On January 2, 2004, NASA’s Stardust spacecraft will fly through a comet
and collect samples of dust for return to Earth.
Philosophers have long sought to "see a world in a
grain of sand," as William Blake famously put it. Now scientists are
attempting to see the solar system in a grain of dust–comet dust, that is.
If successful, NASA’s Stardust probe will be the first ever to carry matter
from a comet back to Earth for examination by scientists. It would also be
the first time that any material has been deliberately returned to Earth
from deep space.
And one wouldn’t merely wax poetic to say that in those tiny grains of
comet dust, one could find clues to the origin of our world and perhaps
to the beginning of life itself.
Comets are like frozen time capsules from the time when our solar system
formed. Drifting in the cold outer solar system for billions of years,
these asteroid-sized "dirty snowballs" have undergone little change
relative to the more dynamic planets. Looking at comets is a
bit like studying the bowl of leftover batter to understand how a
wedding cake came to be.
Indeed, evidence suggests that comets may have played a role in the
emergence of life on our planet. The steady bombardment of the young
Earth by icy comets over millions of years brought some of the water
that makes our brown planet blue. And comets contain complex carbon
compounds that might be the building blocks for life.
Launched in 1999, Stardust will rendezvous with comet Wild 2 (pronounced
"Vilt" after its Swiss discoverer) on January 2, 2004. A rendezvous
with a comet is a little like a rendezvous with a Gatling gun on a
foggy night. As Stardust plunges through the hazy clouds of gas
surrounding Wild 2’s core, dust grains will fly by the spacecraft at
about 13,000 mph, or six times faster than a speeding bullet. The "eyes"
of Stardust, an onboard camera, will peek out from the body of the craft
through a periscope to avoid damage. A Whipple Shield–a stack of five
sheets of carbon filament and ceramic cloths each spaced 2 inches apart–
protects the rest of the spacecraft.
Stardust will use a material called aerogel to capture some of the
fast-moving grains. Aerogel is a foam-like solid so tenuous that it’s
hardly even there: 99 percent of its volume is just air. The ethereal
lightness of aerogel minimizes damage to the grains as they’re caught.
Mission planners hope to catch more than one thousand grains larger than
15 microns in the aerogel.
Wild 2 orbited the sun beyond Jupiter until 1974, when it was nudged by
Jupiter’s gravity into a Sun-approaching orbit–within reach of probes
from Earth. Since then the comet has passed by the Sun only five times,
so its ice and dust ought to be little altered by solar heating. Pristine
dust from Wild 2 can tell us what the solar system was like before it
was baked by 4.5 billion years of sunshine and radiation.
After the encounter, Stardust will loop around the Sun on a two-year
journey back to Earth. In January 2006, home again, the spacecraft will
eject the Sample Return Capsule (SRC), which looks like a miniature
Apollo capsule. The SRC will parachute to Earth and, if all goes as
planned, land in Utah where scientists will be waiting…
To see a world in a grain of sand
And a heaven in a wild flower
Hold infinity in the palm of your hand
And eternity in an hour
William Blake (from Auguries of Innocence, c.1800)
