Oct 21, 1994
Donald E. Brownlee, Principal Investigator, University of Washington,
Peter Tsou, Benton C. Clark, Paul N. Swanson, and Joseph Vellinga.
|( Wild-2, Michael Brown, 4-1-97)|
Comet P/Wild-2 is a 'fresh' comet which has been recently (1974) deflected by
the gravitational action of Jupiter from an earlier orbit lying much further
out in the solar system. Samples from Wild-2 thus, offer an exciting glimpse
of the best preserved fundamental building blocks out of which our solar
system formed. Sample collection will make use of exciting new aerogel
material, the lowest density solid material in the world. Fortuitously, a rare
but opportune orbital design using two Earth gravity assists allows STARDUST
to capture cometary dust intact and parent volatiles as well, at the incredibly
low relative speed of 6.1 km/s. With the aid of onboard optical navigation,
the flyby can take place at an encounter distance as close
as 50 km from the comet's nucleus, permitting the capture of the freshest
samples from within the coma parent molecule zone. This rare trajectory
imposes a very low post-launch fuel requirement and enables launch by a
Med-Lite launch vehicle. As an exciting bonus, STARDUST will also collect
recently discovered by Ulysses and confirmed by Galileo. In additon, a particle
impact mass spectrometer provided by the Germany's DLR will obtain in-flight
data on the compositon of both cometary and interstellar dust, especially the
very fine particles. Excellent images
of the comet's nucleus will be taken by the optical navigation camera, another
bonus. The spacecraft dust shield will also provide coma dust spatial and
temporal distribution, and the X-band transponder may provide an estimate of
the mass of the comet.
1.0 Scientific Goals and Objectives
The primary goal of the STARDUST mission is to collect cometary coma samples,
plus a bonus of interstellar dust samples, and return them to Earth. The
returned samples will be investigated by a global community of researchers at
laboratories capable of analyzing extraterrestrial materials at the levels of
spatial scale and accuracy where the most critical information in these
primitive materials is retained. On a single mission, STARDUST will collect
both ancient pre-solar interstellar grains and nebular condensates that were
incorporated into comets at the birth of the solar system and as a target of
opportunity will capture contemporary particles presently entering the solar
system from the interstellar medium (ISM). The spacecraft, its mission
trajectory and collectors are all designed for the comet encounter but with
little extra effort the mission will also collect contemporary interstellar
The comet samples will be collected during a 6.1 km/s flyby of Comet Wild 2. At this extraordinarily low flyby speed, coma dust in the 1 to 100 micron size range will be captured by impact into ultra-low density aerogel and similar microporous materials. Particle collection at this speed has been extensively demonstrated in laboratory simulations and Shuttle flights [Tsou 1993] and we have shown that the comet dust collection can be done with acceptable levels of sample alteration. The most unique and probably the most important result of the study of the returned samples will be detailed analyses of the elemental, isotopic, mineralogical, chemical, and biogenic properties of cometary matter at the characteristic micron size scale of interstellar grains (IS) and initial nebular condensates. The same collection medium will be used for the collection of coma volatiles. In addition to collection, comet dust will also be studied in real time by a time-of-flight mass spectrometer derived from the PIA instrument carried to comet Halley on the Giotto mission. This instrument will provide data on the organic particle materials that may not survive aerogel capture, and it will provide an invaluable data set that can be used to evaluate the diversity among comets by comparison with Halley dust data recorded with the same technique.
The exploratory aspect of the STARDUST mission is the collection of contemporary IS grains. During selected portions of its cruise phase, STARDUST will use the opposite sides of its comet dust collection modules to collect fresh interstellar grains. Ulysses and Galileo have recently detected a moderately high flux of fresh interstellar grains entering the solar system from the same direction as IS gas, the direction to the constellation Scorpio [Grun et al. 1994]. STARDUST will collect particles from this directional stream. The exploratory aspect of capturing interstellar dust is that the particles are smaller, they impact at higher velocity, and properties such as size distribution, collimation and dynamics in the inner solar system are just now being determined. It is clear, however, that these particles do exist in collectable quantities and the STARDUST mission provides an exciting and unique opportunity to collect samples of materials formed outside our solar system. Interstellar gas samples should be physisorbed in the aerogel capture medium for direct measurement of the isotopic composition of elements such as He and Ne.
Laboratory investigation of the returned samples with instruments such as electron microscopes, ion microprobes, atomic force microscopes, synchrotron microprobes, and laser probe mass spectrometers will provide an extraordinary opportunity to examine cometary matter and interstellar grains at the highest possible level of detail. Remarkable advances in microanalytical instrumentation now provide unprecedented capabilities for analysis on the micron and submicron level, even extending to atomic scale for imaging. These properties will provide direct information on the nature of the interstellar grains that constitute most of the solid matter in the Galaxy, and they will provide a highly intimate view of both pre-solar dust and nebular condensates contained in comets. The comparison of these materials with primitive meteorites and collected interplanetary dust samples will provide the basis for examining the pre-solar solids that were involved in solar system formation, the solids that existed in the outer regions of the nebula where comets formed as well as solids in the inner regions of the nebula where asteroids formed. These data will provide fundamental insight into the materials, processes, and environments that existed during the origin and early evolution of the solar system over a wide range of distance from its center.
Interstellar dust was the initial solid building material used in formation of the solar system and nearly all the atoms heavier than oxygen now in the Sun and planets were in interstellar grains just before the formation of the solar nebula. Typical interstellar grains are micron-size particles that initially formed by condensation around other stars and were later influenced by the various interstellar environments in the Galaxy [Mathis 1993]. The particles are actually samples of other stars and they contain isotopic records of nucleosynthesis in these stars as well as chemical, morphological, and mineralogical records of the environments and processes that influence the formation and evolution of solid grains in the Galaxy. This information is retained on submicron spatial scales can only be adequately studied in terrestrial laboratories where sophisticated analytical instrumentation provides the ultimate precision, sensitivity, and adaptability without serious constraints on mass, power, cost, or high reliability.
At present, interstellar grains are studied mainly by astronomical techniques
that are sensitive only to general physical properties such as size and shape
and provide little information on real physical properties and the records of
formation and evolution that they pertain to. The recent discovery and study
of rare interstellar grains preserved in meteorites [Anders and Zinner 1990
1994] has shown that IS grains do preserve excellent records about the nature
of their parent stars, including details of the complex nuclear reaction
processes that occur within them. The grains that have been extracted from
meteorites are chemically robust phases, such as SiC, diamond, and graphite,
that survive both solar system processing and the chemical processes used to
extract them from the bulk of meteoritic material of solar system origin. The
IS grains that have been identified in meteorites are predominantly grains that
formed in gas outflows from carbon rich stars (C/O>1) such as red giants and
ABG stars and the more typical IS grains from oxygen rich stars have not been
found. In STARDUST we expect to collect grains produced by those types of
stars that are major sources of interstellar dust.
1.1.1 New Developments
This project is made possible by four recent developments and innovations. The
first is the development of an intact capture technology that makes
the effective capture of high velocity particles in space [Tsou 1984]. This
capture is accomplished by impact into aerogel and other low density,
microporous materials. These exotic capture materials have densities as low
as 0.002 g/cm3, and have been proven effective for intact
capture of particles
with speeds even higher than 6 km/s [Tsou 1990]. The second
key development was the discovery of a low energy sample return
trajectory that enables both
the slow, 6.1 km/s, flyby of an active short-period comet and moderately low
encounter speed with the interstellar dust streaming into the solar system.
The interstellar collection occurs when the spacecraft trajectory closely
parallels that of the IS stream vector, thus minimizing the relative impact
velocity. A bonus with this trajectory, is that the target comet was only
recently captured into its present orbit by the gravitational action of
Jupiter, which means it may be a relatively pristine object dating from the
earliest time of solar system history. Thirdly, the discovery by the
and Galileo spacecraft, of an appreciable flux (15/m2-day)
large interstellar grains entering the solar system with the same speed and
direction as the neutral interstellar helium [Grun 1993 and 1994]. This
contribution actually dominates the interplanetary flux beyond 3 AU for
micron-size particles. The fourth development has been the continual
improvement of analytical instrumentation and techniques that now provide good
quality isotopic, mineralogical, elemental and chemical assessment capabilities
at the micron level. These remarkable advances have led to the identification
and analysis of individual SiC and graphite interstellar grains that are
present in trace amounts in primitive meteorites.
STARDUST is primarily a comet coma sample return mission plus a bonus of IS
grains, returning them to terrestrial laboratories. A time-of-flight mass
spectrometer derived from the PIA and PUMA instruments flown on Giotto and Vega
Halley missions will also be included on the payload to provide both
complementary and corroborative data to the sample return results. For the
comet Wild 2 encounter, the objective is to recover more than one thousand
particles larger than 15 microns in diameter as well as volatile molecules
on the same capture medium.
The sample return objective for fresh
interstellar grains is to collect over 100 particles in the 0.1 micron to 1
micron size range. They will be collected in a manner designed to preserve,
at minimum, the elemental and isotopic composition for major elements in
individual submicron particles. We will use trajectory information from impact
track angles to distinguish interstellar grain impacts from those of comet or
asteroid dust that will also impact the collection media. An important
objective with both the cometary and IS collections is to gather and return
samples with minimal modification from their original state. Particles
traveling at km/s speeds are typically decelerated to rest over distance scales
of millimeters to a centimeter. For a comet encounter at 6 km/s, we are
confident that the collection will occur with little modification to solid
components. This impact speed has been exceeded by micron to centimeter
projectiles launched by the NASA light gas gun facilities at the Johnson Space
Center and Ames Research Center. Laboratory impact tests at these facilities,
as well as actual meteoroid captures from Earth orbit, have demonstrated that
solid projectiles of a few microns in size and larger can not only be collected
in low density aerogel, but also extracted for analysis [Brownlee et al.
1994]. We will minimize the damage to these IS particles by using the lowest
density aerogel that can be fabricated and survive launch and recovery
1.3 Value to Science
The wealth of data which will result from the STARDUST mission is due to its
multi-faceted nature. We will collect interplanetary dust, but we have
especially optimized our mission implementation to collect extra-solar system
grains, i.e. the interstellar dust. Cometary samples are of intrinsic
interest for the entire comet science community, but hold considerable interest
for the exobiologists as well.
1.3.1 Cometary Dust
Comets presumably formed in the outer solar nebula, where the temperature
remained low enough that many intact interstellar grains should have survived
nebular processing [Greenberg and Hage 1990]. Yet, infrared spectra of comets
differ from corresponding spectra of IS dust, both in the silicate and
refractory organic components. The cometary 10 micron silicate feature shows
fine structure indicating that it is more crystalline than interstellar dust.
The dust analyzer on the Halley probes detected silicates and carbon-rich CHON
particles [Solc et al. 1987], indicative of a refractory organic component.
At present, we do not know what fraction of cometary dust is presolar, and what
fraction was formed in the solar nebula and transported to the region of comet
formation. It is also not known how the nebular accretion of IS grains into
larger aggregates may have changed their observable properties.
Both comets and asteroids are sources of interplanetary dust particles (IDPs). The majority of IDPs collected in the stratosphere are chondritic aggregates. Among these, the pyroxene-rich class is thought to be of cometary origin, based on their porous structure, high carbon content, and high atmospheric entry velocities determined by He retention and other thermal indicators. It is very important to verify this identification with a directly collected sample of cometary dust. The most striking feature of these porous aggregate IDPs is that they are unequilibrated mixtures of high and low temperature condensates, even on a micron scale. Does this reflect efficient mixing of small grains formed in different parts of the solar nebula and subsequently welded in to physically distinct units before the porous aggregates formed? Or, are these submicron units truly IS grains? Bradley (1994) argues that the major structural submicron units of the pyroxene IDPs show evidence of heavy radiation processing that most likely occurred in the interstellar medium (ISM). The sub-units called GEMS (glass with imbedded FeNi metal and sulfides) are an exotic material composed of silicate glass with large numbers of imbedded 10-nm metallic and sulfide grains. If GEMS are preserved IS silicate grains, they will radically alter our picture of neatly separated components of IS dust. In these particles, carbon occurs as discrete phases and not mantles or coatings on silicates. The detection of the highly distinctive GEMS structure and composition in the STARDUST collection would prove that these are IS silicate grains.
From comet samples that can be captured intact, it should be possible to determine the following:
Interstellar dust forms by condensation in circumstellar regions around evolved stars, including red giants, carbon stars, AGB stars, novae and supernovae. The process gives rise to silicate grains when there is more oxygen than carbon in the star, and carbonaceous grains when the carbon content exceeds that of oxygen. Pristine grains will retain the isotopic signatures of their formation environment and such signatures have been detected as rare components of primitive meteorites. Interstellar dust accumulates volatiles in molecular clouds. Grains are sputtered in intercloud regions, they experience shocks, and they undergo cycles of destruction and re-formation in the interstellar medium. When the grains are subsequently exposed to UV and cosmic rays in the interstellar medium (ISM), processing may convert the icy mantles to refractory organic material.
Dust is the major form incorporating heavy elements in the Galaxy that are not inside stars. Due to its high area to mass ratio, dust plays important roles in interstellar processes. One of its most important properties is its light absorbtion that permits the formation of cold dense clouds, where molecular species can both form and be shielded from the otherwise destructive effects of ultraviolet radiation. The cooling effect of dust in some clouds assists in their collapse to form new generations of stars and planetary systems. Interstellar grains are the major repository of condensible elements in the interstellar medium and dust influences nearly all types of astronomical observations including obscuration of visible light from most of the stars in our Galaxy. While astronomical observations of extinction, polarization, and limited spectral features provide clues to the nature of interstellar grains, such observations are not sufficiently definitive to confidently match the particles with theoretical models. Basic information, such as the abundance of SiC (from carbon stars), the abundance of graphite, grain morphology, silicate mineralogy, the role of radiation processing, grain ages, and the association of silicates and carbonaceous matter, is highly uncertain. Collection of even a few degraded particles would provide a unique and historic opportunity to directly examine solid matter that formed outside the solar system. This information would provide powerful constraints on grain models and provide insight in the relationship of presolar and meteoritic materials.
It will be possible to determine:
Comets, being rich in water and other volatiles, have been postulated to be transporters of volatile and biogenic elements to the early Earth. Clearly, the study of cometary material is essential for our understanding of the formation of the solar system, and most importantly to exobiology, the interstellar contribution of pristine, early-formed organic matter from several different environmental regions. In addition to astronomical observations of silicate dust in interstellar clouds, some 60 compounds have been identified in interstellar clouds, three-fourths of which are organic and the remainder, inorganic. The first five interstellar molecules detected by microwave spectrometry were NH3, H2O, CH2O, HCN, and HC3CN (cyanoacetylene). There is compelling evidence that four of these occur in comets, and the fifth (HC3CN) may be present as well. The volatiles and silicates inferred to be in comets by astronomical observations are also found in interstellar clouds. How the biogenic elements entered the solar system, were transformed by processes operating therein, became distributed among planetary bodies, and what molecular and mineralic forms they took during this history are questions of major importance for exobiology. Comparison of the compositions of the volatiles contained within cometary material with those found in carbonaceous meteorites and interplanetary dust will provide a basis for determining what commonalties in source regions can be attributed to the materials in these putatively related objects. The analysis of minerals like carbonates, clays, and sulfates in comet dust would also be significant for the history of interaction between water and minerals in the early solar system [Bunch and Chang 1980],
Finally, the iridium anomaly in rocks at the Cretaceous-Tertiary boundary
coupled with other evidence has raised the probability that impacts of
asteroid-sized bodies with the Earth have greatly influenced the course of
biological evolution. If true, biological evolution, like prebiotic chemical
evolution, is connected in a fundamental way with the dynamical evolution of
small bodies in the solar system. Although the chance of finding a unique
elemental signature in captured cometary coma material may be slight, such a
discovery would be of enormous value in distinguishing between an asteroidal
and a cometary impactor for this highly significant anomaly.
1.4 History of the Investigation
The first major JPL study of a cometary mission was documented in 1959 [JPL
1959]. At least eight studies of cometary missions during the 1970s exist,
with comets Encke and d'Arrest the focus. This culminated in a Halley Flyby
with Probe and Tempel 2 Rendezvous mission [Atkins 1979]. The development of
intact capture technology was triggered by JPL's Halley Sample Return Mission
[Tsou 1983]. The first comet coma sample return mission with intact capture
was jointly proposed with Goddard Space Flight Center (GSFC) as a NASA mission
for the Planetary Observer Program and proposed direct re-entry via a
Discoverer Capsule [Tsou et al. 1983]. Using the spare Giotto spacecraft, JPL
proposed Giotto II jointly with the European Space Agency (ESA). In the fall of
1988, a Cosmic Dust Intact Capture Explorer mission was proposed to the
Small-Class Explorer Program to capture cosmic dust in Earth orbit for two
years [Brownlee 1988]. Since 1987, low-cost flyby sample return missions to
comets, SOCCER, have been jointly studied in U.S. and Japan [Uesugi et.
al. 1993]. A joint proposal between JPL and ISAS on a Flyby Sample Return via
SOCCER was presented at the 1992 Discovery Mission Workshop. In June 1994,
ISAS made a final decision for an asteroid mission rather than the SOCCER
mission; the opportunity for proposing a timely comet coma sample return mission
gave birth to STARDUST. The first proposed IS dust mission was for the Space
Station Attached Payload Program, an Interstellar Dust Intact Capture
Experiment [Brownlee 1988]. With the startling discovery of IS dust by
Ulysses, a target
of opportunity presented itself to use the same technology as for cometary dust
collection and on the way to and from a comet sample return, incorporate
interstellar dust capture into STARDUST.
1.5 Need for the Investigation
The data from STARDUST will provide the opportunity for significant scientific
breakthroughs in areas of key interest to both astrophysics, planetary science
and exobilogy. The mission will provide much needed direct information on the
solid particles that permeate the Galaxy and the typical particles and
non-volatile organics that dominated the outer regions of the solar nebula at
the earliest stages of its evolution. The single most important aspect of
STARDUST is that it will return comet samples and interstellar grains to the
laboratory, where they can be studied at the highest possible level of detail
and sensitivity. Sample return of primitive solar system materials from
comets has long been recognized as a scientific contribution of extraordinary
importance. This has been emphasized in many studies. The much quoted 1980
National Academy of Sciences COMPLEX report Strategy of the Exploration of
Primitive Solar System Bodies gave the highest priority to determination of the
composition and physical state of a cometary nucleus. The NASA Solar System
Exploration Committee, in its proposed 1986 implementation (Planetary
Exploration Through The Year 2000) of the COMPLEX strategy stated that no
mission short of a sample return could provide the range of detailed analyses
needed for this COMPLEX goal. The importance of comet sample return is
emphasized in the soon to be finished report of NASA's Small Bodies Science
Due to budget constraints and other related problems, NASA has never launched any comet mission, but ESA approved a sample return mission as one of the four major cornerstone missions in its Horizon 2000 program. Again due to funding problems, the lack of a strong international partner, and the cancellation of NASA's CRAF, ESA descoped its Rosetta sample return mission to a CRAF-like rendezvous mission without sample return. Although the potential scientific return would be unprecedented, it is clear that due to complexity and cost the conventional mission design where a subsurface cometary sample is obtained by drilling and then returned to Earth under cryogenic preservation will not be affordable in the foreseeable future. The approach used by STARDUST is vastly less complicated than the original Rosetta design. It can be done at an order of magnitude less cost and will have a high benefit/cost ratio. This type of mission can achieve many of the major objectives set for cometary solids, as well as provide some information on volatiles. The sample size is small relative to the kilogram mass to be returned by a Rosetta-like mission, but we do not view this as a significant problem, at least with regard to the study of presolar grains. Abundant evidence indicates that both cometary and IS solid samples are very fine-grained with typical components being micron and sub-micron in size. Because we are focused on these grains we do not require a large sample mass. Even if a ton of sample were returned, the main information in the solids would still be recorded at the micron level and the analyses would still be done a single grain at a time. A single 100 micron cometary particle could be an aggregate composed of millions of individual IS grains. The key information in these samples is retained at the micron level, and even aggregates of 10 microns in size are considered giant samples.
The value of having actual bona-fide comet samples and IS grains in hand cannot be overestimated. Even though the samples will be small and partly eroded, they will open a significant new window of information on galactic and nebular processes, materials and environments. Having actual samples in hand provides many unique advantages. Just as the return of lunar samples by Apollo totally revolutionized our understanding of the Moon, its properties, processes, origin and evolution, we expect that this sample return will also have a profound impact on our knowledge of comets and stars. Most of our existing information on these materials was obtained by either indirect methods, or by methods that are not sensitive to the most important records contained in them. Polarization, wavelength dependent adsorption and scattering, as well as spectral features such as the shape of the 10 micron silicate feature and the 0.22 micron UV feature, provide valuable insight into physical properties of IS grains such as shape, size distribution and composition, but this information is generally not specific, and usually not uniquely related to the most important physical properties such as isotopic composition and chemical composition. The actual compositional and isotopic differences between particles from AGB stars, M dwarfs and toner from a Xerox machine are vast, although as small dark rounded particles they may be indistinguishable as observed by astronomical techniques. Models of interstellar grains are constructed to reproduce astronomically observable attributes, but just as was the case for lunar surface models, these may be far from reality. STARDUST will provide ground truth to test models and reveal actual properties, as well as provide an opportunity for considerable synergy between directly studied sample properties and the many different ways that IS grains can be studied by astronomical observations.
The most valuable aspect of STARDUST is that it provides directly measured data on comet and IS samples at the sub-micron level. The measurement of elemental, mineralogical, and isotopic composition, as well as morphological and molecular information at this size scale can only adequately be done with the high level of adaptability, precision and control achievable in the laboratory. All of the work done on IS grains by astronomical methods, and nearly all of the data that can be obtained on a CRAF-like rendezvous mission, is sensitive only to bulk (larger than micron) averaged properties of assemblages of large numbers of individual micron components. Since the critical data is really at the micron level, this can only be assessed by sample return. The recent work on IS grains in meteorites has shown decisively that the most important information, namely the extreme isotopic anomalies that relate to nuclear processes are to be found in individual micron- and smaller-sized grains. Isotopic effects in these micron grains are sometimes one million times larger than nucleosynthesis-related anomalies in millimeter samples. Larger samples mix the original smaller components and dilute of the chemical, mineralogical and isotopic signatures of stars and environments that formed and influenced them. IS grains are small and a full deciphering of the records they can contain requires complex use of many analysis techniques on single grains. STARDUST provides a uniquely affordable capability of sampling comets and IS dust, the initial building blocks of planets both in our the solar system and those that exist around other stars.
The importance of sample return missions compared to having to space qualify new science instruments is due to several factors which enormously increase the science value of the STARDUST mission.
Periodic Comet Wild 2 is proposed to be the cometary target of this mission.
The reasons for this comets selection are, in part, the fact that it has only
recently been deflected by Jupiter from a distant orbit into its current orbit
of considerably smaller perihelion distance and, in part, because a remarkably
slow flyby is feasible. The drastic modification of the orbit was caused
by a very close approach to Jupiter, to within 0.006 AU, in September 1974
[Sekanina and Yeomans 1985]. In its pre-1974 orbit, the comet's heliocentric
distance varied between 5.0 AU at perihelion and 24.7 at aphelion, apparently
with no major transformation involved during the past 300 years. The
implications of this orbital evolution are that the comet had been stored in a
fairly cold environment prior to 1974 and, in that sense, it is more pristine
than most short-period comets. Although only a very limited amount of data is
available on its dust-to-gas mass production ratio during the three apparitions
since the orbital transformation, periodic comets freshly perturbed in from
distant orbits are initially relatively dusty, their activity apparently
diminishing with time. This general evolution seems to be confirmed by Wild 2,
judging from an estimated fading of about 1 magnitude between the light curves
in 1978 and 1990. Among
the more interesting possibilities is that only from 1% to 5% of the surface of
the nucleus may be active. With our relatively slow flyby, the existence and
activity of jets should be well observed.
3.0 Measurement Objectives
3.1 Collection of Dust Samples / Aerogel
On this mission, both comet coma samples and the contemporary IS grains must be
captured at high velocity with minimal heating and other effects of physical
alteration. We will use new intact capture technology that has been developed
over the past decade to support flyby comet sample return missions [Tsou 1984]
of the type proposed here. With this technology, hypervelocity particles are
captured by impact into underdense, microporous media such as aerogel
1994] and special polymers [Tsou 1991]. These are not conventional foams, but
rather special porous materials that have extreme microporosity at the micron
scale. Aerogel is composed of individual features only a few nanometers in
size, linked in a highly porous dendritic-like structure. This exotic
material has many unusual properties, such as uniquely low thermal
conductivity, refractive index, and sound speed, in addition to its exceptional
ability to capture hypervelocity dust. Aerogel is made by high temperature and
pressure critical point drying of a gel composed of colloidal silica
structural units filled with solvents. Over the past three years,
been made and flight qualified at the Jet Propulsion Laboratory. We intend to
use the JPL facility because it allows us to have full control over the media
properties and purity. We also intend to include other aerogel-like materials
that are optimized for analysis of specific material properties. For
volatiles collection each collector medium will be dopped with selected
absorbents. Silica aerogel produced at JPL is a water clear,
high purity silica glass-like material that can be made with bulk density
approaching the density of air. It is strong and easily survives launch and
space environments. JPL aerogel capture experiments have flown [Tsou 1993] and
been recovered on Shuttle flights, Spacehab II and Eureca.
(See also Instrumentation: Sample Collection below.
For additional technical information about aerogel see,
When hypervelocity particles are captured in aerogel they produce narrow cone-shaped tracks, that are hollow and can easily be seen in the highly transparent aerogel by using a stereo microscope. The cone is largest at the point of entry, and the particle is collected intact at the point of the cone. This provides a directionality detector and is the basis of our approach of using single slabs of aerogel to collect both cometary and interstellar dust, and being able to differentiate between them because the A side of the collector is exposed in the comet dust impact direction and the B side is positioned toward the interstellar dust stream.
Nominally, the track length is comparable to the column length of aerogel that has the same mass per length as the particle. A 10 micron-size particle will produce a 1 mm track in 0.05 g/cc aerogel when impinging at 6 km/s. With proper illumination, the track of a 10 micron-size particle can be seen with the naked eye. The captured particle is seen optically just beyond the tip of the cone, and it can be recovered by a variety of techniques, ranging from extraction with a needle, to microtoming, and focused ion beam etching. Recovered samples are then treated by sequential analysis techniques that have been developed for the analysis of small meteoritic samples and IDPs.
A critical issue with STARDUST is how well the physical state of samples can be preserved during the capture process. We are certain that solid materials from the comet flyby can be recovered in excellent condition because experiments at this speed and with this particle type have been successfully simulated in the laboratory, and actual samples have been collected in space. The state of captured contemporary interstellar dust is less secure because of the small particle size, higher impact velocity, and unknown material properties. The capture process has been theoretically modeled and there is a growing amount of experience with capture in aerogel and polymer foams from both laboratory tests done with dust accelerators, and with capture of actual micro-meteoroids in space. Using light-gas guns, most laboratory simulation efforts are limited to speeds of 7 km/s or less, and to non-porous projectile materials strong enough to survive launch. Such materials include solid silicates, sulfides, metals, glasses, and composites made of various materials in a binder such as epoxy. These tests have proven that the most common solid mineral grains in meteorites and IP particles can be captured in excellent condition at the 6.1 km/s relative impact speed associated with the comet Wild 2 encounter [Tsou 1990, Zolensky 1994].
One of the processes that enhances the survival of the projectile is the formation of a layer of compressed aerogel on the particles leading edge. This is layer serves as a buffer to insulate the projectile from direct impact with the aerogels solid nanometer-size structural elements. This layer can surround the particle, as evident for the microsphere shown here. Soda lime glass spheres have been captured without melting or fracturing, and a variety of minerals including olivine, pyroxene, troilite, and feldspar have also been collected without melting. Some of the most temperature sensitive tests have demonstrated that lattice spacings in hydrated silicates (serpentine) and even cosmic ray tracks in lunar grain projectiles survive 6 km/s collection in 0.05 g/cc aerogel [Tsou 1994]. Both of these properties are usually annealed out of silicates when they are heated above 600 C for a few seconds. The heating time-scale for capture of 10 micron-size particles into aerogel is less than a microsecond, and it is possible that the peak temperature produced was higher, but the important result is that these temperature sensitive properties survive, including retention of carbon, He and solar flare tracks. Most of the properties of silicate minerals survive brief heating to 600 C and we have full faith that the returned comet sample will retain most of the original properties contained in solid particles. Nearly all of the 10 micron-size IDP collected in the stratosphere were heated to 600 C, or above [Love and Brownlee 1994], and yet they retain most of their original physical properties. While we are confident that solid mineral grains can be recovered in excellent shape the situation is less certain for materials that are micro-porous. These materials may extensively fragment during impact and become dispersed in the capture media. This process cannot be fully simulated in the laboratory, both because weak projectiles do not survive launch, and because the physical properties of cometary solids in the 1 micron to 100 micron size range are unknown.
The best estimate for the nature of typical cometary material is probably that
of interplanetary dust samples recovered from the stratosphere.
While typical IDPs are actually rather solid, some
are porous aggregates of micron- and smaller-size grains with bulk porosity as
high as 50%. These particles probably will fragment during capture at 6 km/s
and may not survive as well as the solid particles used in laboratory
simulations. However, even the most porous IDP s contain solid single mineral
grains ranging in size of up to tens of microns, and these should behave just
as in the simulations. The fine-grained porous fractions may compact and
fragment, but we expect that they will preserve their structural properties at
the individual component level. Fragmentation and mixing with the capture
medium complicates sample extraction since it disperses material over a larger
volume, however individual component properties should survive in analyzable
3.2 Collection of Volatile Samples
Organic components are probably the most sensitive phases of interest in these
collections. Laboratory tests at Ames, conducted at speeds of 6 km/s, have
demonstrated the survival of polycyclic aromatic hydrocarbons (PAHs) and some
amino acids, although certainly many organic compounds will be modified or lost
during the collection process. Because of the importance of organics and the
strong interest in them by the exobiology community, we will make the strongest
possible effort to preserve organic components in analyzable form. The
time-of-flight analyzer, provided by the German space agency, to conduct
analysis of organics
during the mission will provide compositional information on particles either
too small or volatile for effective capture and analysis in aerogel. This
data will complement the collected dust sample analysis and provide data that
can be directly compared with the Halley data.
To be analyzed, dust samples must usually be removed from their collection
substrates. This is not always the case, however, and some studies can be
done while the projectiles are still at the end of their tracks. Synchrotron
x-ray fluorescence, to determine elemental composition, is an example of a
technique that may be possible while the sample is still embedded in the
collection substrate. Most techniques will require at least partial sample
extraction. For meteoroid samples that have already been collected in aerogel
this was done by cutting into the aerogel with either dental drills or scalpel
blades to isolate the region holding the sample and then cutting down to the
actual sample by either microtoming or mechanical grinding. Usually this is
done after impregnating the aerogel with low viscosity epoxy. For studies
where epoxy interferes, other techniques can be used, including direct
4.0 Anticipated Data Return
4.1 Dust Analysis
Following the Apollo example, the returned dust samples will be evaluated by a
Preliminary Examination Team and then be made available to qualified
investigators around the world. We can currently envision a general analysis
scheme and predict the general types of science return, but with continual
advances in analytical techniques, and instrumentation, we cannot fully predict
what the ultimate science return will be. Sample analysis in the laboratory is
highly adaptive, and iterative, and the ultimate returns are not fully
predictable. With the current analytical state-of-the-art, we can expect the
The min/pet studies should provide information about the origin and evolution of IS and cometary samples and their relationships or similarities to meteorites and IDPs. Some of these results could be obtained very quickly. If the comet samples are preserved in even moderate condition it should be possible in a few hours to tell which classes of IDPs are derived from comets and which ones come from asteroids. For example a major class of IDPs and all of the carbon rich chondrites contain hydrated silicates. Some believe that these phases formed inside asteroids by the interaction of silicates and water (melted ice) but is also possible that some formed by low temperature gas-grain reactions in the nebula. If the cometary dust does not contain hydrated phases, this is strong evidence for the hypothesis that primary nebular silicates were anhydrous and that hydration was largely confined to secondary processes.
It is proposed that the Voyger filter wheel with 8 positions carry three gas filters for CN, C2, and 6300 O (to monitor water distribution), three moderately wide dust filters, a polarizing filter, and a clear filter. The gas filters will permit study of the dust jets as possible sources of gas as well as a comparison of the sources of gas and dust on the nucleus surface. The three dust filters and the polarizing filter will allow study of the color and scattering properties of the dust. The clear filter will be used for the study of general nucleus morphology and albedo. A source will be provided for calibration of the imaging system to permit absolute photometry, with calibration images at least every hour before and after closest approach.
A six kilometer nucleus would fill one pixel at 100,000 km, about 4.6 hours
before closest approach. The nucleus may be even larger. Information
relative to size, shape, and jet location should be determined well before
closest approach. In sum, the slow flyby speed and close nucleus approach
distance of STARDUST provide superior imaging, an order of magnitude better in
resolution and an order of magnitude greater in number of images than any
prior cometary mission, without in any way compromising the primary goal of
collecting cometary samples and interstellar dust.
4.4 Coma Dust Activity
Originally the captured dust was to be tagged and cometary dust dynamics to be
studied using an acoustic sensor mounted in the base of the capture media to
register both time and location of the dust capture nondestructively. In order
to minimize the complexity of having an electronics connection to each of the
collector trays, the coma dynamics detection has been removed from the capture
media into an even larger surface area, the spacecraft Whipple dust shield.
Piezoelectric impact transducers are used to detect large particle events.
4.5 Dynamic Investigation
From analysis of the spacecraft telecommunications link, it is expected to be
possible to obtain doppler shifts as the spacecraft moves through the coma
providing a measurement of momentum transfer due to collisions with dust, and
hence the spatial mass density of dust at various locations. If the comet is
low in dust output, it may be safe to fly close enough to place an upper bound
on the mass of the nucleus due to trajectory bending. If the nucleus is
relatively large and massive, that in turn could place a useful bound on the
bulk density. Dynamic science is provided at no hardware cost; it will not be
necessary to fly any special hardware, such as an Ultrastable Oscillator, to
accomplish these measurements.
There are only two dedicated science payloads on STARDUST: the aerogel dust
collectors and the Particle Impact Analyzer. All other science data is
obtained from engineering functions that are required for the operation of the
spacecraft. These engineering instruments are the navigation camera and the
Whipple shield flux monitors. Dynamic science is obtained without special
5.1 Sample Collection
The sample collection subsystem provides the mechanisms which allow exposure as
well as protection and final stowage of all collectors into the Sample Reentry
Extensive experience in both laboratory and space flights of aerogel for collecting hypervelocity particles exists [Tsou 1993]. Eight Shuttle flights have been equipped with aerogel collectors, including Get Away Special Sample Return Experiments as well as aerogel panels mounted above the top deck of the SpaceHab flight Sample return experiments on the Mir space station in 1995 are in the design phase. Get Away Special Sample Return Experiments are on going program. More than 2.4 m2 of silica aerogel capture cells have been flown todate. By the completion of the STARDUST's Phase A work, considerable more flight experience would have been gained.
The camera optics proposed are a spare Voyager wide angle unit, also using a
single Voyager eight position filter wheel and thermal housing, but with a
1024 x 1024 CCD detector used for Cassini Mission with 12 micron pixels rather
than a (now) antique vidicon detector. This will give a pixel size of six meter
pixels at 100 km distance. The 5 ms spare Galileo shutter speed will move the
surface track 30m at closest approach, for the 6 km/s flyby speed.
By coupling the miniature imaging camera electronics with the Voyager flight
spare optics and mechanisms and a modernized Galileo sensor head, the low-cost
science imaging system for the STARDUST mission can be realized within 15
months for very low cost. It will configure into an envelope very similar to
the wide-angle camera that flew successfully on Voyager. The principal difference is that the STARDUST Imager requires a thermal
radiator and some minor modifications to the sensor head. The sensor head uses
the existing Galileo design with the Cassini 1024 x 1024 area array CCD, but
will be modified to use the miniature imaging camera electronics, which was
built and tested as a prototype at JPL during FY 94. Flight qualified versions
of these electronics are presently being built for the MILSTAR program to be
launched in Spring 1995.
Some image motion compensation by means of moving the imaging scan mirror will
be feasible. This will improve the resolution to 5 - 10 m, an order of
magnitude better than Giotto. The lower flyby speed (6 km/s versus 70 km/s)
and less dust opacity guarantee better and more
comprehensive imaging of the nucleus.
5.3 Comet and Interstellar Dust Analyzer (CIDA)
This is the same instrument design as flew on Giotto and two Vega spacecraft,
obtaining unique data on chemical composition of individual particulates in
Halley's coma. It consists of an inlet, a target, an ion extractor, a
time-of-flight (TOF) mass spectrometer (MS) and an ion detector. The inlet is
a baffled system to prevent sunlight from entering the instrument and raising
the background noise in the detector. The target is planned to be a corrugated
Ag or other heavy metal material. It will not be necessary to have a moving
Ag foil for the Wild flyby, as was done for the higher flux at Halley. In
addition, the target area will be increased to 50 cm2 from the
previous 5 cm2.
A light flash which accompanies the initial impact is detected and used to set
the zero for the TOF measurement. Electrostatic grids extract positive or
negative ions, depending upon which polarity they are commanded to, from the
impact microplasma. These ions move down a bent-tube TOF MS, with an
electrostatic reflector to focus ions of similar energies onto the ion
detector. By measuring arrival time, the mass of the ions is determined. It
is expected that at the 6.1 km/s flyby speed, molecular ions as well as
will be important. The instrument is sensitive at least over the range AMU=1
to 150. Even sub-micron sized particles produce observable signals and
The use of a transient recorder mode will allow a much superior data set than was possible from the data-constrained links that were available for the previous PIA flights because it will take all data at the equivalent of Mode 0 data, i.e., the highest time resolution and full peak profiles rather than peak detection. In addition, it will eliminate the criticality of the event detection methods since all signals generated by the ion detector itself will be accepted rather than gated. For the comet flyby, most of this data will be played back slowly over ensuing days or weeks.
The investigator who designed and developed PIA for the Halley missions is a
member of our science team and will lead its re-build and enhancements for
this advanced application. The German DLR is fully backing the flight of this
unit on the STARDUST mission.
5.4 Whipple Shield Dust Flux Monitors
The dust shield which protects the core bus will have small vibro-acoustic
sensors mounted to it in order to sense large impacting comet dust particles.
This is used mainly as a method for early determination of the potential dust
hazard when the spacecraft is first experiencing the coma environment. With
its more than 10,000 cm2 of exposed surface area, this monitor
has 100 times more target area than the CIDA instrument.
6.0 Science Team
The science team is led by Dr. Donald E. Brownlee, Principal Investigator, who
is well-known for his discoveries and work on cosmic particles collected in
the stratosphere; he was also a co-investigator on the Giotto mission.
A team of
co-investigators who are well suited for guiding the development and execution
of the STARDUST mission has been assembled. Dr. Peter
Tsou who invented the intact capture technology for just such a comet coma
sample return mission and introduced the use of aerogel for intact capture will
serve as Deputy, and be in charge of the STARDUST payloads and the fabrication
of the collection media. The other co-investigators include names well known
in the field of planetary science: Ben Clark, Fred Horz, Jochen
Kissel, Martha Hanner, Marcia Neugebauer, Ray Newburn, Scott Sandford, Zdenek
Sekanina, and Mike Zolensky.
6.1 Sample Analysis Advisory Committee
We purposely have not included a long list of sample analysis co-investigators
with the STARDUST PI and co-investigator team because the analysis of returned
samples will come after the end of the mission. The major role of the
STARDUST investigators is to assure that the mission collect the best possible
samples and return them to Earth in the best possible condition. Instead of
including co-investigators with expertise in specific sample analysis areas, we
chose to create a Sample Analysis Advisory Committee composed of experts in all
areas of extraterrestrial material studies. The members of this committee will
not be funded by the STARDUST project but some may have travel costs covered to
attend special meetings. We plan to have most of the committee meeting in
conjunction with annual scientific meetings (i.e. LPSC) to minimize travel
costs. The committee will advise on all aspects of STARDUST that pertain to
the nature of sample collection, preservation, initial examination, curation,
processing and distribution to investigators. The existence of this committee
is very important because of the highly diverse and interdisciplinary nature
of sample analysis. Samples considered to be in pristine condition for some
investigations are hopelessly contaminated or degraded for others. The samples
must be collected, stored and handled in ways that minimize alteration and
contamination of the many different properties that will be studied. The best
use of the STARDUST samples will require analysis of specific samples by many
techniques in many different labs in a logical sequence to preserve the
maximum amount of information about the sample. It is also hoped that this
committee will stimulate the continual advancement of the leading edge of
microanalysis techniques so that they can best be utilized when the STARDUST
samples are first returned.
The following is a list of distinguished scientists that have agreed to serve on the Sample Analysis Advisory Committee. We will expand the membership of this committee as the project progresses.
J. R. Cronin, Arizona State University
P. Eberhardt, University of Bern
R. Pepin, University of Minnesota
R. W. Walker, Washington University
G. J Wasserberg, California Institute of Technology
J. Wood, Smithsonian Astrophysical Observatory
6.2 Science Team Members Experience and Responsibilities
D.E. Brownlee (PI) will oversee all aspects of the project. His specialties
are the collection and analysis of extraterrestrial particulate samples. He
is widely known for his pioneering work on IDPs and has teamed with Tsou in
more than a decade's development of the intact capture technology.
P. Tsou ( Deputy PI) will assist in ovedrseeing all aspects of the project. He will apply the intact capture technology which he invented and his experience in making aerogel for the sample collection system on STARDUST and will produce the flight capture media.
B. Clark will oversee the S/C development as well as a key role in the comet detection collection mechanisms. He was a co-investigator on the Giotto PIA instrument and he will participate in the improvement of PIA for STARDUST.
M. Hanner is an expert on scattering, absorption and emission by cometary and IS grains. She has extensive experience in astronomical observations of cometary and IS grains. She will participate in the trajectory design of cometary and IS encounters.
F. Horz has been PI on several meteoroid collection experiments in space and he directs the JSC hypervelocity gun range. He will participate in performing impact experiments so that sample alteration can be recognized after collection, and will assist in the design of the dust shield.
J. Kissel was the PI on the Giotto PIA instrument and will modify the STARDUST mass spectrometer for analysis of small cometary particles and the direct analysis of interstellar grains. He will optimize the design for organics that survive the 6 km/s impact speed.
R. Newburn is a comet scientist. He will use existing and future data on Wild 2 to model the amount and distribution of dust in its coma, critical for picking an effective but safe flyby distance and will participate in planning the imaging science.
M. Neugebauer was the Project Scientist for the CRAF mission. She will participate in planning the collection of volatiles and as well as in their analysis.
S. Sandford works in astronomical observations and laboratory simulations related to the chemistry of interstellar materials. He will participate in the design of the collection systems so that samples can be related to astronomical observations and models of the solar nebula.
Z. Sekanina studies the distribution and dynamics of particles released from comets. He will participate in the use of comet observations to plan the flyby trajectory, and will also play a key role in determining the exposures for the interstellar dust collection.
M. Zolensky is the Cosmic Dust Curator at JSC. He is a mineralogist with extensive experience with meteorites and IDPs as well as the studies of impact craters on spacecraft. He will plan the handling, processing, and analysis, and will play a key role in curation and sample distribution.
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