DUGWAY PROVING GROUND, Utah, 10 Jan. 2006. 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 failed with a similar mission in September 2004, when the Genesis mission crashed into the desert floor in the same location. The probe had collected particles from the solar wind, and was supposed to deploy a parachute and be snagged in mid-air by a helicopter. But its parachute never deployed, and the craft hit the sand at 100 mph.
For the current mission, 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."
Source: The Washington Post
