1.1 Goals
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 dust particles.
The comet samples were collected during a
6.1 km/s flyby of Comet P/ Wild 2. At this extraordinarily
low flyby speed, coma dust in the 1 to 100 micron
size range was 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
and 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 possible 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 Ulysses and Galileo spacecraft,
of an appreciable flux (15/m2-day) of relatively
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.
1.2 Objectives
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 P/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 environments.
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:
1. the mineralogical, elemental, and
chemical composition of comets at the
submicron scale;
2. the extent that building materials
of comets are found in interplanetary
dust particles (IDPs) and meteorites;
3. the state of water in comets - is it
all in ice or are there hydrated minerals;
4. mixing of inner nebular materials (i.e.
chondrule fragments) to the comet formation
region;
5. the presence of isotopic anomalies;
6. the nature of the carbonaceous material
and its relationship to silicates and
other phases; and
7. evidence for pre-accretional processing
either in the interstellar medium (ISM)
or the nebula (including cosmic ray tracks,
sputtered rims, etc.).
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1.3.2 Cometary Volatiles
Although the dust/volatiles ratio varies greatly
from comet to comet, the volatiles is a significant
fraction of the mass of every comet nucleus. Because
the volatile and refractory components of comets
may have condensed in very different locations
and environments, complete knowledge of the composition
of a comet requires study of both phases. The
objectives of the volatile collection experiment
are to determine the elemental and isotopic compositions
of cometary volatiles. Of special interest are
the biogenic elements (C,H,N,O,P and S) and their
molecules. Some molecular bonds in large molecules
can remain unbroken in a 6 km/s impact, as shown
by laboratory experiment. At the very least, the
obtainable information on gaseous components will
be elemental and isotopic. In addition, the time-of-flight
mass spectrometer carried on Stardust will provide
direct measurements of volatile species in the
impacting dust samples and is expected to obtain
much more information on complex molecules than
for the Halley flybys because impacts with coma
particles are less than 100 times as energetic.
1.3.3 Interstellar Dust
At present, astronomically derived information
on interstellar grains comes primarily from observations
of extinction, scattering, polarization, and infrared
emission. The UV-IR extinction curve requires
several IS dust components: small (less than 0.01
micron-size) grains to explain the far-UV extinction;
graphitic carbon to produce the 0.22 micron bump;
and somewhat larger particles of size 0.1 micron,
giving rise to the visual extinction. A spectral
feature near 10 microns is evidence for small
amorphous silicate grains. Finally, a series of
IR emission bands is ascribed to polycyclic aromatic
hydrocarbon (PAH) molecules [Hudgins et al. 1994]
or hydrogenated amorphous carbon grains. In addition
to astronomically derived data, new information
has come from laboratory studies of interstellar
SiC, graphite and diamond that have been identified
in meteorites as trace constituents. These samples
have been identified by their peculiar isotopic
compositions. With the exception of a few alumina
grains most of the laboratory IS grains appear
to have formed around carbon rich stars. The phases
identified are very robust materials which aids
their survival in the interstellar medium (ISM)
the solar nebula, and the extreme chemical processing
in the laboratory that is used to isolate them
from the bulk of meteoritic minerals. Besides
direct information on the chemical, mineralogical
and isotopic composition of a selected set of
IS grains, these samples provide proof that at
least some wonderfully crystalline IS grains grow
to sizes of at least 20 microns in circumstellar
outflows, and that they survive residence in the
interstellar medium for appreciable amounts of
time with their mineralogical and isotopic compositions
intact.
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:
1. the elemental composition of the grains;
2. the isotopic composition of several
important elements, such as C, H, Mg,
Si, and O;
3. the mineralogical and textural character
of surviving phases;
4. whether all IS grains are isotopically
anomalous,
5. the mineralogy of the silicate grains
- whether glassy or crystalline, as well
as their Si:O ratio;
6. the prevalence of graphite particles,
including whether their abundance is sufficient
to explain the IS 0.22 micron extinction
bump;
7. the extent of physical mixing of the
mineral phases, including whether the
grains have a silicate core/organic refractory
mantle structure, and also if they are
a heterogeneous mixture or not, and
8. whether there is any evidence for grain
processing in the ISM, especially whether
the effects of shock sputtering, collisions,
accretion and chemical alteration can
be identified.
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This work would provide ground truth information
on interstellar grain models and perhaps provide
physical properties or grains of effects of processing
that were previously unforeseen. It would provide
data on the degree of processing after initial
formation in circumstellar regions, and it would
provide information on the relative importance
of oxygen-rich and carbon stars in producing interstellar
dust. Isotopic effects in the samples would be
direct probes of nucleosynthesis processes in
a variety of types of stars. In the case of hydrogen,
isotopic fractionation would provide insight into
the ion-molecule reactions that are a favored
explanation for high D/H ratios in some molecular
clouds and trace components in meteorites and
IDPs.
1.3.4 Exobiology Implications
Comets are now known to contain large quantities
of volatiles, including organic compounds and
a rich variety of microparticles of various types
(pure organic particles, silicates, sulfides,
and mixed particles) with a graduation of sizes
that extends to submicron diameters. Organic particulates
actually consist of several sub-populations [Clark
et al. 1987; Mason and Clark 1989] which can be
assigned based upon their elemental composition.
These include particles containing (H,C,N), (H,C,O)
(H,C), and (H,C,N,O, with and without Mg). The
latter we have termed the CHON particles. Cometary
material is expected to represent a variety of
types. Organic compounds may have been imported
to Earth by comets [Oro 1961]. Also potentially
important to the abiotic beginnings of life is
the complex nature of cometary particles on the
microscale. In particular, we draw attention to
the high surface area and accessibility of nanometer-sized
subunits, the unknown chemical reactivity of the
amorphous grains with cosmic proportions of elements,
and finally, the presence of stoichiometric and/or
well-crystallized mineral grains in close proximity
to chemically-distinct grains. With high surface
areas, juxtaposed chemical constituents, and their
easy transportability, these particulates may
have been critically important for abiotic catalytic
activity, macromolecular synthesis, and the subsequent
chemical synthesis pathways [Clark 1988a and 1988b].
These are the well-known prerequisite processes
for the origin of life.
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
Working Group.
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.
Ultimate Sensitivity,
Accuracy and Precision
The most complex and powerful, state of the art
instruments will be available for this study because
the analyses will be done back on Earth.
Adaptability
One of the most significant aspects of sample
analysis is adaptability. A typical spacecraft
analytical instrument is designed years before
launch to perform a specific task with pre-determined
levels of sensitivity and operation modes. In
contrast, in typical laboratory programs the key
scientific goals constantly evolve as more and
more is learned about the samples. New studies,
done in different ways, with newer and better
methods constantly yield new insights and open
up new investigations which could not have been
originally envisioned. The most important developments
in the laboratory analysis of extraterrestrial
materials were not anticipated before sample requests
were sent in but were only reached after dedicated,
usually interdisciplinary efforts, yielded previously
unanticipated results. The samples will provide
very rich ground for unexpected discoveries.
Time Factor
The samples are a resource. They can be studied
for decades into the future using ever-improving
techniques and analysis technologies limited fundamentally
only by the number of atoms and molecules available.
Many types of analyses now performed on lunar
samples were not even conceived at the time of
the Apollo missions.
Credibility of Measurements
Sample analyses can be replicated to confirm results.
In many cases a property, such as trace element
composition, can be measured by totally different
techniques to verify the accuracy of the finding.
Instrument calibration can be done with nearly
unlimited flexibility. The ability to do real
time calibration of standards and analog materials
as well as evaluation of background and contamination
problems is essential for high credibility results
with many techniques. For example, calibration,
and understanding fractionation effects and interferences,
and evaluation of complex and sample dependent
effects are critical for the most meaningful analyses
with the ion microprobe.
Reliability, Size, Mass, Power and Cost of Instrumentation
Although these are critical limitations for flight
instrumentation, they are not for laboratory instruments.
Cutting edge laboratory instrumentation is notoriously
difficult and demanding. Complex instruments constantly
break down, but instrument reliability is not
a issue since almost all components can be replaced
or fixed. In almost all cases, laboratory instruments
are radically different from those that would
be considered for space flight. They are larger,
more complex, more flexible, and usually consume
large amounts of power. An example is trace element
synchrotron x-ray fluorescence analysis, a technique
that uses synchrotron radiation to do spot x-ray
fluorescence (XRF) analysis of small samples.
In this case the analysis instrument is larger
and more massive that an entire launch vehicle,
more costly that an entire Discovery mission,
and operates at an extraordinary power level.
While this may be an extreme, most analyses done
in the laboratory use instruments that could not
be flown on spacecraft, except for highly specialized
versions with limited capability. The most important
aspect of laboratory instruments is that they
are on Earth where they can be run, modified,
fixed, tested, and calibrated.
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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 
