[meteorite-list] Stardust Capsule Heading Home With Precious Cargo

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Wed Jan 11 12:47:15 2006
Message-ID: <200601111745.k0BHjdc12231_at_zagami.jpl.nasa.gov>

http://www.signonsandiego.com/news/science/20060111-9999-lz1c11comet.html

Stardust capsule heading home with precious cargo that could shed
light on solar system's origins

By Leigh Fenly
San Diego Union-Tribune
January 11, 2006

If you happen to be somewhere between Portland and San Francisco early
Sunday morning, you might see Stardust streaking home. It will be in a
hurry.

The 100-pound space capsule will fire through the skies at 28,860 miles
per hour - that's San Francisco to Los Angeles in less than a minute,
faster than any man-made object has ever gone.

For NASA scientists, waiting seven years on the ground, it will not a
moment too soon.

"Inside that capsule is the most precious treasure," explained Donald
Brownlee, principal investigator for the Stardust mission at the
University of Washington. "This is the end of a fantastical voyage."

Launched in 1999, Stardust has been quite a record breaker. At 2.88
billion miles, it has traveled farther than any Earth-returning
spacecraft. It has collected close-ups that have churned up researchers'
ideas of what comets look like. And it has successfully completed its
main mission, collecting - and now bringing home - particles of dust
from comet Wild 2 that are believed to be as old as the sun.

The real thrill awaits. After it parachutes into a dry Utah lake bed, a
helicopter crew will fly the return capsule with its 1/1000th of an
ounce of comet dust to a clean room at Dugway Proving Ground. From there
it will be moved to a special laboratory at NASA's J Johnson Space
Center in Houston.

"Within a couple of days, scientists will be diligently digging into
these samples to understand the secrets of our solar system," Brownlee
said. "There's enough we'll be analyzing these particles for decades to
come."

Bringing up the past

Tiny Comet Wild 2 (pronounced Vilt), only three miles across, coalesced
4.6 billion years ago at the time the solar system was forming.
Scientists theorize that a supernova explosion was the catalyst that
began turning an enormous gaseous cloud known as the solar nebula into a
sun, planets and comets.

The explosion would have sent shock waves into space that squeezed the
cloud, pushing more and more of its material inward. At some point,
gravity would have overcome the internal pressure and the cloud would
have collapsed and shrunk. This would have caused it to spin faster and
faster. Eventually it flattened into a rotating pancake with a bulge at
the center.

Stardust wouldn't be coming home with comet dust if it weren't for a
California chemist named Samuel Kistler who mixed up a peculiar
substance 75 years ago. The details of his invention are still debated,
but the popular story is that Kistler, while at the College of the
Pacific in Stockton, made a friendly wager with a colleague, Charles
Learned.

The bet: Who can figure out a way to replace the liquid in jelly with
gas without causing the jelly to shrink?

Kistler beat Learned and published his results in Nature in 1931. The
peculiar material he created was aerogel, a silicon-based solid with a
porous, spongelike structure that is 99.8 percent air.

Without it, NASA would have been hard pressed to figure out a way to
capture comet dust, traveling at four miles a second, without it
vaporizing or distorting.

Stardust's collecting arm is outfitted with more than 100 aerogel
collecting capsules, encased in an aluminum grid. Tests on Earth show
that dust grains slow down as they bear through the threadlike silica of
the aerogel. Damage is minimal.

Aerogel is made of water, alcohol and silica that gels into something
that looks like Jell-O. The trick is to dry the "aero-Jell-O" without
collapsing it into a dense slab. This is accomplished by exchanging the
alcohol with liquid carbon dioxide, and then removing the carbon dioxide
at high pressure.

The end result is what's been dubbed "frozen smoke," one of the lightest
solid materials known.

This is not aerogel's first trip to outer space. In 1997, aerogel was
used to insulate Sojourner, the Mars rover. Nighttime temperatures on
Mars dropped to minus 88 degrees. But inside Sojourner, where sensitive
electronics needed protection, it remained a balmy 70 degrees.

Fast forward 100 million years. The pressure and densities of hydrogen
in the middle of the bulge will now be great enough to sustain
thermonuclear fusion reactions. Our sun is born.

Some coalescing clumps of gases and rocks became planets, others
remained as comets - small, hard blobs of rock and ice that are still
the most abundant bodies in the solar system.

Over a trillion comets are estimated to exist in the Kuiper Belt,
outside Pluto, and in the far distant reaches of the Oort Cloud. They
are our origins and will outlast everything else in the solar system.
Even after the sun burns out billions of years from now, vast numbers of
ejected comets will be wandering about the Milky Way, holding within
them the archival evidence of our most distant past.

At the very minimum, say NASA scientists, comets contain some regions
that haven't been heated above freezing for 4.6 billion years. Their
cold, pristine insides make them the very best candidates for collecting
and studying the materia that built the solar system.

"Comets are the relics of the original formation of the solar system and
hold clues to the formation of all stellar systems," said Andrew
Dantzler, NASA Discovery program director. "That's why they're getting
such intense scrutiny."

Surprising landscape

Far from the sun, comets are cold, inactive bodies. But sometimes they
are drawn into the inner solar system, where they can enter the
gravitational streams of planets. Wild 2, discovered in 1978 by Paul
Wild, is one of these. In 1974, it had an ulta-close encounter with
Jupiter that threw it into a new orbit closer to the sun.

As comets approach the sun, their interiors stay cold but their black
surfaces become quite hot. Solar heating causes water ice and frozen
volatiles to vaporize. The liberated gas molecules stream outward from
the surface at near supersonic speed. Dust and rocks that were glued
together by ice are released and driven outward by the wind of escaping
gas.

With each pass close to the sun, a comet's materials are boiled off into
space, until, after about a thousand trips, most of its volatile
materials are lost and it can no longer generate a coma or tail.

Wild 2, an inner solar system newcomer, is a long way from that. When
Stardust reached it in 2004, Wild 2 had made only five solar orbits.

Scientists don't know exactly what comets are made of, but they thought
they knew what they looked like. Stardust's images of Wild 2 taught them
otherwise. The 72 images taken by the spacecraft show a diverse
landscape like no other known in the solar system, with tall spires,
pits and craters.

Stardust flew by Wild 2 at 13,000 mph, six times faster than a bullet.
It flew in front of the nucleus, through a halo of gases and dust. For
several hours, it extended its tennis-racket-shaped arm and collected
thousands of grains of dust into a curious space-age material called
aerogel. Then it stowed its collector and headed home.

"What we got is better than if we had landed on the comet," said
Brownlee. "Then you might see things that have been modified. In my
view, this is the dust that went in to form these things 4.6 billion
years ago."

Having the actual samples may be the only way to tease out their exact
composition. Gaseous emissions from comets can be studied by telescopes
or spacecraft, but dust and rocks are a lot tougher.

Having them in hand is "the difference between a human using binoculars
and getting down to the molecular level where you can study DNA,"
Brownlee said. "We'll be able to get all the details - unique chemical
and physical information - you could never dream of if you were looking
at it remotely."

At half the width of a human hair, the samples will be "huge rocks,
compared to what we can analyze," he said. They will be sliced into
hundreds to thousands of smaller samples for study.

Brownlee doesn't expect the dust to confirm or refute theories of solar
system formation, but rather to start filling in the sketchy details.
"Understanding how comets are made will help us assess their importance,
what role they played. And we'd also like to know how they affected the
prebiotic Earth."

It's widely believed that during a heavy bombardment 4 billion years
ago, comets delivered to Earth the ingredients for oceans and atmosphere
and ultimately life. Our planet is still layered in comet dust, although
it is hardly the pristine stuff of Wild 2.

"You can find more in your back yard than we're bringing home in
Stardust," said Thomas Duxbury, Stardust project manager. "We live
surrounded by comet dust."

Lessons from Genesis

The job now is to get the dust home safely. Here's the plan:

Four hours out from landing, Stardust will release the 100-pound sample
return capsule so that it enters Earth's atmosphere over the Pacific
Ocean. The capsule will be protected through its blistering re-entry by
a special carbon-based heat-resistant shield, like the one that
protected Apollo astronauts. "We call this our knight in shining armor,"
said Duxbury.

As it streaks over the western United States, Stardust will light up the
sky in pink-white light as bright as Venus (although in San Diego we'll
be too far south to see it.)

At 105,000 feet, it will deploy its first parachute; at 10,000 feet, its
second. Expected landing time is 2:15 a.m. Sunday on the moonscape of
the Utah Test and Training Range. The canister with the dust samples
will not be opened until it arrives at Johnson Space Center in Texas.

Not far from the recovery team's mind is Genesis. That spacecraft,
returning with samples of solar wind, crashed-landed in 2004 into the
same Great Salt Lake Desert. Most of Genesis' samples were contaminated
after a parachute failed to deploy, although some studies are continuing.

"We took lessons from Genesesis," explained Ed Hirst, Stardust Mission
systems manager. "We dug out the prelaunch blueprints and went over them
in detail to completely understand the Genesis mishap. Stardust is the
fastest return vehicle ever brought back to Earth, so in a way bringing
it back is the only way to test it. Something could happen, but we think
the probability of that is very low.

"Our return capsule is extremely rugged. In the event the chute doesn't
open or the capsule lands hard, we know we can still finish our science
opportunities."
Received on Wed 11 Jan 2006 12:45:39 PM PST