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Comet Coma Sample Return
Plus Interstellar Dust,
Science and Technical Approach
Oct 21, 1994
by Donald E. Brownlee, Principal Investigator,
University of Washington,
Peter Tsou, Benton C. Clark, Paul N. Swanson,
and Joseph Vellinga.
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Comets are the most dynamic andspectacular solar
bodies. They appear to be well-preserved relics
of the preplanetary material that accreted in
the outer fringes of the solar nebula. Stardust
will be not only the first US mission to a comet,
but the first robotic return of cometary dust
and volatile samples. Stardust is the culmination
of more than a decade's quest for a comet coma
sample return mission. The scientific value of
having actual bona-fide comet samples in hand
cannot be overestimated.
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 63 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
inerstellar dust, 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.
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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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The targets of the Stardust mission are a comet
and the stream of IS dust. The selected trajectory
will flyby Comet P/Wild 2 at a low velocity. Altogether,
three orbits will be made around the sun to minimize
the delta-V requirements for the mission so that
a Med-Lite launch vehicle can be utilized rather
than a Delta rocket and to maximize the time for
favorable collection of interstellar dust. The spacecraft
flew by Wild 2 about five years after launch
and will return the samples two years thereafter.
The encounter takes place at 1.86 AU from the sun
or 75 days past Wild 2 perihelion passage. The relative
velocity at encounter is 6.1 km/s. The flythrough
of the coma is planned on the sun side at a miss
distance selected to balance between the need to
protect the spacecraft and the desire to satisfy
the sampling goals stated above. Imaging the comet
nucleus is planned with the body-fixed camera; motion
compensation will be made with the scan mirror during
the nucleus flyby. IS dust will be collected on
three aphelion legs of the orbits, which allow the
dust to be collected at low velocity by orienting
the spacecraft so that the trajectory velocity subtracts
from the IS velocity. The "B" sides of
the collectors are exposed during this portion of
the flight.
Periodic Comet P/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.
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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
[Tsou 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, aerogel has 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, http://eande.lbl.gov/ECS/aerogels/satoc.htm).
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 P/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 form.
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 mechanical extraction.
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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
following:
Mineralogy/Petrology
The primary identification of minerals and amorphous
phases and their textural relationships will be
determined primarily by electron microscopy (EM),
although many other techniques such as laser micro-Raman
spectroscopy, ion microprobe, time-of flight SIMS,
IR spectroscopy, and UV fluorescence will also
provide valuable inputs. The EM studies will probably
constitute the most widely used techniques because
of their power, ease of use, high resolution,
and sensitivity. SEM studies can reveal general
morphological features, but the main EM work will
be the study of microtome thin-sections in the
analytical transmission electron microscope. These
techniques are fairly standard and are routinely
used for the analysis of interplanetary dust samples
and meteorites. Thin-sections are made, usually
with a thickness of 0.1 micron, and then the combined
results of various types of imaging (bright field,
dark field, energy loss, backscatter) coupled
with various types of electron diffraction, energy
dispersive x-ray analysis and electron energy
loss spectroscopy can provide very accurate analyses
on individual components less than 10 nm in size
[Bradley and Brownlee 1986]. Thicker slices can
also be made for He isotopic analysis, ion microprobe
studies or IR measurements.
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.
Isotopes
Existing techniques used for the analysis of IDPs
and interstellar grains in meteorites provide
powerful capabilities for detection of isotopic
effects in comet samples and interstellar grains.
Much of this work will be done with the ion microprobe
because of its high sensitivity and spatial resolution.
As is the case for the interstellar SiC grains
in chondrites, it is possible that some interstellar
grains will have detectably anomalous isotopic
compositions for all detectable elements. Such
grains will provide clues to element production
in the stars that they formed around. One of the
major goals will be to determine the isotopic
composition of IS silicates. These may carry large
anomalies although if they extensively modified
by cycles of destruction and re-formation in the
interstellar medium (ISM) they might typically
have solar isotopic compositions. One of the non-nuclear
anomalies carried in IS grains is high D/H fractionation,
purportedly due to cold ion-molecule reactions.
This work could lead to links with the host of
interstellar molecules identified in cold clouds
by radio astronomical techniques.
Elemental Composition
With energy dispersive x-ray analysis in the electron
microscope and trace element analysis using synchrotron
x-ray fluorescence, it is possible to do full
quantitative composition analyses on micron samples
for major to trace elements. This is critical
for phase identification but it also might provide
information about the bulk composition of comets
and IS grains. The expectation is that the mean
composition of this material is chondritic although
it is possible that there may be deviations for
volatile elements for which chondrites may not
give true solar abundances. Elements of interest
include N, C, S and Zn. All of these elements
are strongly fractionated among the most primitive
chondrite groups. Although the total sample mass
to be collected by Stardust is small, its analysis
may provide insight into the cosmic abundances
of these elements. It will provide direct data
on the amount of carbon and nitrogen that are
incorporated into comets as non-volatile phases.
The Halley data imply that the bulk of cometary
carbon in is non-volatile material.
4.2 Volatiles Analysis
Stardust combines the most recent advances in
underdense media intact capture technology with
new findings on chemisorption on high surface
area materials. Our proposed volatiles collection
will expand the existing scientific research domain
in the origin and history of biogenic elements
and compounds by returning the first largely unaltered
materials to Earth-based laboratories for detailed
analysis. Analyzing these samples will help to
answer many important exobiology-related questions
about our solar s ystem, the origin and evolution
of comets and life itself. For example;
+ What is the origin,
age and history of comets?
+ What was the biological composition
of the pre-planetary materials which aggregated
into planetary bodies?
+ How do relatively complex organic molecules
form in the interstellar medium?
+ What is the elemental and molecular
contribution of matter from outside our
Solar System?
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Following the approach of Dust Analysis described
above, the samples will be evaluated by the a
volatiles Preliminary Examination Team first and
then be repackaged into portions available to
qualified investigators around the world. We envision
now a general analysis scheme and predict a generic
science return from volatiles samples, recognizing
the continuing advancement in exbiological science
and the techniques of volatile analysis. An advantage
of returning samples is that the science of capturing
of volatiles is not now known. Samples of cometary
volatiles are captured in three forms: entrapped
within dust particles, chemisorped by dopands
within selected capture medium and physisorbted
on the large surface area of the capture medium.
The technique to analyze samples of captured volatiles
will differ. In any case, due to high surface
area of the capture medium, the sample vault must
be sealed against any further deposition of volatiles
or the escape of already condensed volatiles.
On return, the sample vault shall be opened in
a controlled environment capable of capturing
any volatiles that may escape. Analytical techniques
include mass spectrometry, gas chromatography/mass
spectrometry, chemical reaction analysis, auger
electron analysis, and infrared spectroscopy.
4.3 Coma and Nucleus Imaging
The raison d'être for Stardust is its ability
to provide unique new knowledge of comets and
IS particles. The mission will also provide excellent
imaging, however, both optical navigation in support
of the dust collection and for the study of nucleus
morphology. The proposed camera will have the
ability to investigate the large scale distribution
of dust and associated gases in the general coma
as well as in jets. It also will permit observation
of the areas on the nucleus which are the source
of the dust. There are advantages in studying
a comet somewhat less active than Halley in that
the spacecraft can fly closer to the nucleus and
not be so readily overwhelmed optically or physically
by the outflowing dust. The imaging results also
will determine the size, shape, and albedo of
the nucleus, and quite possibly its rotation.
Close to the nucleus, Stardust will provide detailed
nucleus morphology at 10 times better resolution
than the Giotto pictures of Halley.
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.
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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 hardware.
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 Capsule (SRC).
Dust Collectors
The dust collectors are sheets of aerogel (see
Aerogel
above, or Aerogel
Summary) and other low density media that
are simply exposed to the sample flux. Similar
to previous exposures on Spacehab and Shuttle
experiments, the collection media will consist
of modular aluminum cells holding about 1 to 2
cm thick blocks. The cells will form a rosette-shaped
structure that will fold up to a compact configuration
for stowage into the Earth return capsule. One
side of the modules will be used for the comet
encounter and the opposite side will be used for
interstellar collection. The useful collecting
area is 1225 cm2 for interstellar dust
grains and 1225 cm2 for cometary dust.
The density of impacts will be low and this dual
use causes no problem with sample cross-talk.
Several collector materials may be used, but unless
better capture media are discovered, the bulk
of the area will be silica aerogel with an average
density of 0.02 g/cm3. For the interstellar
side of the modules we will try to use a graded
density medium to give even lower density for
the initial impact. These collectors are totally
inert and only have to be exposed and recovered.
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.
Volatiles Collectors
Two approaches have been conceptualized for volatile
capture: physisorption and chemisorption using
the very same capture media as are used for dust
particles. Initial experiments of physisorption
of noble gases under a simulated cometary flyby
encounter environment using getters proved surprisingly
successful [Tsou et al. 1987]. Silica aerogel,
our baseline particle capture medium, has the
high surface area of 500 to 1000 m2/g.
In fact, silica aerogel has been shown to be more
effective in capturing organic volatiles than
activated charcoal [Attia 1994].
5.2 Camera
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 morecomprehensive 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 2 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 atomic ions 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 compositional profiles.
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.
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![sci_team.gif](https://webarchive.library.unt.edu/eot2008/20090119061930im_/http://stardust.jpl.nasa.gov/images/headers/sci_team.gif) |
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
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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.
Last updated November
26, 2003 |
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