to locate the impact point, characterize the
nature of the surface, and assess the general orientation
of the probe at impact.
Second, impact angle affects coupling, thereby affecting flyby observations of the impact vapor plume
(not a prime objective), ejecta composition (a prime
objective) along with excavation depth, and ejecta evolution. Analyses of standard targets (e.g., sand, pumice
powder) from experiments at the NASA Ames Vertical
Gun Range (AVGR) provided pre-mission estimates of
cratering efficiency and crater formation time (critical
for planning the duration of observations).
Third, the number of unknowns for the surface
properties of 9P/T-1 required use more exotic targets.
For example, some pre-mission analyses predicted that
little ejecta would be observed due to compression of
highly porous target materials [6]. Consequently, the
series of AVGR experiments included the effect of
impact angle on crater excavation and the impact flash
for under-dense targets (0.3-1.0g/cc). Instead of compression cratering dominating excavation, low-angle
impacts (30° from the horizontal) significantly increased cratering efficiency due to the reduced effective coupling depth, i.e., shallower effective depth of
burst [2]. Moreover, the evolution of the ejecta curtain
could be used to assess gravity (hence bulk density)
and near-surface density (hence, nature of the surface).
Post-Encounter: After encounter, follow-on experiments provided: an explanation for the unusual
evolution of the impact flash and delayed emergence
of the vapor plume [7,8]; distinction between impact
excavation and explosive chemical reactions [2]; accounting for the total ejected material based on both
telescopic and DI observations; evolution of spectral
compositions [9]; an explanation for the obscured crater during close approach [7,10,11]; and estimates of
crater size and appearance, which would be assessed
later during the Stardust-NExT flyby 5 years later [12].
LCROSS: The LCROSS mission faced very different challenges. First, mission success depended on
sufficient ejecta ballistically launched into sunlight
well above the surface where atomic and molecular
fluorescence would allow spectral analyses [13]. Second, the impactor was essentially a hollow shell with a
very low density. Third, the impact speed was very low
(~2.5km/s). Fourth, all instruments were space proven
but essentially off the shelf, rather than specifically
constructed (or reassembled) for reliability. Fifth, the
impact site was generally controlled but entirely unknown since it was in shadow. And sixth, there was
still debate on how much water ice might be present.
Nevertheless, the mission achieved its goals through its
unique mission design, multiple observing platforms
(LRO), and a series of fortunate events.
Pre-Encounter: Just as in the DI mission, laboratory experiments played an important role for defining
instrument requirements and approach sequence. Simple extrapolations of empirical results revealed that
ejecta reaching the required altitude might not come
from the excavation stage but very early-time coupling, which would significantly reduce the amount
debris reaching sunlight [4]. Consequently, additional
experiments were needed in order to understand the
effects of the under-dense projectile.
Prior to the mission, new experiments at the AVGR
revealed that a hollow projectile would produce an
unusual excavation sequence. First, the experiments
revealed an early low-angle component that could affect a wide region around the crater [14]. Second,
rather than an expanding cone-shaped ejecta curtain,
most of the ejecta emerge in a near-vertical plume due
to collapse of the hollow projectile [15].
Just as in the DI mission, observations of the event
were critical for understanding the impact conditions,
location, and observational results. Without such data,
there would have been ambiguity in interpreting the
results. One of the unique aspects of this mission was
its proximity to Earth which allowed real-time changes
in sensitivity and exposure times during the final
plunge into the shadows. This last-minute adjustment
allowed viewing the impact site just after impact.
Post-Encounter: The ability to observe the LCROSS
collision with multiple instruments constrained conditions of the impact, thereby enabling interpretations
[13,16]. Because of prior experiments, a ghost-like
cloud migrating across the field of view during final
approach could be recognized as a remnant of the highangle plume, which appeared to move due to changing
perspectives from the shepherding spacecraft [16].
Knowing the precise timing of the impact also allowed
interpreting the evolving spectral emissions as an effect of layering. Later impact experiments at the
AVGR provided more thorough analyses of the
LCROSS event (including the use back-up instruments) in order to: constrain the nature of the thermal
signatures, estimate excavation depths, assess the
ejecta-velocity-angle distributions, and reconcile disparate observations [17]. Subsequent analyses of earthbased telescopic observations of the plume (rising
above the lunar limb) confirmed that the collision created a two-component ejecta plume [18].
Lessons Learned for Future Kinetic Probes:
First, scalable impact experiments or experiments designed to isolate a process or variables are necessary
for interpretations and guiding numerical models.
Second, knowledge of the target shape and surface
structure (under-dense, rubble pile, regolith, etc.) is
critical in order to optimize coupling. Third, imaging
on the kinetic probe is needed in order to understand
the pre-impact target surface. Fourth, the kinetic probe
can be tuned to optimize results (e.g., speed or density)
and control coupling process. Because of the large
unknowns related to momentum transfer and complexities of the process, “target practice” may be critical before implementing a strategy for deflection.
References:
[1] A’Hearn et al. (2005), Science 310, 258-264; [2]
Schultz et al. (2005), Space Science Reviews 117, 207239; [3] Richardson et al. (2005), Space Science Reviews 117, 241–267; [4] Schultz, P. H. (2006), LPI
Contribution 1327, 15-16; [5] Korykansky et al.
(2009), Meteorit. Planet. Sci. 44, 603-620; [6] Housen,
K. R. et al. (1999), Nature 402, 155-156; [7] Schultz et
al. (2007), Icarus 190, 295-333; [8] Ernst, C. M. and
Schultz, P. H. (2007), Icarus 190, 334-344; [9] Sunshine, J. et al. (2007), Icarus 190, 284–294; [10]
Schultz, P. H. (2009), Lunar Planet. Sci. Conf. 40, no.
2386; [11] Schultz et al. (2013), Icarus 222 (2013)
502–515; [12] Veverka, J. et al. (2013), Icarus 222,
424-435; [13] Colaprete et al. (2010), Science 330, no.
6003, pp. 468 – 472; [14] Hermalyn, B. et al. (2009),
Eos Trans. AGU, 90(52), abstract U31B-0034; [15]
Schultz, P. H. et al. (2009), Eos Trans. AGU, 90(52),
abstract U22A-08, (2009); [16] Schultz et al. (2010),
Science 330, no. 6003, pp. 463-468; [17] Hermalyn, B.
et al. (2012), Icarus, 218(1): 654–665; [18] Strycker et
al. (2013), Nature Communications 4, article no. 2620.
THE ISIS MISSION CONCEPT: AN IMPACTOR FOR SURFACE AND INTERIOR SCIENCE.
S. R. Chesley1, P.A. Abell2, E. Asphaug3, and D.S. Lauretta4, 1Jet Propulsion Laboratory, Calif. Inst. of Technology,
4800 Oak Grove Dr., Pasadena, CA 91109, 2NASA Johnson Space Center, 2101 NASA Parkway, Houston, TX
77058, 3School of Earth and Space Exploration, Arizona State Univ., Tempe, AZ 85281, 4Lunar and Planetary Lab.,
Univ. Arizona, 1415 N. 6th Ave., Tucson, AZ 85705.
The Impactor for Surface and Interior Science
(ISIS) mission is a concept for a kinetic asteroid impactor mission to the target of NASA’s OSIRIS-REx
asteroid sample return mission. The two missions
would be strong partners in this investigation, and thus
the ISIS name is apt, given that Egyptian mythology
tells us that Isis was the wife of Osiris. The ISIS mission concept calls for the ISIS spacecraft, an independent and autonomous smart impactor, to guide itself to a
hyper-velocity impact with 101955 Bennu while the
OSIRIS-REx spacecraft observes the collision. Later
the OSIRIS-REx spacecraft would descend to reconnoiter the impact site and measure the momentum imparted to the asteroid through the impact, before departing on its journey back to Earth.
The ISIS mission plan includes a number of potential launch options, including a Geosynchronous Transfer Orbit before the end of 2017, a shared launch in
2018, or a dedicated launch in 2019. For all launch
options, the ISIS launch is after the OSIRIS-REx
launch in September 2016. The OSIRIS-REx spacecraft is scheduled for rendezvous with Bennu in November 2018, and the team expects to collect its sample by October 2019, but there is schedule margin for
additional sampling attempts, until early Jan. 2020.
The window for the OSIRIS-REx injection back to
Earth does not open until March 4, 2021, and so the
OSIRIS-REx spacecraft will wait at least 14 months
for its departure window to open. The ISIS impact
would be late in this period, 4-5 months before the
close of the OSIRIS-REx departure window in late
June 2021. The ISIS impact velocity is 9.5-11.5 km/s,
depending on the launch option.
The ISIS concept of operations begins with the
OSIRIS-REx spacecraft entering a radio science orbit
around 1999 RQ36 in order to establish the pre-impact
asteroid ephemeris with increased precision. Well before the ISIS impact, OSIRIS-REx would move to a
safe vantage point from which to observe the ISIS impact and view the ejecta cone as it expands over a period of several minutes. The spacecraft would also monitor for lofted debris at locations far from the impact to
understand the shockwave propagation through the
body. After the impact the ISIS science investigation
includes three phases, each expected to last 2-4 weeks.
In the first phase, OSIRIS-REx would monitor the debris generated by the impact until it has cleared enough
to allow a safe start to the second phase, which consists
of a low pass, or perhaps a series of passes, over the
impact area to obtain spectra and high-resolution imagery (1-2 cm/pixel) of the crater, as well as areas far
from the impact site. The final stage in the ISIS science
investigation calls for the OSIRIS-REx spacecraft to
again enter a radio science orbit, perhaps a terminator
orbit with radius 1-2 km, in order to facilitate the estimation of the asteroid deflection provided by the ISIS
impact. The ISIS science investigation would be complete in 45-90 days plus schedule margin, at which
point OSIRIS-REx would be free to implement its
Earth-return injection maneuver.
The ISIS mission provides tremendous dividends
across a wide range of planetary science disciplines, as
well as meeting significant objectives of human exploration and technology demonstration.
•
The change in the asteroid’s velocity from the
impact (~0.2 mm/s) would be measureable through
radio science observations with signal-to-noise ratios
>10. This would reveal the momentum enhancement
factor β, shedding light on the geotechnical properties
of the interior and surface materials, as well as greatly
informing any future asteroid impact deflection efforts.
•
The cratering experiment will reveal the mechanical properties of the surface and subsurface material and the size distribution of the regolith material.
The study of the ejected material will shed light on the
meteorite formation process.
•
The crater imaging would reveal the subsurface geology and constrain the crater formation process, while spectra of the pristine material from depth
would provide added context for the OSIRIS-REx
sample.
•
The impact would induce seismic waves that
travel through the body and reflect off boundaries. This
would induce lower energy disturbances far from the
impact site, providing a seismic experiment that may
loft material, cause landslides and topple boulders.
This part of the study would be facilitated by images
showing the dynamic effects of the impact, as well as
through the comparison of pre- and post-impact highresolution imagery.
•
The particulate environment produced by a
range of disturbances would be observed. Moreover, it
is not unreasonable to expect that volatiles could be
released, and these could be spectrally characterized.
The ISIS mission concept would leverage NASA
investments in the OSIRIS-REx mission to provide
extraordinary science returns, provide critical information for human exploration, and demonstrate asteroid impact mitigation technology, all for a small fraction of the cost of a Discovery mission. The ISIS concept is of course suitable as an add-on for other future
missions to sub-kilometer asteroids, and thus the opportunity to form future partnerships would be welcome. However, given the low frequency of NASA
near-Earth asteroid rendezvous missions (almost 20
years between NEAR-Shoemaker and OSIRIS-REx),
the availability of OSIRIS-REx as an observer spacecraft represents an extraordinary but time-critical opportunity.
CHANGING THE SPIN OF AN ASTEROID – A KINETIC-IMPACTOR TEST MISSION. L. Drube1, A. W.
Harris1 and S. Schwartz2, 1Institute of Planetary Research, German Aerospace Center ([email protected]), 2Nice
Observatory.
Introduction: We present a concept for a kinetic
impactor demonstration mission that aims to change
the spin rate of an asteroid rather than the orbit of the
asteroid around the Sun or, for example, the orbital
motion of a moon around its primary. Similar information to the aforementioned existing concepts could
be obtained by measuring the change in the spin period
of an asteroid resulting from an impact that transfers
significant angular momentum to the asteroid, i.e. a
large value of mi vi d sinθ (see Figure 1).
The mission can be either a mission with an impactor and an observer spacecraft, or one with only an
impactor, where the change in the spin period will be
measured from Earth-based telescopes.
Estimation of the effect: A simple estimate of the
magnitude of the possible change in rotation period
with such a mission can be calculated by using the
following angular momentum conservation formula:
where Z is the rotation axis of the asteroid, IZ the
asteroid's moment of inertia, i denotes the impactor,
and β the ejecta. For the rest see Figure 1.
Figure 1. Illustration of a kinetic impactor changing the
rotation period of an asteroid. Credit: Robert Gaskell
produced the shape model of Itokawa's north side used
here [1].
Using Itokawa as an example we assume the following: no ejecta (thus underestimating the efficiency);
no angular momentum lost in other processes (thus
overestimating the efficiency); impactor mass = 500
kg; relative velocity = 10 km/s; the distance d*sinθ =
250m; Itokawa's moment of inertia around the rotation
axis = 7.7686*108 km2kg and its rotation rate = 12.132
hr [2]. It follows that the angular velocity after the impact will be:
Which is a change of about 8 minutes from the
original rotation rate. This magnitude of change in
rotation rate will be visible even from Earth-based observatories in the case of Itokawa.
Detailed computer modeling of the impact is in
progress.
Asteroid target shape: An elongated asteroid
would be preferred, because it has a relatively low
mass compared to the possible impact distance from
the center of mass, and if impacted at the right time, it
might be possible to increase the amount of ejecta in
the direction close to the opposite of the direction of
angular velocity change, thereby maximizing the momentum enhancement factor β. A pronounced shape
will also make it easier to measure the rotation rate
from Earth-based telescopes.
However, more regularly shaped asteroids are not
excluded for use as targets, provided the change in
rotation rate is visible from the observer spacecraft for
a two-spacecraft mission, or visible from Earth in the
years following the impact for a one-spacecraft mission.
One-spacecraft mission: The best demonstration
mission would be a two-spacecraft one, in which a
second spacecraft observes the impact itself, the crater
formation, the ejecta, and the exposed surface in the
crater.
If however the budget only allows for one spacecraft, then this would normally reduce the science
greatly, as there would only be very limited knowledge
about the properties of the asteroid relevant to understanding the results of the test impact. However if the
target asteroid were one that had been observed previously with spacecraft, then the lack of an observer
spacecraft would not necessarily be a serious drawback. The most likely candidates to be used as targets
are Itokawa (Hayabusa I mission target), Bennu (Osiris-Rex mission target) and 1999 JU3 (Hayabusa II
mission target). Mission analyses have however yet to
be performed.
In the case of Itokawa the apparent visual magnitude as seen from Earth brightens to between 20-19
every 4 years, with a very close flyby in 2033 [3]. In
2033 the apparent magnitude will be around 16, and it
will also be possible to observe it with the Goldstone
and the Arecibo radar telescopes (SNR 22 for Goldstone and SNR 310 for Arecibo [4]), whereby any
large surfaces changes as a result of the impact might
be visible.
The change in the rotation rate due to the YORP effect is so small that it is negligible compared to the
effect from the impactor. For example, in the case of
Itokawa the YORP effect causes about 0.0072 minutes
decrease in the period per year [5], which is 3 orders of
magnitudes less than that expected from the concept
described here.
Much science can be obtained from observing the
impact itself and the ejecta produced. Therefore, in the
case of a single spacecraft mission, it would be beneficial to separate the impactor into two parts, such that
an imager with data transmission capability could be
ejected to fly by the asteroid and observe the impact
and the ejecta cloud.
Conclusion: Preliminary studies show that a mission concept in which an impactor produces a change
in the spin rate of an asteroid could provide valuable
information for the assessment of the viability of the
kinetic-impactor asteroid deflection concept. However
further studies are necessary to evaluate in detail relevant aspects of the concept, especially the effects of
regolith and subsurface structure.
References: [1] Gaskell et al. (2006), AAS/AIAA
Astrodynamics Specialists Conf., AIAA paper
2006_6660, Keystone, CO, Aug, 2006. [2] Scheeres et
al. (2007) The effect of YORP on Itokawa, Icarus,
188, 425-429. [3] JPL's Horizons web-interface (Aug
2014)
http://ssd.jpl.nasa.gov/horizons.cgi#top
[4]
Near-Earth Object Human Space Flight Accessible
Target Study (NHATS) website (Aug 2014)
http://neo.jpl.nasa.gov/cgi-bin/nhats. [5] Scheeres et al.
(2007) The effect of YORP on Itokawa, Icarus, 188,
425-429.
Acknowledgments: The research leading to these
results has received funding from the European Union's Seventh Framework Programme (FP7/20072013) under grant agreement n° 282703 (NEOShield).
SMALL CARRY-ON IMPACTOR (SCI) AND DEPLOYABLE CAMERA 3 (DCAM3) IN HAYABUSA-2
MISSINON. M. Arakawa1, T. Saiki2, K. Wada3, T. Kadono4, Y. Takagi5, K. Shirai2, C. Okamoto2, H. Yano2, M.
Hayakawa2, S. Nakazawa2, N. Hirata6, M. Kobayashi3, P. Michel7, M. Jutzi8, H. Imamura2, K. Ogawa2, Y. Iijima2, R.
Honda9, K. Ishibashi3, N. Sakatani2, H. Hayakawa2, H. Sawada2, 1Kobe Univ. ([email protected]), 2JAXA, 3PERC/Chitech, 4Univ. of Occupational and Environmental Health, 5Aichi Toho Univ., 6Univ. of
Aizu, 7Observatoire de la Cote d'Azur, 8Univ. of Bern, 9Kochi Univ.
Introduction: Hayabusa-2, the Japanese next asteroid exploration mission, equips a small carry-on
impactor (SCI). The SCI consists of a disk impactor
made of copper with a diameter of 30 cm (Fig. 1). This
disk will be deformed by an explosion to form a semispherical shell and be accelerated to a velocity ~ 2km/s
for the collision onto the asteroid surface (Fig. 2). This
is a novel apparatus to excavate the asteroid surface,
and hopefully it will enable us to observe a fresh surface without space weathering and thermal alteration.
Furthermore, we will be able to recover the asteroid
sample excavated from several 10 cm depth at the deposit of the impact ejecta. Furthermore, the SCI impact
on the asteroid is a very good chance to examine the
crater formation process on a real asteroid surface under the micro gravity. Here we introduce the scientific
purposes, and the operation and observation plans of
the SCI impact.
Scientific Purposes of SCI: The first purpose of
the SCI impact is to clarify the sub-surface structure.
The asteroid surface is continually shaken by highvelocity impacts and changed by the space weathering
and the thermal alteration owing to the solar radiation.
We are interested in this present dynamics working on
the asteroid surface, thus the sample should be recovered from the ejecta deposit in order to elucidate the
dynamics. The depth information of the recovered
sample is very critical to reconstruct the sub-surface
structure. We should use the crater scaling rule, especially for the ejecta scaling, to estimate the original
depth of the recovered sample. Therefore, the crater
formation mechanism should be studied on 1999JU3 to
obtain the impact scaling rule applicable to the surface
on the asteroid.
However, we have no reliable information of the
Fig.1: Appearance (left) and cross-section (right) of SCI.
surface properties of 1999JU3 to apply the impact scaling rule for the SCI impact crater. Thus, the second
purpose of the SCI is to clarify the sub-surface physical
property of the 1999JU3 and to construct the impact
scaling rule applicable to the surface of this tiny asteroid. Moreover, the tiny bodies like 1999JU3 and Itokawa have a large advantage to investigate the effect of
gravity on the crater formation process because on the
earth it is quite difficult to study the gravity effect on
the crater formation, so the SCI impact could be used
to refine the crater scaling rule especially related to the
gravity.
Real Scale Test of SCI: In order to confirm if the
explosion of SCI and launching the liner are successfully carried out, we have conducted real scale SCI
impact experiments in a field, where a large scale explosion is permitted, at Kamioka-area in the middle
Japan. This is also a good chance to conduct a larger
scale impact experiments than laboratory scale impacts.
We succeeded in observing 5 impact experiments with
our high speed videos. Spherical copper liners were
impacted onto a sand mound and typical ejecta curtains
emerged as we expected (Figs. 2 and 3). The measured
crater sizes are almost consistent with the known scal-
Fig. 2: Expected time evolution of the copper liner after explosion of SCI (left) and a real image of spherical shell liner
with a diameter of ~ 15cm accelerated by a real scale explosion of SCI (right).
Fig.5: Appearance of DCAM3
SCI is exploded with an onboard timer and then the
copper liner impacts onto the asteroid. The liner is expected to impact within a circle region of 200 m radius.
Hayabusa-2 will keep escaping over 200 km away from
the asteroid. Then, Hayabusa-2 will be back to the asteroid, spending more than 2 weeks for its safety from
ejecta floating around the asteroid. After coming back,
Hayabusa-2 will find, observe, and touch-down to or
around the crater.
Fig. 3: Time sequence of high-speed video image of the
crater formation process. Elapsed time after the impact is
shown in each image.
ing rule.
Operation: Fig. 4 shows the simple overview of
the operation plan of SCI. SCI is separated from Hayabusa-2 at an altitude of several hundred m (nominally
500 m) from the surface of 1999JU3. Having no thrusters, SCI floats and descends toward the asteroid due to
its gravity. After waiting for 1 minute or so, Hayabusa2 itself starts to escape from the separation point, going
into the shade region of the asteroid, in order to avoid
debris from SCI explosion and ejecta from the newly
formed impact crater. On the way to escape, Hayabusa2 separates a deployable camera (DCAM3, Fig.5) that
aims to observe the floating and explosion of SCI, and
the ejecta curtain emerged from the crater. After several tens of minutes (nominally 40 min) from separation,
Observation Plan: First, the SCI impact will be
observed with DCAM3. Recording images of the ejecta
curtain or the impact fragments by means of DCAM3
will make it possible to investigate the sub-surface
physical properties and the ejection process in impact
cratering. DCAM3 is a palm-sized deployable camera
device. It has two camera components inside: DCAM3A and DCAM3-D. DCAM3-D is planned for the above
scientific observation and its specification is listed on
Table 1. Second, the artificial crater will be explored
by the remote sensing equipments onboard Hayabusa-2,
i.e., Optical Navigation Camera (ONC), Thermal InfraRed Imager (TIR), and Near InfraRed Spectrometer
(NIRS3) to obtain the basic parameters of the impact
crater and the various kinds of information of the fresh
surface and the subsurface structure. Such information
will be used to determine the physical properties of the
subsurface material and to refine the crater scaling rule
for the ejecta velocity distribution and the crater diameter. The cooperation between the laboratory experiments and the numerical simulations is very important
to utilize the obtained results and to conduct the realistic extrapolation of the scaling rule toward the large
scale.
Table 1: Main specifications of DCAM3-D scientific camera
Size of lens tube
Field of View
F value
Observable wavelength
Resolution
Sensor
Spin rate
Fig. 4: Overview of SCI operation plan.
30 x L40 mm
74deg. x 74 deg.
1.7
450 – 750 nm
0.65m @ 1000m
CMOS, 2000x2000pix, 8bit gray
60-120 deg./sec
ASTEROID DEFLECTION USING A KINETIC IMPACTOR. G. R. Gisler1,2, J. M. Ferguson1, C. S. Plesko1,
and R. P. Weaver1, 1Los Alamos National Laboratory, Los Alamos NM 87544, 2Mathematics Department, University
of Oslo, 0316 Oslo, Norway.
Introduction: If an asteroid is known to be competent, deflection by a kinetic impactor is a realistic
and achievable option. We have performed twodimensional hydrodynamic simulations in axisymmetry of kinetic impact scenarios to compute β, the momentum multiplication factor.
Scenario: We imagine a spacecraft sent to intercept a threatening near-earth asteroid. Upon close approach, the spacecraft fires a projectile at the asteroid,
such that the spacecraft’s velocity combined with that
of the projectile is sufficient to excavate a crater and
impart a substantial momentum boost. The spacecraft
itself flies past the asteroid while its sensors record the
result of the impact. A design for such a mission is
given, for example, in [1].
In general, the momentum delivered to the asteroid
will be greater than the momentum of the impacting
projectile because of ejecta from the crater. The ejected
material moves back at high speed in the direction
from which the projectile came. This ablated material
provides an additional momentum boost to the asteroid, according the Newton’s third law of motion. It is
conventional to express this boost as a momentum
multiplication factor, β, formulated as the ratio of the
asteroid momentum after collision to the projectile
momentum before collision [2].
A critical research focus in kinetic energy deflection lies in determining what values of β are possible
or likely for given scenarios.
What determines β? The additional thrust given
to the asteroid depends on the speed and mass of the
material ejected from the crater, and these quantities
are in turn dependent upon the material composition
and strength of the asteroid material. For most, if not
all, asteroids, these are unknown. Some general remarks can be made, however. If the material of the
asteroid’s crust is rich in volatiles, β should be higher
than if not. If the material has very high strength, β
should be lower, because less material would be ablated from the crater on the frontside, and some material would be spalled from the backside. The kinetic
energy of the impacting projectile will also affect β.
The calculations: We have begun a series of exploratory calculations to explore the dependence of β
on the kinetic energy of the projectile and on the composition of the asteroid. In the runs that we present
here, the composition is a dry rock-like material of
relatively low density (1.2 g/cc) and modest strength.
The asteroid diameter is chosen to be 500 m, an approximate match to 101955 Bennu, the target for the
OSIRIS/REx mission. The equation of state for the
asteroid is of Mie-Grüneisen form. We intend in the
future to perform further calculations with other equations of state and strength models for the asteroid, including the effect of volatiles.
The impactor is an iron sphere with radii from 32
cm to 108 cm, and the impact velocity is chosen to be
20 km/s in all cases.
The code used for these calculations is RAGE, an
adaptive-mesh-refinement hydrocode originally developed at Science Applications International and subsequently modified at Los Alamos National Laboratory.
RAGE is a multi-material code well suited to problems
of this nature [3].
The Results: The values of net push velocity and
β for a series of four calculations are shown in the following table.
The larger the impactor (and therefore the larger its
momentum before collision) the larger the push given
to the asteroid, as expected. A little more surprising is
that the momentum multiplication factor, β, drops for
larger impactors. The reason for this is the increased
importance of spall from the asteroid’s backside, which
diminishes the momentum boost.
An illustration of the boost given by ablation and
its diminution by spall is shown in the following plot
of momentum against time for the material making up
the original asteroid. This plot is from the calculation
in the first line of the table above.
The red line shows the total axial momentum given
to asteroid material by the projectile (negative is the
direction of desired push). The green line shows the
momentum of material ablated off the front side, with
positive axial momentum. The cyan line shows the
momentum of material spalled from the back side of
the asteroid. Finally, the blue line is the total momentum of remaining asteroid material (the original asteroid less what has been ablated or spalled), reflecting
both the effect of ablated and spalled material. Ablation of material from the crater produced on the projectile’s impact with the asteroid begins within 0.01 second, spall from the backside begins only after the
shock of impact propagates all the way through the 500
m diameter asteroid, a little after 0.1 second.
A pair of density plots from the same calculation is
shown below. Left is at 0.1 second, when the frontside
ablation is well underway, and right is at 20 seconds,
the end of the calculation, when spall from the sides
and back of the asteroid has started to remove some
asteroid material. Some 10% of the asteroid’s initial
mass is lost, approximately 2/3 of this to ablation and
the rest to spall.
The calculation of mass lost to ablation and to spall
is done with the aid of Lagrangian tracer particles,
placed uniformly throughout the asteroid body and
tracked throughout the calculation. Although selfgravity is not included in the calculation, we consider a
tracer and the mass it represents to be lost whenever its
outward directed velocity exceeds the computed escape velocity from the asteroid.
Conclusions: The calculations so far done suggest
that a kinetic impact is indeed an efficient and effective
method of applying a deflection velocity to a competent asteroid. The scenario illustrated, for a 32-cm ra-
dius iron sphere (effectively a cannonball) fired at 20
km/s (including spacecraft velocity) at an asteroid involves an energy expenditure of 0.05 kT, much smaller
than most scenarios involving nuclear stand-off bursts.
However, if the asteroid is not competent, such a
scheme might instead produce the undesirable effect of
merely disrupting the asteroid, which could at least
partially reassemble itself prior to encountering the
Earth.
References:
[1] Barbee, B.W. et al. (2013) Planetary Defense
Conference IAA-PDC13-04-07.
[2] Housen, K.R. & Holsapple, K.A. (2012) Lunar
and Planetary Science Conference, paper 2539.
[3] Gittings, M. et al., Computational Science and
Discovery, 1, 015005.
MISSION DESIGN FOR THE DOUBLE ASTEROID REDIRECTION TEST (DART) Justin A. Atchison, The
Johns Hopkins University Applied Physics Laboratory, Laurel, MD 20723 USA; [email protected]
Introduction: The Double Asteroid Redirection
Test (DART) will be the first space experiment to
demonstrate asteroid impact hazard mitigation by using
a kinetic impactor to deflect an asteroid. DART is the
interceptor element of the Asteroid Impact & Deflection Assessment mission (AIDA), a joint NASA-ESA
mission which also includes the ESA Asteroid Impact
Monitor (AIM) rendezvous mission. The primary goal
of AIDA is to measure and characterize the deflection of
an asteroid by a kinetic impactor. The results will have
implications for planetary defense, human spaceflight,
and Near-Earth Object science and resource utilization.
Mission Design Challenges: Mission design is concerned with the trajectory design that reaches Didymos
and delivers sufficient approach hand-off conditions for
the impact guidance algorithms. Given that Didymos
is a near-Earth target, chemical-propulsion trajectories
were only considered.
The DART mission is intended to impact the secondary asteroid near its center-of-mass, along the direction of its velocity relative to the primary. Doing
so maximizes the induced change in orbit energy of the
secondary and reduces uncertainty in the experiment parameters. This challenge restricts the approach lighting
conditions and approach angle with respect to the secondary’s relative orbit plane. It is also desirable for the
spacecraft to arrive at the target when observers on the
Earth can image the encounter using radar, which imposes an Earth-range constraint on the impact time and
implies that the encounter must take place during defined windows when Didymos is near to its periapse.
Finally, Because this is a mission with international collaboration (mission partners as well as Earth-based observers), it is useful to have a single constant arrival date
regardless of launch-date. This facilitates planning and
cooperation across partners.
Target Information: The target is Didymos
(1996GT, 65803), a Near-Earth binary asteroid pair
discovered in 1996 [1]. The system is an elliptical
(e = 0.394), inclined (3.407 relative to ecliptic) orbit
with a period of 770.18 days. Scheirich and Pravec [2]
studied light curve data spanning 29 days and computed
the pair’s relative orbit geometry. The secondary orbits
the primary with a period of 11.90 hr. The ratio of
the secondary’s maximum dimension to the primary’s
maximum dimension is 0.21. The pole of the orbit
plane, given as right-ascension
and declination
in the ecliptic plane is computed to be ( = 157+47 ,
+11
= 19+45
= 70+25
15 ) or ( = 329 194 ,
15 ) degrees.
The two solutions are quite different, and each has
large uncertainties. This study uses the second solution,
owing to the fact that it lies closer to the ecliptic.
Direct Trajectories:
The first available
Earth/Didymos conjunction occurs in October 2022.
This arrival date offers low-energy departures and low
Earth-ranges for radar imaging at arrival. Porkchops
plots, which depict mission parameters as a function of
launch and arrival date, were generated for departure
dates starting as early as late 2019[3]. Figure 1 gives
a sample year’s direct launch options, with contours of
launch energy (C3), declination of launch asymptote
(DLA), solar phase angle (✓S ), and impact angle (✓I ).
Feasible departures are possible twice per year. Regions
of the figure that are shaded red represent arrivals with
poor solar phase angles (> 75 ). Regions that are
shaded green represent departures with DLAs outside
of the bounds for a direct launch from a candidate
launch site, Wallops Flight Facility (< 38 or > 38 ).
The DLA constraint is relevant for this particular mission geometry because Didymos is inclined to Earth’s
orbit, and is below (0.055 AU) the ecliptic at the time of
Earth/Didymos close-approach. This inclination tends
to drive the transfer V, since little energy is required
to raise the orbit to the Didymos location (1.036 AU) at
the time of conjunction.
There are four acceptable departure windows over the
four year launch period. Selecting October 1 2022 as
the fixed arrival date, a trajectory was selected for each
region, as summarized in Table 1. A sample from this
table, consistent with Figure 1 is given in Figure 2. The
arrival geometry places Didymos very near to Earth’s
orbit, below the ecliptic. The transfer trajectory’s lineof-nodes is also shown. The favorable trajectories depart when these lines are approximately parallel, and
the arrival location is perpendicular to the transfer trajectory’s line-of-nodes. Again, this geometry indicates
that the inclination change is a driver for V. The arrival
is “early” relative to the Earth/Didymos close-approach,
owing to the arrival solar phase angle constraint. A solar phase angle < 75 requires the spacecraft to arrive
with a velocity outbound relative to the sun. That is, the
spacecraft must be between the sun and Didymos with
a positive flight path angle prior to impact.
Table 1: Trajectory Summaries
A
B
C
D
Launch
Date
Jan 08 2020
Dec 20 2020
Dec 20 2021
Jun 17 2022
C3
[km2 /s2 ]
3.68
3.80
3.74
3.93
DLA
[deg]
35.6
34.2
35.0
-37.4
✓S
[deg]
57.5
59.9
59.8
42.6
✓I
[deg]
30.6
25.8
25.7
26.6
Figure 1: Direct Trajectory Options for 11/2020 to 11/2021 Launches
There is also an option for a resonant return, in which
the DART trajectory would have a period equal to half
of Didymos’s heliocentric period (2:1). In the event of
a missed impact, the spacecraft would complete two orbits before having a second impact opportunity. None
of the previously identified trajectories satisfy this constraint. However, if a vehicle on one of them failed to
intercept Didymos, a second opportunity in Nov 2024
would require a burn on the order of 466 m/s. The costs
and benefits of accommodating this additional V in
the spacecraft design would have to be considered prior
to launch. As an alternative, it may be possible to target an Earth flyby that would lead to a second Didymos
impact opportunity.
Co-Manifest Trajectory: Since the AIM and
DART spacecraft are both targeting the Didymos
system, we investigated options for co-manifested
trajectories. In this case, both spacecraft would launch
on the same launch vehicle and separate once in heliocentric orbit. Although the target is the same, the arrival
conditions are critically different, with AIM seeking a
low arrival velocity for rendezvous and DART seeking a
high arrival velocity for impact. This difference can be
accommodated using a so-called V-EGA trajectory,
in which an Earth flyby is used to separate the two
spacecraft onto their respective favorable trajectories.
One such trajectory departs on 21 Oct 2019 with a C3
of 27.4 km2 /s2 and a DLA of -12.2 . After launch,
the two spacecraft separate and perform a deep space
maneuver (DSM) to set up their respective Earth flybys.
The AIM DSM is 930 m/s on 07 March 2020, leading
to a 20 Oct 2020 Earth gravity assist, and a 10 Jun
2022 Didymos arrival with 1.05 km/s relative velocity.
The DART DSM is 70 m/s on 24 March 2020, leading
to a 23 Oct 2020 Earth gravity assist, and a 01 Oct
2022 Didymos arrival with 6.5 km/s relative velocity.
Additional effort could likely lead to optimized V
requirements for this point-design.
References: [1] P. Pravec, et al. (2003) Proceedings of
the IAU 8244. [2] P. Scheirich, et al. (2009) Icarus 200:531.
[3] J. Atchison, et al. (2014) International Symposium on
Space Flight Mechanics.
Figure 2: Sample Direct Trajectory (B)
Double Asteroid Redirection Test (DART) Systems Overview Z. Fletcher1, E. Smith1, J. Atchinson1, and A.
Cheng1, 1JHU/APL ([email protected])
Introduction: DART is one of the two spacecraft
part of the Asteroid Impact Deflection Assessment
(AIDA) mission. DART’s primary mission is to impact
the binary asteroid system Didymos at hypervelocity.
Predicting the expected momentum transfer from an
spacecraft kinetic impact is vital to designing a hazardous asteroid deflection mission; however, there is uncertainty in created ejecta mass and velocity. DART
will help to reduce uncretainties in momentum transfer,
or the beta factor, resulting from kinetic impacts.
Didymos is a binary asteroid system with a 800m
diameter primary and 150m secondary, orbiting each
other with an orbital period of 11.9 hours. DART will
target the smaller of the two asteroids, casuing the system orbital period to change.
Figure 1 - Didymos System
The primary requirement for DART is to change
the orbital period by more than 0.2% to measure the
beta factor. Over weeks, the mean anomaly change will
build up to more than 10%, allowing for accurate
measurements of the period change. These measurements are taken from the ground or the AIM spacecraft, the other spacecraft comprising AIDA. DART
needs to impact near the center of mass and determine
the final impact location to be able to reduce uncertainty in beta.
The DART spacecraft will use optical navigation to
accurately taget and impact the secondary at a velocity
of ~6.25 km/s. DART is designed to be a simple,
lightweight and low-cost spacecraft with a single instrument.
Table 1: DART Spacecraft Driving Design Requirements
The DART impact on the secondary member of the Didymos
system shall cause at least a 0.17% change in the binary orbital
period.
DART shall autonomously impact the secondary asteroid through
its center of figure with a miss distance of less than 15m (TBR).
DART shall determine the impact location within 1 m.
DART shall determine local surface topography and geologic
context of impact site to meter scale .
Instrument: An optical telescope is the sole instrument on the DART mission. This telescope is a
rebuild of the Long Range Reconnaisance Imager
(LORRI) that flew on the New Horizons mission.
LORRI is a 20cm Ritchey-Critchen telescope with a
passively cooled visible CCD [2].
LORRI can observe Figure 2 - Simulated Image
the Didymos system 2
months prior to impact.
LORRI can discriminate between the primary and secondary ~8
hours prior to impact,
allowing optical navigation algorithms to target
the smaller secondary.
Data & Commuications: DART will
take periodic images
during the cruise to
Didymos. Once terminal guidance begins hours prior to
impact, DART will begin continuously imaging Didymos.
The final image of the Didymos secondary needs to
be high enough resolution to determine the impact location to an accuracy of 1 meter. This requires the
downlink of at least one image in the final 5-10 seconds of approach.
The DART spacecraft has a high-gain antenna and
a radio capable of transmitting high-rate data back to
Earth (.07 AU away at impact). This allows the DART
spacecraft to send images of the approach through impact.
Table 2 - Trajectory Options [1]
Navigation: During the cruise, the spacecraft navigation is done with traditional deep-space navigation us-
ing computed states and correction maneuvers. At two
months prior to impact, imaging of the binary system is
possible. The spacecraft will downlink images of Didymos to the ground. The images will be used to reduce
uncertainty and improve navigation to the asteroid system.
Once the Didymos secondary can be discriminated
from the primary, DART will enter terminal guidance
mode. This is an autonomous optical navigation system
based on proportional guidance algorithms [3]. DART
will steer itself to the secondary by running closed-loop
control of the thrusters.
Spacecraft Bus: The DART spacecraft bus is optimized for final approach of the asteroid system. In order to minimize the number of mechanisms, the instrument and antenna are fixed for the geometry of the
final approach. During cruise, the spacecraft will slew
between pointing the telescope to the target and pointing the high-gain antenna to Earth.
Power comes from two solar arrays with a total area
of 2.7 m^2. The spacecraft maintains a distance of ~1
AU from the sun. Attitude control is performed by a
star tracker, IMU and thrusters.
Figure 3 - Spacecraft Configuration
A hydrazine monoprop propulsion system is sized
for 120 m/s of delta-v. The delta-v is used for attitude
control, eliminating launch vehicle dispersion errors
and terminal guidance maneuvers to impact Didymos.
The launch vehicle places the spacecraft in a trajectory
to the target and the spacecraft makes minimal targeting adjustments to reduce the propellant requirements
of the spacecraft.
Spacecraft mass is less than 300kg. The low mass
allows for launch on a variety of launch vehicles, a key
for maintaining an affordable mission.
Refernces: [1] J. Atchinson (2014) Mission Design
For The Double Asteroid Redirection Test (DART),
AIDA International Workshop [2] A. F. Cheng, H.A.
Weaver, S.J. Conrad, and et al. Long-Range Reconnaissance Imager on New Horizons, Space Science
Reviews, 140:189–215, October 2008 [3] J. Atchinson
(2014) Double Asteroid Redirection Test - Mission
Design And Navigation, ISSFD 2014
ENVIRONMENTAL AND INSTRUMENTATION REQUIREMENTS OF THE ASTEROID IMPACT
MONITORING (AIM) MISSION, A COMPONENT OF THE ASTEROID IMPACT AND DEFLECTION
ASSESSMENT (AIDA) MISSION. P. Michel1, J. Biele2 (DLR), M. Delbo1 (Univ. Nice, CNRS, OCA), M. Jutzi3
(Univ. Bern), G. Libourel1 (Univ. Nice, CNRS, OCA), N. Murdoch, S.R. Schwartz1 (Univ. Nice, CNRS, OCA), S.
Ulamec (DLR) and J.-B. Vincent4 (MPS), 1Lagrange Laboratory, University of Nice-Sophia Antipolis, CNRS, Côte
d’Azur Observatory, CS 34229, 06304 Nice Cedex 4, France, [email protected], 2DLR, Germany, 3Univ. Bern, Switzerland, 4MPS, Germany.
Introduction: AIM is a rendezvous mission that focuses on the monitoring aspects i.e., the capability to
determine in-situ the key properties of the secondary of
a binary asteroid. DART, consisting primarly of an
artificial projectile, aims to demonstrate asteroid deflection. In the framework of the full AIDA concept,
AIM will also give access to the detailed conditions of
the DART impact and its outcome, allowing for the
first time to get a complete picture of such an event, a
better interpretation of the deflection measurement and
a possibility to compare with numerical modeling predictions. We will describe the knowledge gain resulting from the implementation of the European Space
Agency’s Asteroid Impact Monitoring (AIM) as a
stand-alone mission. Then we will consider AIM with
its second component, the Double Asteroid Redirection
Test (DART) mission under study by the Johns Hopkins Applied Physics Laboratory (with support from
members of NASA centers including Goddard Space
Flight Center, Johnson Space Center, and the Jet Propulsion Laboratory). We will then present our analysis
of the required measurements addressing the goals of
AIM to the binary Near-Earth Asteroid (NEA) Didymos. Finally we will present the environmental parameters for AIM. This information is included in the final
report on the environmental and instrumentation requirements for AIM submitted by the AIM team to
ESA [1].
Mission Goals: The mission goal for the AIM study is
twofold. On one hand it will provide the opportunity to
demonstrate, on a minimalistic deep-space mission,
technologies related to autonomous navigation, onboard resources management and close proximity operations. On the other hand it will characterise the secondary of a binary asteroid and demonstrate the technologies required by a simple monitoring spacecraft as
well as establishing the suitability of binary asteroids
as candidates for future explorations and asteroid deflection tests. Both AIM and AIDA address issues that
interest a large variety of communities, such as communities of researchers and engineers working on impact physics, planetary defense, seismology, geophysics (surface and internal properties), dynamics, mineralogy and resources, spectral and physical properties of
small bodies, low-gravity environments and human
exploration.
Figure 1 shows that AIM serves a wide range of objectives. Accompanied with DART it will also serve as a
deflection demonstration and provide important
knowledge on the impact process on asteroids.
Each objective may have a different set of measurement requirements. Therefore, we will indicate a full
set of measurement requirements that would provide
information in all areas indicated in Fig. 1. However,
we will also present what we define as mandatory requirements, i.e. only those that are important for all 4
of these key areas simultaneously.
In the framework of AIM as a stand-alone mission,
images in the visible, the mass and surface (thermal
and material) properties of the secondary, are mandatory (minimum) outputs, as they serve all four areas (Fig.
1) simultaneously. Note that material properties can be
derived either with a surface package and/or a thermal
infrared camera, and/or exploiting the DART impact.
In the framework of AIDA, mandatory measurements
are the precise impact conditions (geometry/environment of the impact) of DART, which will
allow us to interpret the deflection that will also be
measured by ground-based observations. Physical
properties of the secondary (impacted body) and their
modifications after the impact (i.e. the characterization
shall be carried out before and after the impact), the
ejecta properties (size/speed), and the crater morphology shall be measured. The orbital properties of the system and their changes after the impact (change in the
orbit of the secondary around the primary) shall be
determined. The possibility to measure the physical
properties of the primary of the Didymos system is
also considered in the current technical studies.
An environment analysis has also been performed
based on the current knowledge and modeling work
concerning the target properties (physical, compositional, thermal and dynamical properties) and its surrounding, which in the framework of AIDA, concerns
also the time after the impact (e.g. fate of the ejecta
cloud).
Figure 1: The full AIDA concept serves all NEO exploration stakeholders. The same applies to AIM as a
stand-alone mission (bold characters), except for the
deflection demonstration.
References: [1] Michel P. et al. (2014) ESA report.
Acknowledgements: The study leading to the report on AIM requirements and environment was supported by ESA. Some parts of this study are performed
in the context of the NEOShield Project funded under
the European Commission’s FP7 program agreement
No. 282703.
ENABLING TECHNOLOGIES FOR INCREASED SCIENCE OF AIM OBSERVING SPACECRAFT.
J. Gil1, I. Huertas2, and G. Ortega3 1GMV (Isaac Newton 11, Tres Cantos, Madrid, Spain,
[email protected]), 2ESA/ESTEC (Keplerlaan 1, Noordwijk, The Netherlands, [email protected]),
3
ESA/ESTEC (Keplerlaan 1, Noordwijk, The Netherlands, [email protected])
Introduction: ESA has been developing enabling
technologies for asteroid missions in the last decade.
One of the main objectives of these developments is to
increase the science return while keeping a low mission cost. The overall cost involves launch segment,
development of space and ground segments and also
operational costs.
The mission phases in the proximity of the asteroid
are the most interesting for scientific purposes but the
most critical for SC operation (ground-based or autonomous).
ESA is developing low-cost and mass rangefinders
(radar and lidar). However, given the low TRL alternative technology for proximity operations based on optical camera has been developed. Using different image processing techniques and reduced a priori
knowledge (e.g. lightcurve) safe close approach and
station keeping can be performed while properly pointing the science instruments towards the asteroid.
Once the navigation cameras can achieve higher
surface resolution, more complex image processing
techniques can be used to increase the performance.
Ground involvement is of great importance to reduce
the on-board autonomy while maintaining simple, lowfrequency ground control. Based on ROSETTA lessons, fast mission operations are under analysis for
missions like Phootprint or Marco Polo, that can benefit AIM. The main benefit is not only cost reduction
but also increased scientific data return (communication devoted mostly to science and not to navigation
and guidance) and of higher quality (pointing accuracy, resolution because of lower altitudes achieveable).
Enhanced detectors on navigation camera can help
in the proximity operations. A third generation of
CMAS APS detectors are in development at ESA. Using current star-tracker HW with minor modifications,
cost-efficient on-board Guidance, Navigation and Control technologies would improve ground-based performances. The higher reactivity and accuracy permits
flying trajectories closer to the surface of interest and
with better pointing accuracy. For instance, closer slow
velocity flybys or even hovering.
In addition, some techniques for the estimation of
the effect of the impact on the secondary orbit have
been analysed. The main input is the images taken by a
cemara on the observing SC to measure the change of
the orbital period immediately before and after the
impact.
Figure 1. Landing dispersion for a surface package
on the secondary object of a binary asteroid (requirement is 2.6 m 3-sigma)
Figure 2. Worst-case analysis of collision avoidance
maneuver after delivery of a landing package on
the secondary object of a binary asteroid.
Validation of the technologies with representative
HW is a critical step. A methodology based on a stepwise validation, reusing some of the previous step facilities and SW has proved very efficient for complex
GNC systems. Initiall, only SW simulation (Model-Inthe-Loop and SW-In-the-Loop) is used to validate performances. Then real-time Processor-In-the-Loop tests
are executed, along with autocoding when possible to
significantly reduce SW development time. Then, tests
with actual HW could start.
The first step could be validation in an optical navigation laboratory. This facility enables testing the
camera HW and the processing units (FPGA, DSP,
microprocessors) in closed loop with realistic simulation of space conditions in static mounting (motion is
simulated in the image generation).
The second step is the validation in a robotic facility with proper mock-ups of the asteroid. The camera is
stimulated directly with photons from the asteroid
mock-up. The illumination and radiance shall be similar to the mission conditions. These validation ensure
that the developed HW and SW reached a high Technology Readiness Level. This methodology for development and validation of Guidance, Navigation and
Control sub-system (including the Image Processing
algorithms) has proved fast, efficient (detecting and
correcting errors or misperformances) and costeffective.
Figure 4. High-fidelity simulator image of asteroid
during landing
Figure 3. GMV’s HW-In-the-Loop validation facility
Several technologies developed or under development
by ESA for mission to asteroids or minor bodies (like
Phobos) can be (and have been) applied to AIM mission. The main objective of these technologies is to
achieve significant science return while keeping low
overall mission cost.
Figure 5. Real camera image of asteroid during
landing in GMV’s robotic laboratory with asteroid
mock-up
SMALL LANDER CONCEPTS FOR AIDA. S. Ulamec1, J. Biele1, C. Krause, C. Lange2, T.-M. Ho2, 1 German
Aerospace Center, DLR, 51147 Cologne, [email protected] , 2 German Aerospace Center, DLR, Bremen
Introduction: AIDA combines DART, an impactor and AIM an observing spacecraft. It has been
proposed to add a small surface science package as
payload for AIM based on the MASCOT heritage and
allowing in-situ obserbations.
Advantages of surface science: While the primary objective of AIM is to observ the binary system of
Didymos as well as the immediate effects of the DART
impact there are many reasons to study the asteroid at
the surface [1].
Measurements could include the measurement of
the soil mechanical properties, surface temperature
(thus deriving e.g. Yarkowsky and YORP effect from
day-night variations, decoupling thermal inertia from
surface roughness, decoupling thermal inertia from
surface roughness), high resulution imaging of the surface material (and detecting e.g. grain sizes of surface
material), chemical and mineralogical analyses but also
investigations of the internal structure e.g. with seismic
means or with a bistatic radar.
MASCOT heritage: MASCOT is a small (10 kg)
lander to be launched in December 2014 as part of the
scientific payload of the JAXA Hayabusa 2 spacecraft.
It has a “box-like” structure, and will be ejected
from the mother spacecraft at an altitude of about
100m above the surface of the target asteroid, 1999JU3
[2]. MASCOT will be operated with the power provided by a primary battery, allowing operations of
about 12-15 hours. The payload consists of a (visible)
camera, a magnetometer (MAG) a radiometer (MARA)
and an imaging IR spectrometer (MicrOmega). [3]
Figure 1 shows a sketch of MASCOT including its
payload.
Possible Lander Designs as relevant for AIDA:
Small landers, delivered by the AIM spacecraft could
well be based on the MASCOT design but include different payload. There have been proposals to use such
a lander to operate a bi-static radar, detecting the internal structure of the asteroid by transmitting radiowaves
through the body beween Lander and Orbiter. This
concept is based on CONSERT (an instrument aboard
Rosetta and Philae) and has been proposed for asteroids as FANTINA [4].
Other options for surface measurements include
seismometers, detecting impacts (small ones with dedicated impactors or the big impact of the DART spacecraft). These measurements, although fascinating in
concept require further studies, e.g. regarding the coupling of a seismometer to a low gravity body.
MARA
CAM
MAG
μOmega
Figure 1: MASCOT with payload elements
References:
[1] M. Cheng, A., Galvez, A., Reed, C., Carnelli, I.,
Abell, P., Ulamec, S., Rivkin, A., Biele, J., Murdoch,
N., AIDA: Asteroid Impact and Deflection Assessment, International Astronomical Union, IAU, 28th
General Assembly, Beijing, 2012; [2] Ulamec, S.,
Biele, J., Bousquet, P.-W., Gaudon, P., Geurts, K.,
Ho, T.-M., Krause, C., Lange, C., Willnecker, R. and
Witte L.; Landing on Small Bodies: From the Rosetta
Lander to MASCOT and beyond; Acta Astronautica,
Vol. 93, pp. 460-466, 2014; [3] R. Jaumann, J.P.
Bibring, K.H. Glassmeier, M. Grott, T.-M. Ho, S.
Ulamec, N. Schmitz, H.-U. Auster, J. Biele, H. Kuninaka, T. Okada, M. Yoshikawa, S. Watanabe, M. Fujimoto, T. Spohn, A. Koncz, A Mobile Asteroid Surface Scout (MASCOT) for the Hayabusa 2 Mission,
Lunar Planetary Conference, Houston, 2014; [4] A.
Herique, J. Biele, P. Bousquet, V. Ciarletti, T.M. Ho,
J.L. Issler, W. Kofman, P. Michel, D. Plettemeier, P.
Puget, J.C. Souyris, S. Ulamec, T. van Zoest, S. Zine,
FANTINA: Fathom asteroids now: tomography and
imagery of a NEA-payload for MarcoPolo-R CV3/ESA
mission; EGU General Assembly, Vienna, 2012.
Radar: a direct access to Didymos' structure from deep
interior to regolith for science and planetary defense.
Alain Herique The internal structure of asteroids is still poorly known and has never been measured directly. Our knowledge is relying entirely on inferences from remote sensing observations of the surface and theoretical modeling. Is Didymos a monolithic piece of rock or a rubble-­‐pile, an aggregate of boulders held together by gravity and how much porosity it contains, both in the form of micro-­‐scale or macro-­‐scale porosity? What is the typical size of the constituent blocs? Are these blocs homogeneous or heterogeneous? The body is covered by a regolith whose properties remain largely unknown in term of depth, size distribution and spatial variation. Is it resulting from fine particles re-­‐
accretion or from thermal fracturing? What are its coherent forces? How to model its thermal conductivity, while this parameter is so important to estimate Yarkowsky and Yorp effects? After several asteroid orbiting missions, theses crucial and yet basic questions remain open. Direct measurements of asteroid deep interior and regolith structure are needed to provide answers that will directly improve our ability to understand and model the mechanisms driving Near Earth Asteroids (NEA) deflection and other risk mitigation techniques. There is no way to determine this from ground-­‐
based observation. Radar operating from a spacecraft is the only technique capable of achieving this science objective of characterizing the internal structure and heterogeneity from submetric to global scale for the benefit of science as well as for planetary defence. The deep interior structure tomography requires low-­‐frequency radar to penetrate throughout the complete body. The radar wave propagation delay and the received power are related to the complex dielectric permittivity (i.e to the composition and microporosity) and the small scale heterogeneities (scattering losses) while the spatial variation of the signal and the multiple paths provide information on the presence of heterogeneities (variations in composition or porosity), layers, ice lens. A partial coverage will provide "cuts" of the body when a dense coverage will allow a complete tomography. Two instruments concepts can be considered: a monostatic radar like Marsis/Mars Express (ESA) that will analyze radar waves transmitted by the orbiter and received after reflection by the asteroid, its surface and its internal structures; a bistatic radar like Consert/Rosetta (ESA) that will analyze radar waves transmitted by a lander, propagated through the body and received by the orbiter. Imaging the first ~50 meters of the subsurface with a decimetric resolution to identify layering and to reconnect surface measurements to internal structure requires a higher frequency radar on Orbiter only, like Wisdom developed for ExoMars Rover (ESA) with a frequency ranging from 300 MHz up to 2.7 GHz. At larger observation distance, this radar working in SAR mode maps surface and sub-­‐surface backscattering coefficient. In the frame of AIDA mission, this is a unique opportunity to estimate regolith rearrangement in the impact area. This paper reviews the benefits of direct measurement of the asteroid interior. Then the radar concepts for both deep interior and near surface sounding and their return for the AIDA mission are shown. ASTEROID SURFACE GRAVIMETRY OF 65803 DIDYMOS VIA A LANDER CARRIED BY AIDA’S
AIM SPACECRAFT. K. A. Carroll1, H. Spencer2 and R. E. Zee2, 1Gedex Inc., 407 Matheson Blvd. East, Mississauga, Ontario, Canada L4Z 2H2, [email protected], 2Space Flight Laboratory, University of Toronto Institute for Aerospace Studies, 4925 Dufferin Street, Toronto, Ontario, Canada M3H 5T6, [email protected],
[email protected].
Introduction: The internal structure of the two
bodies making up 65803 Didymos could be probed by
making a series of gravity measurements at multiple
locations on the surfaces of those bodies. A small (15
kg) spacecraft, GRASP (GRavitational Asteroid Surface Probe) has been designed that could accomplish
that, if carried to that asteroid by a carrier “mothership,” which would also provide communicationsrelay support. The AIM rendezvous spacecraft component of the proposed AIDA mission could carry
GRASP to Didymos as a secondary payload. Gravimetric surveying of the satellite body Didymos-B
would enhance the ability of the AIDA mission to
achieve its objectives, by providing knowledge about
the mass of that body, and of its internal structure,
which could help interpret the effects of the impact on
that body of the DART spacecraft. Subsequent gravimetric surveying of the primary body Didymos-A
could add information about how Didymos B formed.
AIDA Mass-Determination Objectives: The
main objective of the proposed Asteroid Impact &
Deflection Assessment (AIDA) mission is to demonstrate asteroid deflection by spacecraft kinetic impact
[1], by colliding the proposed Double Asteroid Redirection Test (DART) spacecraft with a target, which is
the smaller of the two bodies that together comprise
asteroid 65803 Didymos. Related objectives are to
determine various characteristics of the target asteroid
(which we will refer to as Didymos-B) before, during
and after the impact event, using the Asteroid Impact
Monitoring (AIM) spacecraft.
These objectives include determining the mass of
Didymos-A and Didymos-B, analyzing their geology,
and deriving collision and impact properties. In particular, in order to properly assess the effectiveness of the
momentum transfer from DART to Didymos-B, it is
important to have a good estimate of the mass of Didymos-B; that is also needed to determine its density,
which is a fundamental parameter to understand the
asteroid’s internal structure and composition.
Methods for Determining the Mass of DidymosB: While the mass of Didymos-A can be determined
from the orbit period of Didymos-B, the mass of Didymos-B is much more difficult to determine from orbital observations and it is currently poorly known; it
is assumed in [1] to be 3x109 kg. There are several
possible methods for determining its mass using the
AIM spacecraft as originally conceived --- i.e., a
spacecraft that would rendezvous with Didymos but
not land on it, making observation using stand-off,
remote sensing instruments only. However, all of these
methods are quite challenging:
 A method is described in [2] whereby camera observations of both bodies of Didymos by AIM,
presumably in conjunction with radio tracking of
AIM from Earth and detailed multi-body orbital
modeling, could be used to determine the semimajor axis of the Didymos system, and the distance from its barycentre to the mass centre of
Didymos-A, from which the mass ratio between
the two bodies of Didymos would be estimated.
Given an expected mass ratio of 1:100 and a semimajor axis of about 1 km, in order to estimate the
mass of Didymos-B to within (say) 10%, the distance between the barycentre and the mass centre
of Didymos-A would have to be determined to
within about 1m, which may be difficult.
 Should AIM fly, radio tracking of AIM from
Earth would be done, and those data would be fit
to a Didymos system model to estimate various
parameters, including masses for both bodies. A
confounding factor, discussed in [3], is the uncertainty in the amount of solar radiation pressure
acting on AIM. AIM would need to loiter at an altitude of 1 km or less above Didymos-B in order
for the solar radiation force to be small enough to
enable a mass determination accuracy of 10%.
Given a Didymos-B orbital semi-major axis of
about 1 km, accomplishing that could be extremely challenging, without colliding with one or the
other asteroid body.
 A similar challenge would face any attempt to use
the same technique as Hayabusa at 25143 Itokawa, which determined the asteroid’s mass to within 5% by dropping from an altitude of 1 km, while
measuring altitude by LIDAR. Since the mass of
Didymos-B is expected to be only 10% that of
Itokawa, this measurement would be difficult to
make with useful accuracy even without the much
more complex orbital dynamics issue.
We propose an alternate concept, suggested first in
[3], of emplacing a gravimeter on the surface of Didymos-B in order to “weigh” that body. While this
would add complexity to the overall AIM mission de-
sign, it might be the only feasible means of determining the mass of Didymos-B to the level of accuracy
needed for AIDA to accomplish its overall mission
objectives; it could also allow additional science objectives to be achieved, enhancing the impact assessment
effectiveness of the overall AIDA mission. Such a
lander, which we refer to here as GRASP (for GRavitational Asteroid Surface Probe) could be added to the
AIM component of AIDA as a secondary, deployable
payload. If implemented as a “microspace” class
spacecraft, its cost could be relatively low.
GRASP Design Concept: Gedex and SFL have
completed an initial preliminary design study of a
GRASP asteroid gravity geophysics lander, whose
design is suitable for measuring the mass of DidymosB. The GRASP spacecraft would be carried to Didymos as a secondary payload by AIM, and would be
released in the near vicinity (within a few km) of Didymos. GRASP includes full 3-axis attitude control
capabilities, as well as a propulsion subsystem capable
of 13 m/s delta-V --- in order to avoid the failure mode
suffered by Hayabusa’s MINERVA lander. GRASP
would make its way to the surface of Didymos-B,
where it would land. It would carry the Gedex VEGA
(VEctor Gravimeter for Asteroids) instrument, to
measure the surface gravity at the landing site with an
absolute accuracy of 1-10 nanoG, with which the mass
of Didymos-B could be determined with an accuracy
much better than 10% from a single measurement,
when compensated with asteroid rotation data determined from imagery provided by AIM.
GRASP would be supported by a “mothership,”
which could be the AIM spacecraft itself, or could be
an additional microsatellite secondary payload, also
released by AIM; using a separate microsat for the
mothership would reduce the impact on the main AIM
spacecraft of supporting GRASP to a very low level.
The mothership would act as a communications relay
between GRASP and its ground controllers (possibly
via AIM), as well as providing navigation support to
GRASP as it descends to the asteroid surface,
manoeuvring within the Didymos system all the while.
The GRASP
spacecraft, shown
in the accompanying figure, is a
30 cm cube, with
6 legs each about
30 cm long. It has
an estimated mass
of 15 kg, including a mass margin of 30%; an
additional 4 kg of
GRASP interface equipment is expected to be left behind on AIM and/or the mothership microsat. Its power and thermal subsystems are designed to withstand
thermal conditions on an asteroid surface, including
eclipses of as long as 12 hours, while producing 6W of
power in sunlight. It can receive and transmit data at 5
kBaud, and its propulsion system has an impulse capability of 200 N-s. Payloads include the VEGA instrument as well as several cameras, and a pair of magnetometers. GRASP is also designed to be able to rove
about the asteroid surface, using a combination of
tumbling (actuated by its reaction wheels), and hopping (using its propulsion system).
Enhanced AIDA Science Potential: The main objective, of measuring the mass of Didymos-B to high
accuracy, could be accomplished quickly by GRASP
by making a single gravimetry measurements at its
initial landing site. Using its roving capability, GRASP
could move about the surface of Didymos-B, conducting a gravimetry survey of the entire body. Extrapolating from terrestrial geophysical surveying practice,
inversion techniques could be applied to infer the
body’s internal density distribution from these gravity
measurements. This could help characterize the internal structure of the target asteroid, potentially identifying compositional or morphological inhomogeneities.
If done in advance of the DART impact, this information could help guide the choice of the DART target
impact point on the surface of Didymos-B.
If left on the surface of Didymos-B during the
DART impact, VEGA will function as a seismometer.
Capable of measureing accelerations at up to 1 kHz, it
could provide information about the details of how
DART interacts with the asteroid during impact, and
how Didymos-B responds. A second gravimetry survey could be conducted following the impact, looking
for changes in the mass distribution of Didymos-B.
GRASP’s propulsion capability is large enough to
allow it to manoeuvre from Didymos-B to Didymos-A,
following the impact event. Additional AIDA-relevant
science could be done by surveying the surface of
Didymos-A, gravimetrically and otherwise.
The precise details of the first GRASP design study
were not optimized for the needs of the AIDA mission.
They could be improved upon. This design serves to
illustrate the size and mass of a secondary payload that
could add considerable value to the AIDA mission.
References:
[1] Cheng A. F. et al. (2013) 64th IAC, paper IAC13-A3.4.8.
[2] Murdoch N. et al. (2012) AIDA Mission Rationale Interim Release, ESA.
[3] Carroll K. A. (2014) LPS XLV, Abstract #2352.
BLOCK MAPPING AND ANALYSIS IN ASTEROID REGOLITH AS A PROXY FOR DETECTING AND
CHARACTERIZING IMPACT-TRIGGERED SEISMIC EVENTS. J. L. Noviello & E. Asphaug, School of
Earth and Space Exploration, Arizona State University. ISTB4, Room 795, 781 E. Terrace Mall, Tempe, AZ 852876004. [email protected].
Introduction: Attempts have been made to determine the internal structures of spacecraft-targeted asteroids, such as Itokawa, Eros, Lutetia and Vesta [1-4]
using a variety of methods. Definitive understanding of
asteroid internal structure will have to wait for seismological investigations, something that has not yet happened due to the perceived high complexity and cost.
A reliable, relatively inexpensive method for determining the internal structure of an asteroid would provide
considerable benefits to the general knowledge of asteroids, and to the processes of planetary formation and
mechanisms of evolution.
The Asteroid Impact Deflection Assesment (AIDA)
mission is in a unique position to directly measure the
physical response of the surface of a small body to an
impact event. AIDA’s target is the 150 m diameter
moon (“Beta”) of the 800 m diameter asteroid 65803
Didymos. Here we propose that it is possible to generate an approximation of seismic moment of that body
using the displacement magnitude of blocks resting on
the surface of a small planetary body as a function of
distance from the impact site.
The change in the size frequency distribution
(SFD) index of blocks, before and after a seismic event
quantifies the physical response of a small body, allowing us to answer some fundamental questions:
What particle velocities were achieved by the regolith
substrate? Did the impact completely remove the fine
regolith? Have blocks moved to attain an equilibrium
at the new angle of repose?
Methods: While the primary emphasis of the
AIDA mission is to study the change in velocity of the
center of mass of an asteroid, AIDA can maximze itsvalue by collecting data related to understanding the
body’s geophysical response to the collisions. To attain
these objectives, the success of AIDA hinges on establishing a precise coordinate system for image tracking.
Therefore prior to the Double Asteroid Redirection
Test (DART) craft’s impact onto Beta, the Asteroid
Impact Monitor (AIM) is expected to take as many
high resolution images of Beta as possible. These images will be used to provide the baseline for a coordinate system on the body. They will also establish the
original orientation and positions of blocks on Beta’s
surface, enabling the seismology experiment we now
propose.
After DART’s impact, AIM should return to Beta
and repeat the imaging processes, looking for shifted
blocks and pre-existing structures such as scarps or
photometric markings. Using techniques already developed for Eros and Itokawa [1, 4-6], we would map
discrete blocks as tracers of mass movement, and as
subjects of size- and spatial-distribution studies.
Predicted movement of blocks: Mass movement of
material on Beta is expected to occur in the form of the
crater itself (whose diameter cannot yet be easily predicted), regional landslides, localized avalanches,
nearby crater collapse, and/or discrete block movements following the impact. Large blocks will be carried by the flow, and the largest discrete blocks will
serve as tracers of the flow trajectory. From their before and after positions and time-lapsed images, we
will construct 3D flow maps tracing the block movement, for use in constraining impact, cratering and
dynamical models.
We propose that these blocks can function as
‘dumb seismometers,’ similar to the way precariously
balanced rocks are used by seismologists on Earth as
markers of peak seismicity [7]. This analysis can proceed in complex detail, modeling the moments of each
block and interpreting the conditions that moved it, or
else more simply assuming a small acceleration triggers mass movement [8, 9] or a small velocity triggers
ballistic movement.
In the ballistic approximation, a seismic impulse
with peak particle velocity 𝑣! encounters the surface
and accelerates materials to distances of order ℎ = !! !
!!
where g is the surface gravity. The ejection of fine
particles from the surface will obscure the impact
event, but the large blocks will be easily visible afterwards. Working backwards we will put together a map
of peak particle velocities.
Block Displacement Measurements: The postimpact images will show the effects of the impact on
the blocks’ displacements. Using the coordinate system
created from the first surveys, it is our intention to
measure absolute displacements of identifiable rocks
and to determine a correlation between displacement
magnitude and linear distance from the center of the
impact site. The presence of a negative but distinctly
non-zero correlation between these two measurements
implies that the seismic waves were propagated inside
of Beta. If displacement of blocks is observed at the far
end of Beta relative to the impact, then that is evidence
of a coherent body.
Block Distribution Measurements: The methods of
previous block distribution studies [5, 10-13] follow
those initially described in [14] and produce SFD indices for selected blocks within a defined size range [5,
12, 13]. An example of this graph and its source image
is shown in figures 1 and 2. These figures show data
from Itokawa.
Figure 1: The SFD of AMICA image ST2516129281,
with the vertical line at D=2.0. The two slopes represent the break at D=6.0. Without considering this
break, the slope is -2.85 ± 0.1.
Figure
2:
The
stretched
AMICA
image
ST2516129281 projected onto the three dimensional
shape model within the JHU APL SBMT. The ellipses
(in pink) each surround exactly one block; these blocks
served as the data in the SFD shown in Figure 1.
It is unclear how the SFD indices will change after the
impact; it is possible that large ejecta blocks will repopulate the area immediately surrounding the impact
site, while the smaller blocks fully escape the asteroid.
The physical expression of an impact within the block
distributions has never been quantified directly, and
the potential scientific gains from this analysis could
create a method to allow quantitative seismology to be
done with a simple flyby mission.
Data Collection: The data will be collected using an
interactive mapping tool that is able to import images,
draw figures (lines, ellipses, polygons, etc.) on the images with respect to a coordinate grid, and overlay
multiple images to measure displacement. We suggest
using a tool such as the Johns Hopkins University Applied Physics Lab (JHU APL) Small Body Mapping
Tool (SBMT), a tool that allows a user to directly map
images onto a three-dimensional shape model of an
irregularly shaped body [15].
Analysis: The average displacements of the large
blocks will be used to compute a seismic moment for
Beta. On Earth, the seismic moment is linearly proportional to the average displacement. On an asteroid
where other variables, such as the shear modulus, may
be unknown, the average displacement values could
help constrain the value of the seismic moment. In the
case of AIDA, the ancillary knowledge collected could
pinpoint a value and provide a reference point to contextualize data collected from other small bodies.
To analyze the SFD indices, we will graph the data
on a log-log plot, similar to the plot shown in Figure 1.
This is the same method used in other studies. This
will make comparing values among planetary bodies
easier, and aid in interpretation and contextualization.
Conclusions: AIDA presents a rare opportunity to
directly measure the effects of a controlled impact on
the surface appearance of a small body. Details such as
average block displacement, peak particle velocity,
block size frequency distribution indices, and potentially seismological observations can be directly calculated from the deliverables of the mission. AIDA will
also allow us to test the methods of using block displacement and distribution to discern the internal properties of an asteroid.
References: [1] Buczkowski D.L. et al. (2008), Icarus,
193, 39-52. [2] Jutzi, M., et al. (2013), Nature 494,
207-210. [3] Kuppers, M., et al. (2012), Icarus 66, 7178. [4] Barnouin, O.S., et al. (2014) LPS XLV, Abstract
#2221. [5] Noviello, J.L., et al. (2014) LPS XLV, Abstract #1587. [6] Thomas, P.C., et al. (2001), Nature,
413, 394-396. [7] Harp, E.L. & Jibson, R.W. (2002),
BSSA 92, 3180-3189. [8] Richardson, J.E., et al.
(2004), Science 26, 1526-1529. [9] Walker, J.D. &
Huebner, W.F. (2004), Adv. Space Res. 33, 1564-1569.
[10] Lee, S.W., et al. (1986), Icarus, 68, 77-86. [11]
Lee, P., et al. (1996), Icarus, 120, 87-105. [12]
Michikami, T. et al. (2008) Earth Planets Space, 60,
13–20. [13] Mazrouei, S. et al. (2014) Icarus, 229,
181-189. [14] Crater Analysis Techniques Working
Group (1979), Icarus, 37, 467-474. [15] Kahn, E.G. et
al. (2011) LPS XLII, Abstract #1618 (sbmt.jhuapl.edu).
SPECTROSCOPY AND IMAGING OF IMPACT VAPOR PLUMES: APPLICATIONS TO KINETIC
PROBE MISSIONS. M. Bruck Syal1, P. H. Schultz2, and P. L. Miller1, 1Lawrence Livermore National Laboratory,
Livermore, CA 94550 ([email protected]), 2Brown University, Providence, RI 02912
Introduction: The earliest observable information
from an impact event lies within the vapor plume generated by shock heating of projectile and target materials. The vaporized component contains the hottest and
brightest material observed during an impact, including
the initial impact "flash" imaged during the Deep Impact cratering experiment at Comet Tempel 1 [1, 2].
Understanding the variables controlling the formation
and evolution of such plumes is necessary for the refinement of strategies to extract maximal information
from kinetic probe missions such as Asteroid Impact
Deflection Assessment (AIDA). Input from laboratoryscale impact experiments using velocities (~5 km/s)
comparable to the planned velocity for the AIDA impactor can constrain specific, early-time predictions for
the mission, including plume luminosity and morphology; such inputs can then inform choices for camera
exposure times, frame rates, and gain settings. Additionally, high-speed spectroscopy of laboratory scale
impact plumes demonstrates the types of compositional and temperature information that can be acquired
through spectroscopic monitoring of the impact event.
Self-luminous vapor plumes produced in laboratory
scale impacts have long been known to contain a
wealth of spectral information [3, 4]. Planetary scale
impact experiments, e.g., the Deep Impact and
LCROSS missions [5, 6, 7], have demonstrated the
utility of spectroscopic observations of impact plumes
for extracting compositional information about the
subsurface. Typical remote sensing techniques, e.g.,
reflectance spectroscopy, only provide compositional
information from the uppermost layer (few microns or
less) of planetary surfaces. Excavation missions provide a unique opportunity to mine deeper material. For
primitive bodies such as comets and asteroids, extracting compositional data from the interior is particularly
valuable, as these bodies may serve as crucibles of
early solar system materials.
Here we describe recent vapor plume experiments
carried out at the NASA Ames Vertical Gun Range
(AVGR) in Moffett Field, CA, emphasizing how these
and future results can can be applied to both the observational strategy and interpretation of data from impact
missions such as AIDA. Advancing sensor technologies and increasing sophistication in the types of target
materials used in impact experiments provide a clearer
understanding of the expected sequence of impact vaporization for planetary scale impact experiments.
AVGR Instrumentation: Spectroscopic instrumentation at the AVGR allows targeting up to six re-
gions of interest within the vapor plume during each
impact experiment. Six separate telescopes, each viewing the impact through target chamber windows and
providing a 2.5 cm field of view at the impact plane,
collect light for spectroscopic analysis. Fiber optics
direct the focused light signal from the telescopes to a
pair of McPherson Czerny-Turner monochromators
(each fed by three telescopes). A range of diffraction
gratings may be used; recent studies have employed
300 lines/mm gratings, which provide maximal spectral range (200 nm) and 0.2 nm spectral resolution.
Spectra are recorded by two Andor iStar 340T
ICCD detectors (2048x526 pixels), mounted to each
monochromator. Triggering of the ICCDs to <100 ns
accuracy is achieved with a laser interrupt signal from
the impact event. A digital delay generator, internal to
each ICCD, allows targeting vapor plume components
at slightly later times. Duration of ICCD exposures is
chosen to optimize both the signal to noise ratio and
the resolution of distinct areas of the plume; typical
exposures range from 2-20 µs. Calibration is carried
out manually for each fiber. An array of Oriel spectral
calibration lamps (Ar, Xe, Ne, and Hg(Ar)) provide
strong emission lines across the entire observed spectral range for wavelength calibration. In-chamber observations of a Labsphere integrating sphere source are
used to apply spectroradiometric corrections.
Concurrent with the acquisition of impact spectra,
high-speed cameras image impact vapor plumes with 1
µs time resolution, from both the side and above. Data
from these images provides context for the precise
areas of the plume being targeted by each spectrometer
in a given experiment. This complementary information is essential for the eventual comparison of experimental data to numerical simulations and established analytical approximations for vapor plumes.
Experimental Conditions: Previously published
studies on impact spectroscopy have focused largely
upon dolomite [(CaMg)(CO3)2] targets. Dolomite is a
useful material for experimental studies of vapor
plumes, due to its volatility. Vapor products from dolomite, including CaO, MgO, Ca, and Mg, possess
many strong emission lines and bands in the optical
region, allowing for accurate temperature and compositional measurements. However, more realistic asteroid proxy materials are necessary to investigate the
vapor products likely to be produced by a mission such
as AIDA. Recent AVGR experiments use targets comprised of powdered (<500 µm) serpentine [(Mg,
Fe)3Si2O5(OH)4], a hydrated silicate commonly detect-
ed at asteroids. Like dolomite, the threshold velocity at
which serpentine vaporizes is known to be low, <1.5
km/s, well below the ~6 km/s velocities achievable
with light gas gun experiments [8]. The presence of
significant porosity, a critical material property for
predicting the momentum impulse imparted to an asteroid during an impact [9] or standoff nuclear detonation [10], also enhances vaporization in these targets.
Pyrex projectiles (6.35 mm) are used to suppress any
spectral signal from the impactor. Most impacts were
carried out at ~5 km/s, although velocity effects are
also probed by using slightly lower speed impacts (3.5
and 4.3 km/s).
Results: Impact angle strongly controls the luminosity and morphology of the observed vapor plumes
from porous serpentine targets, as shown in Fig. 1.
More oblique angles allow the plume to be observed
over longer time scales, while vertical impacts produce
a plume of limited spatial extent and luminosity. The
spectra from these impacts reveal the presence of FeO
emission bands and many Fe lines, in addition to MgO
bands and Na emission. Preliminary results from the
vaporization of serpentine projectiles impacting refractory targets (reverse case of experiments shown in Fig.
1) are shown in Fig. 2. As the serpentine composition
is well-known, analysis of the full suite of data from
different impact conditions will provide ground truth
for impact spectra obtained from remote planetary surfaces.
5"μs"
90o"
60o"
45o"
30o"
10"cm"
10"μs"
20"μs"
30"μs"
Fig. 1. Impact plume evolution for 5 km/s impacts
into porous serpentine targets at different impact angles. Note that vertical impact (far left) produces
plume of limited luminosity, spatial extent, and duration, compared to more oblique impacts. Emission
spectra from six different points within each of these
plumes, along with lower velocity cases for 45o impacts, will be presented at the AIDA Workshop.
Implications: Laboratory experiments demonstrate that porous targets with asteroid-like compositions are readily vaporized at even modest (< 5 km/s)
velocities. While high-speed imaging provides important constraints on the expected behavior of the
vapor plume for larger, planetary-scale impacts, spec-
troscopy of the evolving vapor plume is a particularly
powerful technique for gleaning compositional information from the subsurface of remote bodies. The
AIDA impact is also expected to produce a luminous
vapor plume; spectroscopic monitoring of the impact
provides high science return at relatively low additional cost. The efficiency with which asteroidal material is
vaporized during an impact event is of central importance to the refinement of impulsive strategies for
asteroid deflection; careful observation of planetary
scale plumes holds value for both fundamental science
and planetary defense purposes.
Fig. 2. Spectra obtained from 5 km/s impact of serpentine projectile into refractory target; note that this is
the reverse case (refractory projectile into serpentine
target) shown in Fig. 1, for which data are still being
calibrated and analyzed. Left panels show entire wavelength range; right zooms in on FeO "orange bands"
near 590 nm. Top and bottom sets probe downrange
and uprange plume components, respectively. In addition to FeO, atomic iron lines and the 589 sodium doublet lines are indicated; MgO and Mg are also present
in the serpentine spectra.
This work was performed under the auspices of the
U.S. Department of Energy by Lawrence Livermore
National Laboratory under Contract DE-AC5207NA27344. IM release number: LLNL-ABS-660334
References: [1] Ernst C. M. & Schultz P. H.
(2007) Icarus, 191, 123-133. [2] Schultz P. H. et al.
(2007) Icarus, 191, 295-333. [3] Gehring J. W. &
Warnica R. L. (1963), 6th HVIS 2, 627-682. [4] MacCormack R. W. (1963), 6th HVIS 2, 613-625. [5]
A'Hearn M. F. et al. (2005), Science 310, 258-264. [6]
Colaprete A. et al. (2010) Science, 330, 463-467. [7]
Schultz P. H. et al. (2010) Science, 330, 468-472.
[8] Shen A. H. et al. (2003) J. App. Phys., 93, 51675174. [9] Holsapple K. A. & Housen K. R. (2012) Icarus, 221, 875-887. [10] Bruck Syal M. et al. (2013)
Acta Astronaut., 90, 103-111.
AIDA SURFACE INTERACTION EXPERIMENT: ACTIVE STUDY OF IMPACT PHENOMENA
A. F. Cheng1, S. Ulamec2
1
The Johns Hopkins University Applied Physics Laboratory, Laurel, Maryland, ([email protected]). 2DLR,
Koeln, Germany
Introduction: The Asteroid Impact and Deflection
Assessment (AIDA) mission will be a joint NASAESA mission to demonstrate asteroid deflection and to
characterize the kinetic impact effects. AIDA, currently in pre-Phase A study, consists of two independent
but mutually supporting mission concepts. These two
missions are the Double Asteroid Redirection Test
(DART), which is the US kinetic impactor, and the
Asteroid Impact Monitoring (AIM) rendezvous mission, which is the ESA characterization spacecraft. The
AIDA target for the first demonstration of asteroid
deflection will be the binary asteroid 65803 Didymos,
to be intercepted around the time of its close approach
to Earth in October, 2022.
The AIM spacecraft will arrive in the Didymos system before the DART spacecraft impact, to characterize the binary and to observe the effects of the kinetic
impact, including asteroid deflection and surface modifications. AIM may include a landed, surface science
pakage and a surface interaction experiment to study
the impact response of the object as a function of its
physical properties.
It is proposed to include on AIM an Active Study
Of Impact Phenomena (ASIP) experiment to determine
fundamental surface physical properties and study internal structure of the target asteroid. This information
is necessary to help understand the system’s dynamic
response to high-speed impacts and to validate the
technique of asteroid deflection by high-speed impact.
At present, the magnitude of the deflection resulting
from kinetic impact is highly uncertain, owing to the
poorly understood contribution of recoil momentum
from impact ejecta. Much of the uncertainty arises
from unknown physical properties of the asteroid surface material, such as density, strength, microporosity
and/or voids. In addition, better understanding of highspeed impact dynamics and fragmentation processes,
adapted to a wide range of materials and impact velocities, will help address fundamental issues of planetary
science, such as collisional evolution, fragmentation,
cratering, and regolith formation and evolution.
Rationale: A surface interaction experiment investigates the physics of high-speed impacts on an asteroid over a wide range of impact energy and velocity, to
address the primary objectives of the AIDA mission:
•
analyze the asteroid surface geology, surface
properties and internal structure
•
observe the impact craters resulting from
known impacts to study impact dynamics and derive
surface properties.
In the joint AIDA mission concept, the AIM spacecraft will study the impact made by the DART spacecraft, a >300 kg object colliding with the target body at
6.1 km/s. A surface interaction experiment is proposed
to provide the AIM spacecraft with the capability to
make additional, smaller impacts on the target asteroid
over a wide range of scales, so as to meet the second
primary objective even in the absence of the DART
mission.
Surface Interaction Experiment: The ASIP experiment can provide impacts on a much smaller scale
than the DART spacecraft impact on the asteroid.
Therefore, the ASIP impacts are not of a scale large
enough to potentially deflect an asteroid. The ranges of
impact energies and velocities explored by the combination of the ASIP and DART impacts will be greatly
expanded compared to an AIDA mission with only the
DART impact.
With ASIP, seismic stimuli can be provided, independently of DART, for a landed surface science package with a seismic instrument. Also, the non-linear
effects of momentum transfer and impact dynamics
can be studied in situ at the asteroid. ASIP would be
complementary to the DART experiment. Moreover,
the ASIP impacts would be at scales reproducible in
terrestrial laboratory experiments, and hence they provide the vital link to terrestrial modeling and experimental impact databases.
ASIP can provide impacts at two scales. The ASIP
medium speed impactors are designed to make impacts
at ~130 m/s with ~200 J kinetic energy; using the
compact, pyrotechnic-powered rock chipper units developed for the MarcoPolo-R mission. The ASIP highspeed impactor is designed to make an impact at ~4
km/s with ~200 kJ kinetic energy, using an explosively
formed projectile; this device is already spacequalified and has extensive flight heritage on many
missions. These impacts complement the DART impact at 6.1 km/s and ~6000 MJ energy.
NUMERICAL SIMULATIONS OF SPACECRAFT-REGOLITH INTERACTIONS ON ASTEROIDS. R.-L.
Ballouz1, D.C. Richardson1, P. Michel2, and S.R. Schwartz2, 1Department of Astronomy, University of Maryland,
College Park, USA, 2Lagrange Laboratory, University of Nice Sophia Antipolis, CNRS, Observatoire de la Côte
d’Azur, Nice, France.
Introduction: NASA's OSIRIS-REx mission will
rendezvous in 2018 with the near-Earth asteroid
(101955) Bennu and attempt to touch down and obtain
a sample from its surface. The regolith surface's behavior in response to the spacecraft's intrusion is difficult
to predict due to the asteroid's extremely low-gravity
environment (on the order of 10 micro-g's.) We have
been carrying out high-resolution (N > 100,000) numerical simulations of the intrusion of a realistic physical model of the sampling device into a bed of cm-size
spherical particles to explore the relationship between
the spacecraft's response and the dynamical behavior
of the regolith. If the granular bed is too compliant,
then the spacecraft may sink into the asteroid. If the
granular bed is not compliant enough, then the spacecraft may not be able to obtain an appropriate sample.
This is further complicated by the fact that the degree
of compliance is also dependent on the material properties of the regolith surface (size distribution, local
slope, friction coefficients, shape effects). The ultimate
goal of this study is to construct a library of touchdown
outcomes as a function of the potential observables
(local slope, estimated maximum angle of repose, and
to a limited exited particle size distribution). We study
the effect of varying the regolith's material properties
(cohesive, frictional, and dissipative parameters) in
order to place limits on the range of possible outcomes.
The library will be useful for sample-site selection
based on available observables, and, upon sampling,
may aid in interpreting the physical properties of the
regolith (e.g., depth and density) by comparing measurements from on-board instruments with simulation
data. Preliminary results show that grains with low
coefficients of friction (smooth particles) provide little
resistance and the spacecraft sinks into the asteroid.
For high coefficients of friction (effectively mimicking
grain angularity), the regolith is much less compliant,
and the spacecraft is only able to penetrate the first few
centimeters of the surface layer. This result suggests
that for high grain angularity, the regolith directly underneath the sampler is able to shear thicken due to
particle interlocking.
Figure 1 A device penetrates a `gravel'-like regolith
with a power law size distribution of particles. The
device causes a fluid like splash response of the ejecta,
but the material directly underneath the device shearjams into a solid-like state.
ANALYTICAL CALCULATIONS OF EJECTA MASS AND CRATER SIZE PRODUCED BY THE
IMPACT OF THE DART SPACECRAFT INTO THE MOON OF DIDYMOS. C.M. Ernst, O.S. Barnouin,
and A.M. Stickle, Johns Hopkins University Applied Physics Laboratory ([email protected]).
Introduction: When NASA’s Double Asteroid
Redirection Test (DART) spacecraft impacts the moon
of asteroid 65803 Didymos, the event will be observed
remotely by Earth-based observatories and in situ by
ESA’s Asteroid Impact Mission (AIM) spacecraft. The
objective of the DART mission is to cause a measureable change in the period of the moon’s orbit about
Didymos that can be used to constrain the efficiency of
momentum transfer from the impactor to the target.
The DART impact will join Deep Impact and
LCROSS as planetary-scale impact experiments. As
with these earlier impact events, the initial impactor
parameters are well known; however, the physical
properties of the target are not well constrained. Understanding the conditions of the DART impact is essential for interpreting the ability of the kinetic impactor to deflect the asteroid. A priori estimates of the
scale of the event are essential for planning Earthbased observations of the impact event.
We use crater scaling rules [1–4] to calculate the
amount of ejecta expected to escape the moon and the
asteroid system and to predict the size of the final
crater. Because the physical properties of the target are
not well constrained, we explore the possible outcomes
of the impact event for a range of material properties.
Initial Conditions: For simplicity, we take the impactor to be an aluminum sphere of equivalent mass to
the DART impactor and assume a vertical (90º) impact
angle (normal to the moon’s surface).
Strength vs. Gravity The transition between
strength- and gravity-dominated cratering events can
be estimated by equating non-dimensionalized crater
scaling terms π2 and π3:
Impactor parameters:
• Mass (mi) = 300 kg
• Density (δi) =2780 kg/m3 (aluminum)
• Radius (ai) = 0.3 m
• Velocity (vi) = 6.25 km/s
• Angle = 90º
Target parameters:
• Mass = 3.0 x 109 kg
• Density (ρt) = 1700 kg/m3
• Radius (at) = 75 m
• Gravity (g) = 3.56 x 105 m/s2
• Escape velocity (vesc) = 7.3 cm/s
• Effective strength ( Y ) = see Table1
Didymos system parameters:
Radius (r) = 2548 m
Mass = 2.6 x 1012 kg
Escape velocity (vesc) = 36.8 cm/s
where where c1 ~0.25 to 0.33 and R is the transient
crater radius. We multiply R by 1.25 to convert from
transient to rim-to-rim radius.
Ejecta: Again, following the work of [1–4], we investigate the amount of ejecta that will be produced by
the event. We first calculate the ejecta velocity as a
function of emergence position. Because the cratering
activity needs to be “shut off” when the power-law
scaling no longer works, we calculate an effective
velocity (vef) at a given distance from the impact point
(x) after losses due to gravity and strength:
! Y $
! ga $
π 2 = # 2i & and π 3 = # 2 &
" vi %
" ρt vi %
In order for the cratering process to be gravity
dominated, the effective strength of the moon would
have to be ~4 Pa. Therefore, the DART impact into the
moon of Didymos will be a strength-dominated event.
We consider a range of possible material strength values for the moon, which are listed in Table 1. Gravitydominated crater scaling rules provide upper-limits for
the calculations. We ignore porosity effects.
Crater Size: Following the work of [1–4], the size
of the crater is calculated using the cratering efficiency
term, πv, to calculate the excavated crater volume,
which is then used to determine a transient crater radius:
−3µ
2+µ %
(2+µ )
"
π v = K1 $ π 2 π 4−1/3 + π 3 2 '
#
&
where µ is an empirically determined variable between
1/3 and 2/3 and π4 is the ratio of the target and
impactor densities. We can equate two equations to
determine crater volume:
!m $
V = π v # i & = c1π R 3
" ρt %
and
!V $
R =# &
" c1π %
1
3
1
"
%2
2
2 Y
vef ( x ) = $vej2 − Cvpg
gx − Cvps
'
ρt &
#
where Cvps and Cvpg are constants related to µ and the
shape of the transient crater. To calculate the mass of
excavated material in a given shell of the transient
crater located between xn+1 and xn from the crater
center, we use the equation:
1
3
mri = πρt ( xn+1
− xn3 )
9
where it is assumed that the excavation depth of the
transient crater is approximately equal to 1/9 of the
transient crater diameter. Figure 1 illustrates the
relationship between the calculated vef and the
cumulative mass of material ejected at or below a
given vef.
Results: The results of the calculations for all
scenarios are listed in Table 1. The most likely range
of scenarios is outlined by the box and result in a final
crater size between 8 and 17 m and a cumulative mass
of escaping ejecta between ~104 and 105 kg.
References: [1] Housen, K.R. et al. (1983) JGR
88, 2485–2499. [2] Holsapple, K.A. (1993) Ann. Rev.
Earth and Planet. Sci. 21, 333–373. [3] Richardson,
J.E. et al. (2007) Icarus 190, 357–390. [4] Housen,
K.R. and Holsapple, K.A. (2011) Icarus 211, 856–875.
Figure 1. Cumulative ejected mass versus ejecta velocity for a range of DART impact scenarios.
Table 1. Parameter space calculations. Values of µ, K1, and Y for strengthless sand, dry and wet soil, and soft and
hard rock are taken from [2]. The box indicates the most likely range of scenarios.
MODELING MOMENTUM TRANSFER FROM THE DART SPACECRAFT IMPACT. A. M. Stickle, O.S.
Barnouin, A. Cheng, C. M. Ernst, and A.S. Rivkin, Johns Hopkins University Applied Physics Laboratory, Laurel
MD ([email protected])
Introduction: There are roughly 1000 near-Earth
objects with sizes > 1 km, any of which would have
civilization-wide consequences if they were to impact
the Earth. Even smaller objects, such as that responsible for the recent Tunguska fireball can devastate entire regions. Given sufficient notice, however, there
are several hypothesized mechanisms for deflecting
these: “gravity tractors”, exploitation of Yarkovsky
effects, and more active techniques such as nuclear
weapons and kinetic impactors.
The Asteroid Impact and Deflection Assessment
(AIDA) mission is a joint concept between NASA and
ESA designed to test the effectiveness of a kinetic impactor. The mission is composed of two independent,
but mutually supportive, components: the US-led Double Asteroid Redirect Test (DART), and the ESA led
Asteriod Impact Monitoring (AIM) mission. The
spacecraft will be sent to the near-Earth binary asteroid
65803 Didydmos, which makes unusually close approaches to Earth in 2022 and 2024. These close approaches make it an ideal target for a kinetic impactor
asteroid deflection demonstration as it will be easily
observable from Earth. The kinetic impactor (the 2-m,
300 kg DART spacecraft) will impact the smaller
moon (~150-m diameter) of the binary system at 6.25
km/s. The deflection of the moon will then be measured by AIM and from ground-based observations by
measuring the change in the moon’s orbital period. In
support of this mission concept, a modeling study was
performed to determine the expected momentum transfer to the moon following impact. The combination of
CTH hydrocode models and analytical scaling predictions help to constrain the expected results of the kinetic impactor experiment.
CTH Model Description: To better understand
the large parameter space, simulations of the DART
impact were performed using the CTH hydrocode from
Sandia National Laboratories [1]. Because the physical properties of the Didymos system are not well
known, we examined a variety of target properties (e.g,
material strength and porosity) and impact scenarios to
put constraints on the expected results of the DART
impact. Mission design constraints provide an impact
velocity and a likely impact location with respect to the
center of figure (COF) of the moon. However, impact
angle (whether due to an inclined trajectory or to local
surface topography) is not well constrained. To better
inform mission design, we examined the effects of
impact location (e.g., where DART impacts with re-
spect to the center of figure), and impact angle on momentum transfer.
Figure 1. Schematic of impact model setup, showing
the possible impact trajectories, the location of the
tracer grid (dotted lines), and offset direction (dx) from
the Center of Figure (COF).
The models utilize both 2D and 3D rectangular coordinate spaces to simulate the impact, which allows
for normal or oblique impact trajectories, and allows
impacts offset from the center of figure (Figure 1).
Adaptive Mesh Refinement [2] is used to obtain highresolution coverage of important areas while lessening
computer resource usage. These simulations have a
maximum resolution of ~3 cm, with nominal resolution of ~15 cm. Currently, the spacecraft is modeled as
a solid aluminum sphere (ρ=2.793 g/cm3) with a radius
of 32 cm, which is sized to the expected impact energy
of the 300-kg DART spacecraft impacting at 6.25
km/s. The spacecraft is modeled using an ANEOS
equation of state for Al-2024 and a Johnson-Cook
plasticity model. The moon is modeled as a 150-mdiameter sphere using a sesame equation of state for a
basalt-like material (ρ=2.82 g/cm3 for fully competent,
strong material). In some runs, the p-α model is include to simulate initial porosity to match the measured density of the Didymos system (ρ=1.7 g/cm3).
The simulations are run to a time of 0.4–0.5 seconds, a
time by which the crater has stopped forming and reverberations in the body from the shock wave have
nearly stopped. This minimizes noise in the velocity of
the tracers.
Calculating Momentum Transfer: The momentum transferred to the asteroid by the spacecraft is parameterized by β, which is the momentum enhancement of the asteroid by adding the spacecraft momentum to the momentum of the ejecta excavated during
crater formation. In each simulation, the moon is seeded with Lagrangian tracer particles (schematically
shown in Fig. 1). These particles track the velocity
and state of the material (e.g., pressure, temperature,
density) through time following impact. Because the
moon is initially at rest, the velocity of the tracers following impact is tracked and used to calculate the
moon’s momentum after impact. The resultant momentum transfer, β, to the moon can then be calculated by:
β = 1+
pmoon
pspacecraft
where pmoon is the momentum of the moon following
impact, and pspacecraft is the initial momentum of the
spacecraft.
Analysis: Analytical scaling predicitons were
compared to the predictions from the CTH model to
better constrain material parameters and understand the
simulations.
Analytical Scaling Predictions: Analytical calculations of momentum transfer based on well-known scaling relationships developed for studying laboratory
impact craters [e.g., 3-4] give reasonable estimates for
cratering efficiency. Here, we computed the momentum transfer and change in velocity (Δv) for four possible target types representing the moon of Didymos: a
strong rock, a weakly cemented rock, a highly porous
weak rock, and a cohesionless sand-like surface, following the approach outlined in [5] to account for ejecta retained by the target body. Results of these calculations for the different target types are shown in Table
1. These calculations assume a 90-degree impact by an
object with the mass and velocity of the DART spacecraft.
Table 1. Semi-analytical predictions of momentum
transfer and velocity imparted to a target asteroid after
impact by the DART spacecraft
Material
Asteroid Effective
β
Δv
density
strength
mm/s
kg/m3
kPa
Competent
3000
300
4.1
1.45
rock
Weak rock
2600
200
1.28
0.52
Highly po1000
2
1.45
1.54
rous,weak
Cohesionless
1700
0
1.75
1.10
sand
These results show that the Δv imparted by the spacecraft can range from 0.5–1.52 mm/s. The most realistic target properties for the binary are likely porous and
weak rock, though this is not known for certain. Even
though it has significant porosity, the rock still retains
enough strength relative to the small gravity of the
moon to allow a significant amount of ejecta to escape
the moon, which increases the momentum transfer to
the body. The other three cases likely represent material that is too strong (competent rock), too dense
(weak rock), or too unrealistic (cohesionless) for the
low density of the system and the low gravity of the
moon. The other target types help us place constraints
and bounds on the expected Δv generated by the
DART spacecraft, though, and will be examined in
CTH modeling efforts.
CTH Models: Preliminary results from the CTH
models generally agree with predictions from theoretical studies. For fully dense, competent, strong rocks,
we calculate values for β between 3.8 and 5.5, with an
imparted Δv of 1.47–2.29 mm/s to the moon for impacts at various distances (dx) from the COF (dx=0–30
m). These β values are within ~5–25% of the analytical solutions shown in Table 1, indicating that the impact location with respect to the COF will affect the
momentum transfer in the direction of interest (in the
ecliptic plane, along the direction of motion, here, assumed to be the z-direction).
Due to the small diameter of the moon and the resulting surface curvature, impacts offset from the COF
are also, effectively, oblique impacts with respect to
the surface. This will affect the momentum transferred
along the preferred axis of the moon because the ejecta
distribution and the resulting momentum enhancement
from ejecta motion will will not be symmetrical. Further, any impact offset from the COF of the body will
likely impart some rotation to the body, lessening the
linear momentum transferred by the impact. These
effects, and their significance, are currently under investigation.
References: [1] McGlaun et al. (1990) Int. J. Imp.
Engr. 10(1-4), 351-360; [2] Crawford, D.A. (1999)
Paper presented at the 15th US Army Symposium on
Solid Mechanics, Myrtle Beach, SC, April 12-14; [3]
Housen, K. R., R. M. Schmidt, and K. A. Holsapple
(1983), Journal of Geophysical Research, 88, 2485–
2499; [4] Holsapple, K.A. (1993) Annual Review of
Earth and Planetary Sciences, 21, 333-373; [5]
Holsapple, K. A., and K. R. Housen (2012), Icarus,
221(2), 875–887.
Consequences of the DART impact on the orbital dynamics of the moon of Didymos. O. S.
Barnouin1, D. C. Richardson2, A. Rivkin1, A.M. Stickle1, C.M. Ernst1 and A. Cheng1, 1The Johns
Hopkins University Applied Physics Laboratory, Johns Hopkins Rd, Laurel, MD 20723-6099,
USA, [email protected], 2Department of Astronomy, University of Maryland, College
Park, MD 20742-2421, USA.
Introduction: The Double Asteroid Redirection Test (DART) is one of two independent spacecraft that make-up the Asteroid
Impact & Deflection Assessment (AIDA)
mission, a joint effort of ESA, JHU/APL,
NASA, OCA, and DLR. The overall purpose
of the mission is to demonstrate the deflection
of an asteroid by a kinetic deflector. DART,
the NASA contribution, is the impactor
spacecraft, while the Asteroid Impact Mission
(AIM) spacecraft is the rendezvous probe.
The mission intends to deflect the moon of
the binary asteroid Didymos, henceforth
called Didymoon.
Here, we present the expected momentum
and velocity changes at Didymoon for various
target conditions, employing semi-empirical
models [1]. We then investigate the consequences of the velocity changes on the orbital
dynamics of Didymoon using both analytical
models and a more sophisticated N-body
code, pkdgrav [2]. Pkdgrav permits consideration of more complex geometries than is
possible with the analytical solutions. This
includes investigating changes in the velocity
of the moon out of its orbit plane, and the effects of a realistic primary shape on the
moon’s orbital dynamics.
Scaling Predictions: The analytical calculations of momentum transfer are based on
well-known scaling relationships developed
for studying laboratory impact craters [3] that
give reasonable estimates for cratering efficiency. We compute the momentum transfer
and change in velocity (Δv) for four target
types: a strong rock, a weakly cemented rock,
a highly porous weak rock, and a cohesionless sand-like surface, following the approach
outlined in [1] that accounts for ejecta retained by the target body. The momentum
transferred to the asteroid is parameterized by
β, which is equal to the ratio of the spacecraft
momentum to the sum of the spacecraft momentum and thrust generated by ejecta excavated during crater formation and lost to
space. Results of these calculations for different target types are shown in Table 1. These
calculations assume a vertical impact and
nominal values of 300kg and 6.25km/s for the
mass and velocity of the DART spacecraft.
Table 1. Estimates of β and velocity imparted to Didymoon after impact by DART.
Material
Competent
rock
Weak rock
Highly porous, weak
Cohesionless sand
Asteroid
density
(kg/m3)
3000
Effective
strength
(kPa)
300
β
Δv
(mm/s)
4.1
1.45
2600
1000
200
2
1.28
1.45
0.52
1.54
1700
0
1.75
1.10
These results show that the Δv imparted by
the spacecraft can range from 0.5–1.52 mm/s.
Altough not known for certain, the moon of
Didymos is most likely a porous and weak
rock [see 5 for rationale]. Though it is porous, the rock still retains enough strength
relative to the small gravity of the moon to
allow a significant amount of ejecta to escape
the moon, increasing the momentum transfer
to the body. The other three cases are likely
too strong (competent rock), too dense (weak
rock), or too unrealistic (cohesionless) in light
of the low density of the system and the low
gravity of the moon. However, the other target types help us place constraints and bounds
on the expected Δv generated by the DART
spacecraft.
Orbital Implications: We employ the resultant Δv calculated in the previous section
to evaluate its consequences on the subse-
quent orbital evolution of Didymoon. For a
start, we consider two simple models of the
Didymos binary system: a 2 particle system
and an aggregate primary (Fig. 1). We assume the moon of Didymos is a 150m diameter sphere in all cases, located at 1.095km
from the center of the 0.8km-diameter primary. We considered, as a baseline, the case
where Didymoon had no eccentricity or inclination (e, i = 0). The primary was assigned a
rotation rate of 2.6h, and the secondary an
orbital period of 11.9h [4]. We do not model
any small body forces such as YORP or Yarkovsky that could influence the dynamics of
the secondary but over much longer time
scales than the time expected for both AIM
and Earth-based observations following
DART’s impact.
The Δv explored includes adjustment in
the secondary’s velocity both in and opposite
to the secondary’s orbit direction, and at some
angle to the orbit direction both in and out of
the binary’s orbit plane. Fig. 2 shows some
examples of our results and illustrates that for
most impact conditions considered in the 2particle system, changes in orbital dynamics
of the Didymoon can be observed easily from
both AIM and Earth-based facilities. Preliminary investigation making use of a more realistic primary aggregate appear to enhance
some of the changes in the orbital evolution
of the secondary.
References: [1] Holsapple K. A. and
Housen K. R. (2012), Icarus, 221, 875–887.
[2] Richardson D. C. et al. (2000), Icarus,
143, 45–59. [3] Housen K. R. et al. (1983),
JGR, 88, 2485–2499. [4] JPL Small-Body
Database Browser on 65803 Didymos. [5]
Walsh K. J. and Richardson D. C. (2008), Icarus, 193, 553–566.
Figure 1. Some examples of consequences of Δv imparted by DART to the moon of Didymos.
GENERATING HYPERVELOCITY IMPACTS INTO SMALL BODIES: SIMULATING THE FATE OF
EJECTA. S. R. Schwartz1,*, P. Michel1, M. Jutzi2, and D.C. Richardson3, 1Lagrange Laboratory, University of Nice
Sophia Antipolis, CNRS, Observatoire de la Côte d'Azur, C.S. 34229, 06304, Nice Cedex 4, France, 2University of
Bern, Switzerland, 3University of Maryland, College Park, MD, USA, *e-mail: [email protected].
Introduction: Asteroids and comets collide with
the Earth, profoundly influencing life on the planet
with stochastic impact events that may occur over
timescales of millions of years. It has been estimated
that about every year, asteroids measuring 4 meters
across enter Earth’s atmosphere and detonate [1], and
that asteroids of 100 meters diameter tend to impact
the Earth every 5,000 years. On average, collisions
with 4-kilometer asteroids are occurring about every
13 million years [2]. In 2005, a United States Congressional mandate called for NASA to detect, by
2020, 90 percent of Near-Earth Objects (NEOs) with
diameters of 140 meters or greater [3].
As sky surveys are performed, and detection strategies are developed, for discovering small bodies that
may be on trajectories to collide with the Earth, it is
prudent to concurrently be developing and refining
mitigation strategies, including those that could alter
the paths of hazardous objects. (Strategies that do not
involve affecting a hazardous object’s path would include civil defense approaches.) However, any discussion of efforts to deviate the path of hazardous NearEarth Objects (NEOs) must incorporate an analysis of
the makeup of the body. The assumption that asteroid
surfaces consist of granular material is based on the
results of several observations, including confirmation
by space missions that have visited asteroids in the last
few decades [5,6]. It appears that all encountered asteroids thus far are covered with some sort of granular
regolith. To date, this includes a large range in asteroid sizes, from the largest one visited, by the Dawn
spacecraft, the main belt asteroid (4) Vesta, which
measures about 500 kilometers across, to the smallest
one, sampled by the Hayabusa mission, the NEO
(25143) Itokawa, which measures about 500 meters
across [7]. Thermal infrared observations also support
the idea that most asteroids are covered with regolith,
given their preferentially low thermal inertia [8].
One of the four main types of mitigation strategies
explored by the United States’ National Research
Council’s (NRC) “Committee to Review Near-Earth
Object Surveys and Mitigation Strategies” involves
using an impactor spacecraft to deflect an NEO from
its path by crashing into it at speeds of up to 10 km s-1
or more [9]. The three other strategies outlined by the
NRC include detonations on or beneath the asteroid
surface (nuclear and non-nuclear), gravitational
tractors, and broad civil defense approaches. The
NEOShield Project aims to design a general NEO
defense strategy based upon momentum transfer via
kinetic impact [10]. Begun in 2012, the NEOShield
Project is being funded for 3.5 years by the European
Commission in its FP7 program. It is primarily, but
not exclusively, a European consortium of research
institutions that aims to analyze promising mitigation
options and provide solutions to the critical scientific
and technical obstacles involved in confronting threats
posed by small bodies that cross Earth’s orbit. Here,
we study numerically some of the details involved in a
kinetic impactor approach to NEO threat mitigation as
part of a specific work package of NEOShield.
Figure 1: Initial phase of a numerical simulation involving a 400 kg, 1 g cm-3 spherical projectile impacting a target, 300 m in diameter, at 10 km s-1. Since the
target body is large compared to projectile and resulting crater size, only a region measuring 30 m across is
shown in the image.
Numerical Method: We perform the majority of a
given impact simulation using PKDGRAV, a parallel Nbody gravity tree code [11] adapted for particle collisions [12,13]. A soft-sphere collisional routine was
added recently [14]; with this option, particle contacts
can last many timesteps, with reaction forces dependent on the degree of overlap (a proxy for surface deformation) and contact history—this is appropriate for
dense and/or near-static granular systems. The code
uses a 2nd-order leapfrog integrator, with accelerations
due to gravity and contact forces recomputed each
step. In the spring/dash-pot model used in PKDGRAV’s
soft-sphere implementation, described fully in [14], a
(spherical) particle overlapping with a neighbor or
boundary wall feels a reaction force in the normal and
tangential directions, as well as a host of particular
damping effects that can optionally impose kinetic,
static, rolling, and/or twisting friction. Plausible values for these various material parameters are obtained
through comparison with laboratory experiments.
Figure 2: An artistic representation of a kinetic impactor mission to a hazardous object. This approach is
one of those being studied by the NEOShield consortium for its efficacy in mitigating an NEO threat. As the
impactor collides with the target, it is observed by a
companion spacecraft in orbit around the object.
(Credit: ESA)
The numerical approach has been validated in terrestrial contexts; e.g., [14] demonstrated that PKDGRAV,
using the soft-sphere collisional routine, correctly reproduces experiments of granular flow through cylindrical hoppers, specifically the flow rate as a function
of aperture size, and found that the material properties
of the grains affect the flow rate as well. Also successfully simulated were laboratory impact experiments
into sintered glass beads [15], taking into account interparticle cohesion, and into regolith in support of
asteroid sampling mechanism design [16].
To carry out the initial phase of an impact, we generally use smoothed-particle hydrodynamics (SPH)
software [17] to handle the potion of the evolution of
the impact that involves supersonic motion. The com-
puted positions and velocities of the simulated material
are then ported into PKDGRAV (Fig. 1), taking advantage of its gravity tree solver to find neighbors and
to resolve gravitational forces, and its soft-sphere collisional routine to resolve contact forces. The momentum imparted to the model NEO is analyzed and the
early evolution of the impact ejecta is computed, accounting for the mutual gravitational attraction between ejecta particles as well as the gravitational attraction due to the presence of the target. This is performed in order to determine ejecta fate, i.e, whether it
re-collides, escapes, or enters into orbit around the
target body. This has inherent scientific interest, as it
pertains to the constitutive properties of these small
bodies. In addition, this information is particularly
important for the mission profile of an orbiting artificial satellite injected into the system in order to record
the event (Fig. 2) in light of the potential observational
and mechanical effects of lingering dust and debris.
Results of simulations over a first set of impact conditions and material paramters will be discussed.
References: [1] Collins G. S. et al. (2005) Meteorites & Planet. Sci., 40, Nr 6, 817–840. [2] Harris A. W.
(2009) AGU Fall Meeting, Abstract #PP33B-05. [3]
National Research Council (2009) Near-Earth Object
Surveys and Hazard Mitigation Strategies: Interim
Report. The National Academies Press. [4] Veverka J.
et al. (2000) Science, 289, 2088–2097. [5] Fujiwara A.
et al. (2006) Science, 312, 1330–1334. [6] Russell C.
T. et al. (2012) Science, 336, 684–686. [7] Miyamoto
H. et al. (2007) Science, 316, 1011–1014. [8] Delbó M.
et al. (2007) Icarus, 190, 236–249. [9] National Research Council (2010) Defending Planet Earth: NearEarth Object Surveys and Hazard Mitigation Strategies. The National Academies Press. [10] Harris A. W.
et al. (2013) Acta Astronautica, 90, 80–84. [11] Stadel
J. G. (2001) Ph.D. thesis, University of Washington.
[12] Richardson D. C. et al. (2000) Icarus, 143, 45.
[13] Richardson D. C. et al. (2011) Icarus, 212, 427–
437. [14] Schwartz S. R. et al. (2012) Granul. Matter,
14, 363–380. [15] Schwartz S. R. et al. (2013) Icarus,
226, 67–76. [16] Schwartz S. R. et al. Planet. Space
Sci., submitted. [17] Jutzi M. and Michel P. (2014)
Icarus, 229, 247–253.
Acknowledgements: This study is performed in
the context of the NEOShield Project funded under the
European Commission’s FP7 program agreement No.
282703. Most of the computation was performed
using the computing cluster YORP) administered by the
Center for Theory and Computation at the University
of Maryland's Department of Astronomy. For data
visualization, the authors made use of the freeware,
multi-platform ray-tracing package, Persistence of Vision Raytracer (POV-RAY).
Advances in Meshless Modeling Applicable to Simulation of Impacts on Asteroids.
J. Michael Owen1 , Jared Rovny1,2
1 Lawrence
Livermore National Laboratory ([email protected]), 2 Yale University
Damage modeling: We have augmented the damage models described in [1] (based on the statistical fragmentation theory of [2]) in a few important
ways. First we have extended the concept of the
damage to a tensor formalism (Di → Diαβ ), which
gives us directionality in the damage on each node.
We have also developed a pair-wise limiter based
on the gradient of the damage to determine when
a given pair of ASPH nodes should apply damage to their interactions. Our changes are designed
to focus the damage into evolving fractures and
relieve strain on the bulk of material more effectively, avoiding a common failure mode where the
computational damage becomes too wide-spread
(the most extreme form of these problems result
in all the material becoming damaged and turning
into an undifferentiated dust). We have found these
handful of extensions very helpful in demonstrating convergence of the models and successfully
matching experiments.
Introduction: We present extensions of the SPH
(Smoothed Particle Hydrodynamics) formalism
designed to aid predictive simulation of the response of asteroids to mitigation techniques, such
as kinetic impactors or energy deposition from a
nuclear device. Toward this end we have developed three important improvements to SPH: Adaptive Smoothed Particle Hydrodynamics (ASPH),
extensions of damage and failure modeling in
(A)SPH, and a rigorously energy conserving
“compatible” form of (A)SPH.
(a) SPH
(b) ASPH
Figure 1: SPH vs. ASPH models of a steel rod undergoing tension, colored by damage.
ASPH: Adaptive Smoothed Particle Hydrodynamics (ASPH [3, 4]) differs from SPH in the
choice of sampling volume. SPH assumes a per
point scalar smoothing scale, which is evolved
such that each SPH point samples a constant of
neighbors. However, this approximation is inappropriate if the local point distribution evolves
anisotropically. ASPH instead associates a symmetric tensor smoothing scale with each point,
corresponding to unique ellipsoidal sampling volumes. These sampling volumes are evolved such
that each point keeps the same number of neighbors in each direction, a distinction that can be important in avoiding unphysical numerical failure.
Fig. 1 compares SPH and ASPH models of a steel
rod being stretched in the horizontal direction –
SPH’s inability to adapt to anisotropic distortion of
the points causes the SPH model to preemptively
fall apart numerically. The ASPH model successfully follows the deformation of the material until
it fractures at the points indicated by the damage
model.
(a) Experiment
(b) Calculation (damage)
Figure 2: Simulated damage vs. the experimental
photograph in the gas gun experiment.
Experimental comparisons: We have applied our
methodology to a series of laboratory experiments
to test their validity. In Fig. 2 we model an experiment [5] in which a steel tube (5cm long by
1.27cm diameter) is fitted to an anvil on a gas
gun. Half the length of tube is filled with a plas1
tic plug, while a similar plastic plug is fired into
the open end of the tube at ∼ 2km/sec. When
the projectile impacts the plug in the tube the two
expand outward, fracturing the surrounding steel.
Fig. 2 compares the experimental photograph to
the simulated tube at 35 µsec. We have been able
to successfully match quantities such as expansion
velocities, fragment mass statistics, and fragment
morphology in experiments such as this.
radiating fractures through the core outside of this
fully damaged region. Fig. 3b plots our fracture detection switch used to decide which ASPH points
should decouple, demonstrating the effectiveness
of this switch at picking out the macroscopic fractures. It is important to capture these sorts of differences for our applications in order to predict
both the deflection due to material which is ejected
and results in deflection of the bulk material, as
well as the fragmentation state of the remaining
body. This has implications both for the practical
considerations of deflection and/or disruption, as
well as the fundamental science in determining the
fate of interacting small solar system bodies and
their resulting characteristics after numerous impacts such as these.
References
(a) Materials 0.1 sec after impact
[1] W. Benz & E. Asphaug, (1995) Computers
Phys. Comm., 87, 253-265.
[2] D. E. Grady & M. E. Kipp, (1980)
Int. J. Rock Mech. Min. Sci. Geomech. Abstr., 17, 147-157.
[3] J. M. Owen, J. V. Villumsen, P. Shapiro, &
H. Martell (1998), Ap. J. Supp., 116, 155209.
[4] J. M. Owen, Proc. Fifth Int. SPHERIC Workshop (2010), Manchester, U.K., 297-307.
Figure 3: 2D simulations of 2m Al slug impacting
a 50 m radius idealized asteroid.
[5] T. J. Vogler, T. F. Thornhill, W. D. Reinhart,
L. C. Chhabildas, D. E. Grady, L. T. Wilson, O. A. Hurricane, and A. Sunwoo, (2003)
Int. J. Imp. Eng., 29, 735-746.
Small body results: We are applying our methodology to understand the response of small solar
system bodies to attempts to divert them, such as
kinetic impactors or nuclear devices. Fig. 3 shows
a 2D calculation of a 2×1m Al slug traveling at 12
km/sec impacting a 50 m radius granite asteroid,
consisting of a 40 m radius solid core surrounded
by 10m of strengthless regolith. Our model captures the disruption of the regolith layer, pulverization of the solid core near the impact point, and
Acknowledgments: This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory
under Contract DE-AC52-07NA27344, and partially funded by the Laboratory Directed Research
and Development Program at LLNL under tracking code 12-ERD-005. The authors would like to
acknowledge the LLNL LDRD asteroid team and
their participation in this work.
LLNL-ABS-660337
(b) Fractures in solid core
2
Development of Hands-on Introductory Planetary Defense Course
N. Melamed
The Aerospace Corporation
[email protected]
The Aerospace Corporation is a California nonprofit corporation that provides its clients with scientific
and engineering support on space and space-related
systems. The Corporation’s technical staff has prepared
an introductory planetary defense class to explain the
unique hazards posed by Near Earth Objects (NEOs)
and describe efforts for discovering and tracking NEOs
and mitigating their danger. The class begins by reviewing asteroid and comet threat types and mitigation
strategies, and culminates with a hands-on exercise
using an interactive NEO deflection simulator tool developed by The Aerospace Corporation for NASA HQ.
The development of the simulator was in response to
findings and recommendations from International
Academy of Astronautics (IAA) Planetary Defense
Conferences and studies highlighting the need to identify NEO deflection options and design and test techniques that might be used to mitigate future collision
events. The tool provides the user with insights on how
to slightly alter the velocity of a threatening NEO using
one or several impacting spacecraft and put it on a trajectory that misses Earth. The user selects an object
from a representative set of fictitious, simulated Earth
impacting objects defined by NASA’s Jet Propulsion
Laboratory (JPL). By interacting with this on-line tool,
the user learns valuable lessons for designing feasible
NEO deflection strategies. The web-tool promotes public awareness and preparedness, provides insight into
the nature of the threat and its mitigation, increases the
general knowledge on the topic and its unique challenges, and aids in finding feasible NEA deflection
solutions. The tool has been delivered to NASA in
September 2014 and will be added to JPL’s public
online area including detailed documentation. The
NEO simulator project was developed as part of the
author’s Aerospace Systems Architecting and Engineering Certificate Program (ASAECP), a program that
is designed to ensure appropriately trained staff to support mission success at The Aerospace Corporation.
The Planetary Defense class has been instructed at The
Aerospace Corporation several times between June and
October 2014. The author also volunteers in Science,
Technology, Engineering and Math (STEM) school
outreach activities to inspire the next generation of
scientists and engineers by talking to students about
asteroids and space debris, introduces them to the NEO
deflection simulator, and inspires them to excel in
School, believe in themselves, and make good career
choices.