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Comet, Come Home
February 04, 1999

It’s not hard to see why the launch of a probe to capture a scoop of stardust has NASA engineers waxing romantic. They plan to send off "Stardust" to the music of Hoagy Carmichael’s famous 1929 melody. And scientists hope the first mission to bring samples of stardust back for study in earthly labs will contribute as much to astronomy as Carmichael’s song did to American music.

The Stardust spacecraft, to be launched on a Delta II rocket from Cape Canaveral, Florida, is the first mission dedicated to a comet and will be first to return extraterrestrial material from outside the orbit of the moon. If all goes as planned, the probe will encounter Comet Wild-2 (pronounced "Vilt-2") in January of 2004, when the comet is 242 million miles from Earth.

"And then in 2006 the spacecraft will return to earth and drop a capsule back to earth with these precious samples containing the material from the comet," explains Marc Rayman, a chief engineer at NASA’s Jet Propulsion Laboratory in Pasadena, which is managing the mission. "And this will give us tremendous insight into what the comet is made of and therefore what the early solar system was like."Scientists say Wild-2 is the ideal target because it spent most of its history as a long-period comet in the far reaches of the solar system. In 1974 a close approach to Jupiter knocked it into an orbit that comes close to earth. Thus, the particles Stardust captures will represent the Solar System at its earliest.

"The particles that are going to be captured are roughly the size of the dust particles in a cloud of cigarette smoke," says comet expert Donald Yeomans, a senior scientist at JPL. And Stardust will capture the comet dust in a material scientists describe as "solid smoke." Aerogel-- a suspension of silicon in air -- is as seemingly magical as its mission. "It’s 99 percent nothing... it’s about as close to air as you can get and still have something," Yeomans says.

But the beauty of this stuff is because it’s mostly air and it’s a very low density matter the particles from stardust mission will hit this Aerogel at a velocity of 6 kilometers per second-- that’s about 6 times the velocity of a bullet shot from a highspeed rifle-- and because of this underdense material the particles will actually be trapped in the Aerogel without destroying themselves."

The temperature of the impact will melt the silicon, trapping the precious particles in glass. Poetically, Aerogel was invented in 1933-- Hoagy Carmichael’s era-- and developed for space missions by Dr. Peter Tsou at JPL.

"Once the dust impinges on the Aerogel it penetrates and then these collectors are closed up and sealed while still in the vacuum of space," says Rayman. The 101-pound sample return capsule will end its mission with a landing at the the U.S. Army’s Dugway Proving Grounds at the Utah Test and Training Range near Salt Lake City in January, 2006.

The samples "will be opened up in laboratories where the conditions are carefully controlled and we can be sure what we’re studying is from out there and not just some dust picked up from the laboratory floor," Rayman adds.

Reflecting NASA’s mandate for cost-effective missions, Stardust won’t waste any time or space-- its Aerogel sampling device is two-sided. "It’s not just comet dust they’re going to capture, on the way to the comet they’re going to capture particles in inter-planetary space, stardust, hence the name of the mission, and then that slab will be turned over when they get to the comet and then they’ll run through that material and so they’ll have both inter-planetary dust particles and cometary dust particles to study when they get back."

Indeed, if Stardust succeeds, it will be a stellar credit to NASA’s Discovery Program of low-cost, highly focused science missions. The mission is both on-time and on-budget at a total cost of $165.6 million, not including the launch vehicle.

NASA scientists say the agency’s interest in comets is a direct result of the public’s interest. Comets "may have been the source of water on earth and some of the organic molecules," says Rayman. "It’s possible these organic molecules were delivered to earth by comets early in the solar system’s history. Certainly Hale-Bopp and Hyakutaki over the last few years gathered a great deal of attention as did Comet Halley. And yet we’ve sent very few spacecraft to comets in years past."

But according to Yeomans that’s now changed for good, in part due to movies like "Deep Impact." If an interesting-- or hazardous object appears-- he’d like to be ready to investigate it on short notice. "Now beginning in 1995 we have actually seven missions to comets and asteroids, that will fly by 13 different objects in as many years. It’s a booming business for comet and asteroid studies... they don’t hit the earth very often but when they do, it’s catastrophic."

Yeomans sees comets and asteroids as not only a possible threat, but also as a future boon for humans. He foresees them being mined for resources in the coming millenium. "In the 21st century when we begin to colonize the inner solar system asteroids will likely supply the minerals to build structures in space and comets because they are mostly water will supply water that is necessary to sustain life, probably supply the hydrogen and oxygen used for rocket fuel, so in the next century comets will be the watering holes and gas stations of the inner Solar System."



by STN2


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