Sweeping
up Stardust
Our solar system is moving through a
cloud of gas and dust from between the stars. NASA's
STARDUST probe began collecting samples of the cloud
last week.
February 28, 2000 -- Most science fiction movies portray
space as cold, black and empty. In fact, the voids
between the stars are anything but vacuous. Interstellar
space is permeated by clouds of gas and dust. These
dusty clouds are places where stars are born and where
complicated chemical reactions form organic molecules,
including amino acids.
Many years ago, Earth-orbiting
satellites noticed a stream of cosmic dust shooting past
our planet. It's been detected again and again by NASA
spacecraft, including Pioneer, Voyager and Galileo.
Astronomers now believe that the source is a cloud of
gas from between the stars. Dust grains enter the solar
system moving 30 km/s, about 30 times faster than a
speeding bullet.
On February 22, 2000, NASA's STARDUST
probe -- currently orbiting beyond the orbit of Mars --
took a historic step. Instead of just watching the dust
shoot by, it began collecting samples for a probable
return to Earth in 2006.
Interstellar dust grains are composed
mostly of carbon, silicon, and oxygen and are usually
less than about 1/1000 of a millimeter across. They may
be small, but they're very important. Outside of stars,
dust grains are the major repository of heavy elements
in our Galaxy. One of their most important properties is
opacity. When astronomers peer through telescopes across
the galaxy, they can't see very far because interstellar
dust absorbs light. 
This is frequently regarded as a nuisance by
astronomers, but it is important for life as we know it.
The absorption of light by dust in cold dense clouds
between the stars permits the formation of complex
molecular species -- possibly even the building blocks
of life -- that would be otherwise destroyed by
ultraviolet radiation from nearby massive stars. The
cooling effect of dust in some clouds assists in their
collapse to form new generations of stars and planetary
systems.
Right: Nobody really knows what a
typical interstellar dust grain looks like. By studying
how dust absorbs, emits, and reflects light, astronomers
do know that interstellar dust is much different than
the cell and lint based dust found around a typical
house. Recent work indicates that most dust grains are
not spherical. The above picture shows the result of a fractal
adhesion model for dust grains involving random
conglomerates of spherical compounds of different
properties, here artificially highlighted by different
colors. [more
information]
Although the primary mission of
STARDUST is to sample
dusty material spewing away from comet Wild-2,
scientists are anxious to obtain a sample of
interstellar dust as well. So, while the spacecraft is
still 4 years away from its rendezvous with the comet,
mission controllers commanded STARDUST to deploy its
cosmic dust collector and point the device toward the
stream of incoming interstellar gas.
Bugs on the Windshield
A
unique substance called aerogel
is what STARDUST uses to catch and preserve the high
speed dust samples. Composed of over 95 percent air,
aerogel is the lightest man-made material on Earth. When
a high-velocity dust particle hits aerogel, it buries
itself in the material, creating a carrot-shaped track
up to 200 times its own length. This method of sweeping
up particles has been likened by some scientists to
"bugs on the windshield" of a fast-moving car.
In fact, aerogel is low enough density to collect the
"bugs" without destroying them.
Left: Looking like comets
themselves, microscopic tracks in an aerogel sample lead
to dust particles captured in ground tests to validate
the STARDUST "catcher's mitt."
The aerogel collector on STARDUST has
two sides: one designed to gather interstellar dust and
one for comet dust collection. Engineers can control
which side of the collector is exposed to a dust stream
by orienting the spacecraft. Right now, STARDUST is
oriented so that the interstellar dust particles are
hitting the backside of the collector. Mission
controllers take care to avoid pointing the collector
toward the Sun to minimize the number of interplanetary
dust grains captured on the interstellar side of the
collector.
The current interstellar dust
collection will continue through at least May 25. After
that, it will be returned to its stowed position until
mid-2002, when another period of interstellar dust
collection is scheduled. Mission planners have scheduled
the two sampling periods to minimize the relative
velocity between the spacecraft and the dust stream.
Even so, dust particles will strike the aerogel sampler
at a blazing 25 km/s.
After
STARDUST flies by comet Wild-2 in 2004, it will return
to Earth bearing its cargo of cometary and interstellar
dust samples. Two years later, the craft's aerogel
sample collectors will descend by parachute toward the
U.S. Air Force Test and Training range in Utah, about
100 miles southwest of Salt Lake City in the desert.
Following touchdown, the sample container will be
recovered by helicopter or ground vehicles and
transported to a staging area at Test and Training
range. The canister will then be transported to the
planetary materials facility at Johnson Space Center.
After 7 years in space, the contents may be a little
dusty, but scientists won't mind that at all.
Right: This artist's rendering depicts
the aerogel sample capsule after it descends to the
snow-covered desert in Utah in 2006.
For more information about the
STARDUST mission, see the STARDUST
Home Page from the Jet Propulsion Laboratory
Related Links:
STARDUST
Mission home page -- from JPL
The
Science of STARDUST -- from JPL
STARDUST
Education web page -- from JPL
Why
comet Wild-2? -- from the JPL STARDUST team
Where
is comet Wild-2 right now? -- from the JPL
STARDUST team, updated every 5 minutes
Orbital
elements of comet Wild-2 -- from the JPL STARDUST
team
More
about aerogel -- from JPL
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