The Secrets Within Cosmic Dust | Science | Smithsonian
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The NASA mission, called Stardust, brought back the only material—other than moon rocks—taken directly from a extraterrestrial body. (NASA)

The Secrets Within Cosmic Dust

Dust captured by a spacecraft from a comet's tail holds clues to the origin of the solar system

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At the threshold of a sterile lab at NASA's Johnson Space Center in Houston, I pull on a white clean-room suit, a surgical cap and mask, booties and latex gloves. My host, a mineralogist named Mike Zolensky, swabs my digital voice recorder with alcohol to remove flakes of skin and pocket lint. He doesn't want any detritus to contaminate the precious dust in the room.

Once inside, Zolensky retrieves a palm-size glass box from a cabinet. The box holds a rectangular chunk, less than two inches across, of eerily translucent material. I lean in and squint at it but can't quite focus on anything. Zolensky turns off the lights and hands me a laser pointer. The red beam reveals thin streaks in the chunk that start at its surface and penetrate fractions of an inch, like the traces of tiny bullets. "Those are the comet impacts," he says. "It's beautiful to look at."

The tracks were made during the world's first—and only—attempt to chase a comet and bring a bit of it home. The NASA mission, called Stardust, sent a spacecraft to Comet Wild 2 (pronounced "VILT-too") on a seven-year journey that ended in 2006. It brought back the only material—other than moon rocks—taken directly from an extraterrestrial body.

Scientists expect the comet dust to yield clues about how our solar system and planet were formed. Earth has been through so much in its 4.5 billion years—volcanoes have erupted, mountains have risen and fallen, heat and water have chemically transformed rocks time and again—that scientists have trouble getting their hands on the earliest possible geologic evidence in what might be called showroom condition. It has all been buried or altered. In contrast, comets, which are about the same age as Earth, are pristine relics. "There's nothing left on Earth's surface that's nearly as old as these bits of crystals and minerals," says Carlton Allen, astromaterials curator at the Johnson Space Center. A comet, he continues, harbors the "starting material for our solar system, the stuff that came together to make everything we see."

Comets originated on the outskirts of a vast cloud of gas and dust that coalesced into our solar system more than 4.5 billion years ago. Crystals of ice far from the Sun gradually combined with dust to form trillions of comets, which have orbited slowly around the Sun ever since in a deep freeze far beyond Neptune. Only when gravity from a nearby star or some cosmic disturbance nudges them do they approach the Sun; then we see streams of gas and dust as the ice vaporizes—the signature tail.

The most famous, Comet Halley, is barely ten miles across, but it produces a beautiful tail tens of millions of miles long when its looping orbit brings it between Earth and the Sun every 76 years. (Earth will see it again in 2061.) Asteroids, too, are ancient, but most orbit between Mars and Jupiter, where the Sun has baked them for billions of years. Most of them are dense and rocky, some even metallic, shedding pieces that sometimes land on Earth as meteorites.

The travel plan of NASA's Stardust sounds like a Buck Rogers adventure. Swing far past Mars to within 150 miles of Comet Wild 2, which streaks by at 13,700 miles per hour. Catch fragments without destroying them. Travel back to Earth and drop a capsule into the atmosphere for a blazing nighttime re-entry over Utah. Parachute to the ground without crashing. "We were scared up to the last second," Zolensky says. "We still can't believe it really worked."

For all the impressive rocketry, the mission's most significant technology may have been the strange substance that caught the comet dust. Called aerogel, it's the lightest solid ever created. A slab the size of an average person would weigh less than a pound. A scientist once let me crumble some in my hand; it felt like brittle talc. Yet aerogel is tough. It's made of silica compounds arranged in an elaborate web that can snare particles plowing into it at up to six times the speed of a rifle bullet. The spacecraft carried two aluminum collecting trays shaped like oversize tennis rackets. Each tray held about 130 rectangular pockets filled with aerogel blocks smaller than ice cubes.

In the clean room, Zolensky returns the aerogel chunk to the storage cabinet and removes a bolted aluminum case. Inside lies an entire comet collection tray. It's worth about $200 million. "I'm always nervous about this part," he says. "I try not to drop it." He places the case on a stainless steel table, unbolts it and lifts the lid. "Air flows down from the ceiling, so the rule here is that we never stand over the tray," he tells me. Even with the mask on, I am acutely aware that I must not sneeze.

The aerogel cubes, tinged lu­minescent blue, look like puffs of breath exhaled on a winter morning and frozen solid. The laser pointer reveals scores of tracks marring the cubes' surfaces and interiors, some like sharp pinpricks, others like splaying roots.

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