White Paper for the NRC Decadal Study Prepared by the Community Panel on Near-Earth Asteroid Sample Return

SUBJECT AREA:   Primitive bodies

TITLE:   Near-Earth Asteroid Sample Return

AUTHORS (AFILLIATIONS):
D. W. G. Sears (Univ. Arkansas)
; C. Allen (JSC)
; D. Britt (Univ. Tennessee)
; D. E. Brownlee (Univ. Washington)
; A. F. Cheng (Johns Hopkins Univ.)
; C. R. Chapman (Southwest Research Inst.)
; B. C. Clark (Lockheed Martin Astronautics)
; B. G. Drake (JSC)
; R. Fevig (Univ. Arizona)
; I. A. Franchi (Open Univ.)
; A. Fujiwara (ISAS)
; S. Gorevan (Honeybee Robotics)
; H. Kochan (German Space Agency)
; J. S. Lewis (Univ. Arizona)
; M. M. Lindstrom (JSC)
; K. Nishiizumi (Univ. California, Berkeley)
; M. S. Race (SETI)
; D. J. Scheeres (Univ. Michigan)
; E. R. D. Scott (Univ. Hawaii); H.Yano (ISAS).

 

Primitive meteorites

Primitive solar system bodies (comets and asteroids) have the
potential to provide unique information about the early solar system and
the material it contained.  Meteorites and cosmic dust are samples
of these bodies that are falling naturally to Earth and they have been
intensely studied.  While much can be learned from cosmic dust, considerably
more has been learned from meteorites, which can be studied on the macroscopic
scale by a wide variety of very sophisticated.  Primitive meteorites
have ages comparable to the age of the solar system, they have bulk compositions
very similar to that of the Sun, and they have unique textures, sometimes
being referred to as “cosmic sediments”.  The major components are
chondrules and refractory inclusions, metal and sulfide grains, and a fine-grained
matrix.  It has been argued that trace components in primitive chondrites,
such as graphite, diamond, silicon carbide, and alumina, probably have
an interstellar origin.

Detailed chemical and physical studies of primitive chondrites
enables subdivision into a number of discrete classes, the largest of which
are the ordinary chondrites, the H. L and LL chondrites, but especially
significant scientifically are the rare carbonaceous chondrites, some of
which can be up to 20 vol% water.  The classes show subtle but significant
deviations in composition from those of solar abundances.  The existence
of these classes and the physical and chemical trends they represent are
important clues to processes occurring in the early solar system. 
One such process is the separation of silicates and metal.  Another
is volatile loss.  Yet another is associated with the formation of
the chondrules, glassy silicate droplets containing conspicuous crystal
structures.  Early solar system processes also resulted in variations
in elemental abundance and isotopic proportions of oxygen.  It is
not clear what caused these variations in property or how they relate,
but it is clear that they represent fundamental processes in the early
solar system.

A variety of dating techniques have not only shown the great antiquity
of primitive meteorites, but have made it possible to resolve a great many
events, some of them involving small time intervals for events occurring
many years ago, such as the time interval between the end of nucleosynthesis
and meteorite formation.  Other dating techniques have identified
the times of major and lesser break-up events.

Asteroids

The parent bodies of the meteorites are the near-Earth asteroids
that, in turn, originate in the main asteroid belt or, to a lesser extent,
as the nuclei of extinct comets.  The spectra of sunlight reflected
from the asteroids indicates that they are compositionally very diverse,
ranging from carbonaceous material not unlike the carbonaceous chondrites
(although in both the asteroid and meteorite case the spectra are bland
and not particularly diagnostic) through silicate rich material superficially
resembling the ordinary chondrites (but in detail more closely resembling
a variety of rare metal-rich meteorites) to metallic asteroids resembling
iron meteorites (but again the spectra are bland an insensitive to the
features that distinguish the iron meteorite classes).  In a few cases,
it is possible to link meteorite classes with asteroid types by a convincing
match of their spectra, but in most instances the match is imperfect and
assumptions have to made about “space weathering”, alteration of the surface
of the asteroid by processes on the surface of an airless body, in order
to propose a match.  The apparent mismatch between asteroid and meteorite
spectra was for many years referred to as the “S asteroid paradox” but
may be resolved if processes thought to occur on the Moon also occur on
asteroids, notwithstanding differences in impact regime and target composition. 
However, lunar studies demonstrate that the processes are complicated and
in some ways controversial even when ample samples are available.

Thus many new insights have been obtained over the last one-hundred
years or so of primitive body research, but many questions have arisen
and unequivocal answers to some fundament questions – like the origins
of the classes and their characteristic properties -are still lacking. 
Our panel suggests that missions to return samples from near-Earth asteroids
will provide new insights and a new impetus to this area of research.

Why bring back asteroid samples when we have meteorites?

Attempts to interpret the meteorite data and link meteorites with
asteroids are are currently handicapped by (1) an incomplete sampling of
primitive materials, (2) a lack of knowledge about the geological context
of meteorites available for study.  Meteorites are, in effect, cosmic
jetsam biased towards the fragments of bodies that recently broke up and
towards materials strong enough to survive the rigors of reaching Earth,
being studies without any knowledge of their geological context.

(1)  The meteorites coming to earth are not representative
of primitive solar system material, and it is the water-rich, mechanically
weak, most primitive, and scientifically most significant material, that
is underrepresented.  While the Yarkovsky effect ensures that material
reaching the vicinity of the Earth is fairly representative of the small
(~10 cm) objects in the main belt, most meteorites in our terrestrial collections
come from one or two bodies that were broken up by impact relatively recently. 
The existence of this event is demonstrated, and its age apparent, from
preferred values in their cosmic ray exposure ages, in the case of the
H chondrites, and their Ar-Ar ages, in the case of the L chondrites (Fig.
1).  Thus H and L chondrites dominate the terrestrial meteorite flux. 
The more primitive fragile material is does not survive such processes. 
Additionally, meteorites must survive passage through the Earth?s atmosphere,
which means that only tough material reaches the surface of the Earth while
the fragile water-rich and particularly primitive materials are consumed
(Fig. 2).  It seems almost certain that many kinds of currently unknown
primitive materials exist in the asteroid belt.  Perhaps relevant
to this are the measured densities of asteroids which are lower than those
of ordinary chondrites and only the largest of which are comparable with
carbonaceous chondrites.  Whether this reflects abundant volatiles
or an unusual internal texture is unclear, but it does indicate that meteorites
are giving an incomplete story about the nature of primitive material.

(2)  Studying cosmic jetsam means that not only do we not
know what type of asteroid the meteorites are from, we do not know the
geological context from which the samples came.  We do not know whether
the samples are from inside a crater, from the crater rim, from the ejecta
blanket of a crater, from bedrock, from the surface, from depth, from rare
veins of particularly tough material, or from some other undreamed-of geological
feature.  No terrestrial geologist would think of discussing the origin
and history of a rock without knowing its geological context.  Similarly,
such information is critical to unlocking the scientific potential of the
meteorites.  The NEAR-Shoemaker spacecraft presented a whole new world
of structures and features and a number of scenarios in which meteorites
could have formed.  There were craters and ejecta blankets, linear
structures and grooves, boulder strewn fields, regions of uninterupted
regolith and curious flat regions of apparently fine-grained material that
was raised relative to other local features – often, but not always in
crater bottoms – called “ponds”.  A wide range of environments in
which meteorites might have formed.

So what could be learned?

There are a great many examples of how returned samples from known
context might resolve long-standing question in meteorite and asteroid
studies and thus our understanding of conditions and processes in the early
solar system.  Two of the most fundamental questions in meteorite
studies which were alluded to above are (1) how did chondrules form, and
(2) how were the various metal to silicate ratios caused.  These questions
relate to the formation of the chondrite classes in as much as every class
has unique combination of metal-silicate ratio and chondrule and metal
abundances and sizes.   Many researchers have argued that chondrules
are impact melt droplets produced when a meteorite impacted the parent
body, while others have argued that a process in the nebula produced the
chondrules, perhaps lightning, perhaps one of many other proposed mechanisms. 
If it were found that samples rich in crater ejecta were rich in chondrules,
while samples from the interior or inter-crater plains were free of chondrules,
then it would be clear that chondrules were impact melts.  Similarly,
many authors have argued that metal silicate ratios reflect some unknown
process that occurred in the nebula.  However, if metal-silicate ratios
on asteroid surface samples varied in some way, say with depth in the regolith
or distance from major impact sites, then we would conclude that the metal
silicate ratios were caused by processes on the surface of the asteroids. 
These are just two examples of specific questions that would be addressed
by knowing the context of the samples.  There are many more.

Samples from the surface of asteroids will also enable us to characterize
space weathering, just as lunar samples enabled us to understand space
weathering on the Moon.  While one might expect certain similarities,
differences due to the higher impact rate and velocity on asteroids and
differences in target chemistry, especially the presence of volatiles,
are to be expected.  In fact, space weathering effects in samples
returned from asteroids of different classes could be compared to such
effects for lunar samples in an excellent example of comparative planetology. 
A fundamental understanding of space weathering would facilitate the interpretation
of spectra for asteroids not sampled.

The multiplying effect

While the samples returned from near-Earth asteroids will be of great
scientific values in themselves, there will actually be a multiplying effect
since they will form a bridge between rock samples investigated in great
detail in the laboratory and the astronomical objects that have until now
only been observed from great distances through a telescope.  Thus
the data from returned samples will provide new insights into meteorite
formation and history that will improve the interpretation of all meteorite
data, and data from the returned samples will also help in our interpretation
of astronomical spectra for asteroids (Fig. 4).  There will be an
influx of new data and a refreshing reevaluation of ideas on a scale that
has not been seen since the return of lunar samples by the Apollo program.

Perhaps the best reason for returning samples from NEA is that
while we can be confident that there is a good chance the samples will
shed new light on fundamental questions, exploring new terrains always
stirs new insights and new questions that could not have been predicted. 
The return of samples from the Moon resulted in a complete overturn of
ideas about the origin of the Moon and its history.  The existence
of a magma ocean was not predicted and few would have predicted the widespread
acceptance of an impact origin for the Moon prior to the return of Apollo
samples.

So can it be done?

It is often said that sample return from asteroids is “high science,
high risk”.  In other words, the scientific value of returning new
samples of primitive solar system material from known context on known
asteroids is beyond question, but that the mission is a challenge to current
technology.  The panel suggests that while this was true a few years
ago, events of the last year or two have changed this.  The events
are:

(1) The success of the Deep Space 1 mission and the new confidence it
places in Solar Electric Propulsion.  As a result of the end of the
primary mission phase of Deep Space 1, JPL recommended and NASA Headquarters
approved new specifications for the NSTAR SEP thrusters which bring a number
of asteroid missions into the capability of current technology using small
mission capabilities.

(2) The spectacular rate of discovery of near-Earth asteroids. 
Almost 80% of the known asteroids in near-Earth orbits were discovered
in the last two years.  Nearly 1000 are now known.  Among them
are about 30 in orbits that have lower Dv than the Moon.  In a pilot
study for the Hera mission, Leon Gefert has identified over 60 trajectories
that would take a NEAR-type spacecraft powered by SEP thrusters to three
asteroids and return to Earth.

(3) The spectacular success of the NEAR-Shoemaker mission.  In
many respects, NEAR was a dry run for an NEA sample return mission, accomplishing
many crucial operations flawlessly.  These were, going into orbit
around an asteroid, maintaining a stable orbit for a year, maneuvering
repeatedly with high precision while in orbit, and finally landing.

(4) The pending launch of the technology development mission MUSES-C. 
The Japanese MUSES-C mission is not a science driven mission but it will
return a few grams of sample from a NEA.  For a budget far below the
current Discovery cap (about $150k compared with $250k), it will rendezvous
with NEA 1936 SF6, station keep, descend to the surface momentarily, fire
a projectile into the surface, collect the ejecta in a cone that will channel
it into a container inside a sample return capsule, return to Earth for
a recovery in the USAF Utah test range.

The crucial steps in asteroid sample return are (1) getting to
the asteroid, (2) maneuvering in the vicinity of the asteroid, (3) taking
the sample, (4) returning the sample to Earth.  The first is not a
problem given the large numbers of NEA and the availability of SEP. 
We now have considerable experience and confidence in maneuvering in the
vicinity of asteroids through the NEAR experience, and while this was not
exactly the same as hovering to take a sample, work by Scheeres and others
associated with the NEAR mission and Japanese colleagues working on MUSES-C
mission has resulted in algorithms for controlling a spacecraft during
hovering operations.  Sample return from space has been commonplace
since the spy satellites of the early sixties were captured in the air
over the Utah test range.  More recently, sample return procedures
have been developed for Stardust and Genesis missions and workers at NASA
Langley have developed even simpler methods of sample return from deep
space by direct reentry.  The most challenging aspect of near-Earth
asteroid sample return is taking the sample.

There are a number of sample collection techniques with flight
heritage, such as the automated drill cores of the Luna missions and the
trowels of Viking and Surveyor.  For human missions there are a number
of techniques developed by the Apollo mission.  Of course, the MUSES-C
team have already developed a technique for sample collection which is
flight-ready.  In addition to this, the Lockheed Martin Astronautics
company have developed a large collector which relies on a fly wheel and
screw and Honeybee Robotics have developed a collector that uses two counter-rotating
auger bits on the end of a flexible rod that can be withdrawn to haul the
sample into a container mounted on a lazy-susan.  The Honeybee collector
was recently given SBIR II funds and before the end of 2001 will be tested
under microgravity conditions on the NASA?s KC 135 reduced gravity facility
and under vacuum in the new environmental chamber at the Arkansas-Oklahoma
Center for Space and Planetary Sciences.  Sometime in early 2002,
the collector will be tested from a helicopter.  It is expected that
a sample collector will be flight ready within the next few years that
will be considered low-risk.

Any sample return mission must address planetary protection issues. 
Samples falling naturally to Earth will have received sufficient radiation
dose to sterilize them, and this is probably true of samples obtained from
the surface of asteroids, but it will not be true of samples taken from
depth.  Fortunately, NRC panels have considered this matter at great
length and suggested that sample return from any asteroids except p, d,
and f asteroids (the asteroids probably containing free organic compounds)
does not pose a serious threat to Earth.  Nevertheless it will be
necessary to be sensitive to planetary protection issues during the initial
design of any mission returning samples to Earth.

Why sample NEAs, rather than main belt asteroids?

Clearly, the energetics of reaching NEA with spacecraft are less
demanding than main belt asteroids and thus will be accessible to scientific
research sooner than main belt asteroids.  As stated above, some are
easier to get to than the Moon.  Available evidence suggests that
NEA are representative of the main belt, at least the distribution of asteroids
over the spectral classes is the same for NEA as it is for the main belt
so the potential science returns are greater.  NEA have suffered an
event not experienced by the main belt, namely transfer from the main belt
to the near-Earth vicinity, but there is no reason to expect that this
has changed their mineralogical or chemical properties and they are still
pristine material from the earliest days of solar system history.

However, there are actually reasons why we would prefer to explore
NEA before going to the main belt.  They are part of the near-Earth
space environment, and NASA?s current plans for exploring the solar system
with robotic and human missions involve a steady progression outwards,
from low-Earth orbit, to the Moon, to Mars, to main belt asteroids, to
the outer solar system.  NEA exploration fits neatly between the exploration
of the Moon and the exploration of Mars.  Missions to NEA would have
similar durations to missions to Mars, but would considerably less demanding. 
Second, NEA could ultimately provide local and relatively cheap resources
(water, for instance) for the International Space Station or lunar colonies. 
Third, NEAs include potential Earth-impactors and there is widespread interest
in identifying and characterizing asteroids that could potentially impact
Earth.

Why sample return when we have in situ techniques?

The depth and breadth of analysis on Earth will always be many
times greater than will be available from in situ techniques as even the
most cursory glance at the literature will demonstrate.  The recent
fall of a primitive meterorite in Canada was the subject of a report in
Science which included data for 78 elements using 10 techniques – most
of them requiring sophisticated procedures that could only be performed
on Earth – while a few years before the Pathfinder mission to Mars yielded
data from 7 elements using one technique (Tables 1 and 2).  Aside
from the volume of the data, the quality of the data were very different.

Returned samples also have the potential to be archived for future
reference, pending new techniques and new ideas.  A given asteroid
need never be visited again, and this, given the number of asteroids, could
be a significant advantage.

It is sometimes argued that one needs only to collect data of
a quantity and quality needed to address specific questions and that if
in situ data is adequate for the problem then the additional expense of
obtaining better data is not justified.  This is true, and there are
many science questions for which in situ data has been sufficient to make
major advances.  But this is not true of questions relating to the
composition, mineralogy, petrology, and isotopic properties of primitive
solar system materials where the full arsenal of data are required. 
The problems are complex and diverse, and not amenable to a few very simple
measurements made by simple automated instruments.  This has been
demonstrated repeatedly in meteorite studies – in situ techniques could
never have discovered extinct nuclides or complex diversity in elemental
and isotopic properties of chondrules, for instance.  It is also apparent
from the studies of the NEAR spacecraft, that in situ measurements are
not able to answer the fundamental questions, such as the Mg/Si or Fe/Si
ratio of the surface, the presence and type of chondrules, the oxygen content
or the oxygen isotopic ratios.

There is a cultural problem that must be overcome in placing an
appropriate value on asteroid sample return, which is that because sample
return has only recently become viable the planetary science community
has a long history of finding ways to get round this.  There are a
great many techniques for in situ analysis that have been painstakingly
developed over the last twenty years that we would like to see fly. 
This tends to cloud the issue when the difficulties of sample return are
discussed.

KEY SCIENCE QUESTIONS: