From: National Research Council
Posted: Saturday, September 1, 2001
SUBJECT AREA: Primitive bodies
TITLE: Near-Earth Asteroid Sample Return
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).
CURRENT STATE OF KNOWLEDGE:
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.
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:
The NASA already has in place a facility for astromaterials curation at the Johnson Space Center. The facility has over twenty years experience of handling extraterrestrial materials in a way acceptable to the scientific community and society-at-large. They are currently working with teams for the Genesis and Stardust missions, and they are responsible for handling Antarctic meteorites and Apollo samples.
Judging from the cost of the Japanese MUSES-C mission, asteroid sample return from multiple asteroids in a single mission could be accomplished within the Discovery price cap, especially if there is there is a modest increase in the cap or an augmentation to the funds by overseas partners.
The Panel believes that the near-Earth asteroid sample return should be considered primarily as a science-driven project suitable for the Discovery program. However, it could also be considered as a technology development mission and submitted to the New Millenium Program or as an impact mitigation mission for submittal to the United States Air Force who have been given the mandate by congress to consider impact mitigation. NASA has the mandate to characterize potential earth impactors, but does not have a specific mission program for that purpose.
The community responsible for extraterrestrial material (meteorites, cosmic dust, Apollo samples) research is funded primarily by NASA?s Cosmochemistry program. Another argument for near-Earth asteroid sample return is that a large, well-coordinated and well-funded community already exists for the analysis of the returned samples and existing resources would be adequate for their characterization. Lagging, however, is major equipment, and a sample return mission should assign resources for updating the aging instrumentation in the field.
Priority relative to other activites
The panel argues that the return of samples from near-Earth asteroids is a mission of highest priority relative to other solar system activities currently under study as part of the NRC decadal review. The scientific value of studies of these small solar system objects has been recognized by NASA in the "Space Science Enterprise Strategic Plan". Seven of the 11 goals laid out in the strategic plane can be uniquely addressed by sample return. These are:
a) Primitive asteroids contain evidence for processes occurring during
b) The pre-solar grains they contain help us understand stellar evolution and the relationship between stars and planet formation.
c) Organic compounds they contain can shed light on the origin of molecules necessary for life.
d) Chemical processes that preceded life on Earth can be detected from chemical trends in the samples that can help understand possibilities of life on other planets.
e) Solar wind and solar energetic particles trapped in these surface materials will contain a record of solar activity for bodies in known orbits.
f) Small body sample return (and the data from the encounters) will enable us to design devices to deflect potentially hazardous objects and predict the effects should they reach Earth's atmosphere.
g) Robotic missions to NEA will be pathfinders for human missions that might use asteroid resources to facilitate human exploration and the development of space.
Building Blocks and Our Chemical
Primitive Body Explorers*
Small-Body Sample Return
Giant Planet Deep Probes
Pre-Biotic Chemistry in the
Outer Solar System
Europa Ocean Explorer
Titan Biologic Explorer
Formation and Dynamics of
Lunar Giant Basin Sample Return
Io Volcano Observer
Mars Surface Network
Venus Surface Mission
Evolution of Earth-Like Environments
Mars Sample Returns
Mars Water and Mineralogy Mapper
Mars Mobile Science Labs
Venus Geoscience Aerobots
Mars Geoscience Aerobots
Venus Atmospheric Probes
Venus Surface Science Labs
The Solar System as a Large-Scale
Saturn Ring Observer
Jupiter Polar Orbiter
Outer Planet Multiprobes
Mercury Magnetospheric Multi-Sats
Comet Coma/Tail Multi-Sats
"Portrait" missions are shown in
* Multi-flyby "Visitors", Large Asteroid Orbiter; "New Comet" Encounter
Missions to recover samples from near-Earth asteroids, the only feasible targets for macroscopic sample return with currently technology in the small mission budgets, addresses some of the most fundamental questions in planetary science - questions relating to the origin of the solar system and all materials in it - which underpin many of the activities currently under consideration.
MAINTAINING CAPABILITIES FOR THE FUTURE:
Samples need to be subjected to sufficient preliminary examination to ensure that information reaches appropriate researchers and that appropriate research is done.
Samples need to be need to be distributed to the scientific community in a way that ensures maximum scientific return from the samples, in other words while every care should be taken to ensure that the samples are not used irresponsibly, an effort should be made to see that all scientists with the appropriate credentials receive samples.
A significant fraction of the material (one-quarter?) should be placed in long-term storage pending further developments and instrumentation.
Human Space Activities
Human flights to Mars are an ultimate aim of the space program, yet costs and complexity are considerable. Mission planners at the Johnson Space Center have been investigating human missions to NEAs as a useful way to test Mars-bound technologies. The missions would be of shorter duration (one year vs. three years), simpler (no gravity well or atmosphere at the target), launch windows are numerous, and lower energy would be required than for a missions to Mars. The International Space Station might make such missions even more economic
Archive and Access Issues
See above about archiving samples for future use.
The EPO possibilities of NEA missions are considerable because of the relevance to the impact hazard and popularization of the topic by its relevance to the termination of the dinosaurs and several recent movies. On the strategic and political front there is considerable interest in impact prediction and mitigation and society has every right to expect that the scientific community will make a significant contribution to these efforts. We can do so, while still addressing the demands of fundamental planetary science, by near-Earth asteroid sample return.
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