[Mb-civic] Spacecraft to Bring Home Comet Dust and Clues - Washington Post

[William Swiggard]

Spacecraft to Bring Home Comet Dust and Clues

By Guy Gugliotta
Washington Post Staff Writer
Monday, January 9, 2006; A07

For thousands of years, stargazers on Earth have marveled at the ghostly 
beauty of comets as they streak across the night sky trailing tails of 
fire, wondering what they are and where they come from.

Today's scientists have many of the answers, but on Jan. 15 in the Utah 
desert, barring a mishap, they will have another. The first-ever mission 
to fly to deep space and steal samples from one of these cosmic 
travelers will be returning home.

NASA's Stardust spacecraft flew two years ago through the halo of gas, 
dust and debris, or "coma," enveloping comet Wild 2, and before dawn on 
Sunday it will send a capsule containing the samples on a parachute 
descent into the U.S. Army's Dugway Proving Ground.

The capsule will enter Earth's atmosphere at 28,860 mph -- the fastest 
reentry in spacecraft history. Its sample compartments are filled with 
an ultra-frothy spun glass called aerogel, which has caught, and 
contains, thousands of particles of comet dust, virtually all of them 
considerably less than the width of a human hair.

Even though all the captured material would easily fit in a teaspoon, 
the samples are "more than we can analyze in a decade" with the 
available high-precision scientific instruments, said project lead 
scientist Donald Brownlee of the University of Washington.

"We're using the comet as a library," Brownlee added during a recent 
news conference. "This is a fantastic opportunity to collect the most 
primitive particles in the solar system."

Comets come from the chill reaches of deep space beyond Neptune and are 
composed of ice, dust and debris in a mix virtually unchanged since the 
solar system's birth about 4.5 billion years ago.

Besides what Stardust may reveal about cosmic origins, this effort is 
the latest in a distinguished series of unusual missions to smaller 
celestial bodies. Last year, NASA's Deep Impact dropped a projectile in 
the path of comet Tempel 1, setting up a spectacular Independence Day 
collision. And in November, the Japan Aerospace Exploration Agency's 
Hayabusa spacecraft "docked" with an asteroid to collect dust from it.

Each mission presented daunting engineering hurdles. Deep Impact 
returned spectacular images of the projectile impact and a river of data 
from the resulting debris plume, but mechanical problems have hampered 
Hayabusa to such a degree that project managers were forced to delay its 
trip home until 2007.

Stardust has had its own challenges. Under NASA's cost-saving Discovery 
program, the project spent only $168.4 million, using off-the-shelf 
components where possible and a relatively small Delta II launch rocket 
for the 850-pound spacecraft.

The probe lifted off from Cape Canaveral Air Force Station on Feb. 7, 
1999, "parked" in a holding orbit 115 miles above Earth for half an 
hour, then ignited its upper-stage rocket to escape Earth's gravity.

Its target, Wild 2, is a comet that originated beyond Pluto and migrated 
to the inner solar system after receiving a gravity push from Jupiter in 
1974. Wild 2 currently revolves around the sun every 6.39 years in an 
elliptical orbit between Mars and Jupiter. On launch day, it was 508 
million miles from Earth.

But Stardust traveled 2.1 billion miles over nearly five years to catch 
its quarry, receiving a gravity assist from Earth on the first of three 
loops around the sun to widen its orbit and intercept the comet during 
the third loop.

Stardust carried a two-sided, tennis-racket-shaped particle collector 
with about 100 small aerogel-filled compartments, "like a giant ice cube 
tray," Brownlee said. Aerogel is 99 percent empty space, and has a 
unique capacity to "swallow" high-speed particles and bring them to a 
gentle stop without causing damage to either the particles or itself.

On each of Stardust's first two orbits, engineers deployed one side of 
the collector to catch interstellar dust, composed of tiny grains of 
material from beyond the solar system. Scientists can use these grains 
to study the composition of the stars where the particles originated.

The encounter with Wild 2 took place on Jan. 2, 2004. Engineers had 
opened the collector nine days earlier and had pointed the unused side 
in the direction of the comet's coma. When Wild 2 hurtled by Stardust at 
a relative speed of 13,684 mph, particles whooshed into the aerogel.

The spacecraft's cameras captured 72 images of the comet's surprisingly 
craggy cliffs, house-size boulders, pinnacles, craters and overhangs, 
and 20 jetlike "geysers" spewing gas and dust into space.

Despite the high-speed flyby, however, it was a not a particularly 
violent encounter in cosmic terms, said University of Maryland 
astronomer Michael F. A'Hearn, Deep Impact's lead scientist. The 
relative delicacy of the maneuver gave Stardust an entirely different 
perspective on comets.

"What came out from the [Deep Impact explosion] was small stuff," 
A'Hearn said in a telephone interview. "We think that on the surface the 
particles are bigger, and that's what you get from the outgassing 
[geysers]. It's a gentler process."

A'Hearn said that if the aerogel holds the particles together, it will 
"give us a clue" about whether large pieces of comet were composed of 
homogeneous organic, silicate or ice compounds before lumping together 
in heterogeneous "aggregates," or whether the pieces of comet were a 
chemical hodgepodge from the beginning.

If everything goes according to plan, the Stardust spacecraft will 
release the 101-pound return capsule about 68,000 miles from Earth, then 
fire its thrusters to put itself into orbit around the sun. Four hours 
later -- at 4:57 a.m. Jan. 15 -- the capsule will arrive at the top of 
Earth's atmosphere at an altitude of 410,000 feet.

Fifty-two seconds after that, the capsule will have dropped halfway out 
of the sky, and the heat-shield temperature will peak at 365 degrees 
Fahrenheit. After another minute, a drogue parachute will deploy at an 
altitude of 20 miles. The main chute will bring Stardust down the last 
10,000 feet to the ground.

"Our return capsule is very small but very rugged," said project manager 
Tom Duxbury of the Jet Propulsion Laboratory, and it is unlikely to be 
critically damaged by the descent. Helicopters will fetch the capsule 
and bring it to a Dugway "clean room" where the sample collector will be 
removed.

The collector will then be flown to Houston's Johnson Space Center and 
taken to a specially prepared laboratory. "By [physically] having the 
samples, we can look at a comet in a completely different way," Brownlee 
said. "We'll get all kinds of details that you would never dream of 
having with remote sensing."

http://www.washingtonpost.com/wp-dyn/content/article/2006/01/08/AR2006010800723.html
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