Scientists trying to catch a comet

Sample: A space probe aims to gather dust from the speeding extraterrestrial snowball - 242 million miles from Earth.

Medicine & Science

December 29, 2003|By Frank D. Roylance | Frank D. Roylance,SUN STAFF

Want to learn what comets are made of? Just fly a spacecraft to a passing comet, sweep up some of the dust that swirls around it and fly it back to Earth.

It may seem fanciful, but that's exactly what a team of scientists from NASA's Jet Propulsion Laboratory, Lockheed Martin and the University of Washington are attempting this week.

On Friday, the 770-pound spacecraft is expected to zoom to within 186 miles of the icy nucleus of comet 81P/Wild 2. No easy target, the comet is cruising between the orbits of Mars and Jupiter, 242 million miles from Earth on the other side of the solar system.

Flashing through Wild 2's dusty coma at five times the speed of a rifle bullet, Stardust will open its clamshell-like trap and capture particles of comet dust like flies on flypaper.

In January 2006, Stardust will fly by the Earth again and eject its treasure in a capsule that will streak into the atmosphere and parachute into the waiting arms of scientists in the Utah desert.

"It's a very dramatic mission. We are going to a comet to collect basically pieces from our galaxy [beyond] the solar system," said University of Washington astronomer Donald Brownlee, the principal Stardust investigator.

The payoff begins Friday, as Stardust transmits photographs of the comet's icy nucleus - a three-mile-wide hunk of gravel and ice that appears in telescopes to be throwing off geysers of dust and gas.

Comets have been visited and photographed before. The European Space Agency's Giotto spacecraft snapped pictures of Halley's comet from 12,000 miles away in 1986. NASA's Deep Space I spacecraft flew within 1,400 miles of comet Borelli and sent back pictures in 2001.

But Stardust's images, from 186 miles, should be the sharpest ever made of a comet's nucleus. "We may see cracks and craters, pockets and jets. ... Who knows?" Brownlee said. "We'll see what nature provides for us. It should be interesting."

The Friday flyby may be upstaged by NASA's attempt on Saturday to land Spirit, the first of two rovers headed for the surface of Mars in January.

"It never occurred to me that this would happen," Brownlee said of the coincidence. "It sort of snuck up on me. But our goal is to get the mission done successfully and safely."

Stardust was launched in February 1999. It cruised by the Earth again in 2001 for a speed boost from the planet's gravity, and 13 months ago it flew by asteroid 5535 Annefrank, transmitting pictures that revealed the asteroid to be bigger and darker than astronomers thought.

Since then, blasts of atomic particles launched by solar flares have buffeted the spacecraft. It lost contact with Earth for several days last year after one such outburst, and had its camera temporarily knocked out in the fall by another.

"We worked all these things out, and we're in very good shape" for the flyby Friday, Brownlee said.

Comet Wild 2 (pronouced Vilt 2) is thought to have formed from frozen gas, water and interstellar dust in stars before the formation of the sun and the rest of the solar system 4 1/2 billion years ago. For billions of years it circled the sun in the Kuiper Belt, a region beyond the orbit of Neptune.

Scientists think comets from that region have evaded the warming, vaporization and collisions that have altered matter in the inner solar system, including the Earth, Brownlee said - and thus they may contain "fundamental building blocks" of the solar system.

"Some of the particles will be older than the sun and the planets, and formed around another star," Brownlee said. "We're quite confident of that."

Such interstellar dust has been studied before but never this closely. "It's like having a compact disc in binoculars, from a distance," he said. "You can tell it's round and has a rainbow effect. But if you're going to get any information from it, you have to put it in a CD player."

In 1974, Wild 2 was deflected by Jupiter's gravity and steered into a new orbit between Jupiter and Mars. That made it the perfect target for the Stardust mission - close enough for the spacecraft to reach it, but still far enough from the sun that it will not have been appreciably altered by the sun's heat.

Once the comet dust returns to Earth, scientists expect to find mineral grains, glassy silicates and carbon-based materials. They will be composed of sub-grains smaller than one micron in diameter, one-hundredth the width of a human hair.

"Most people think of these particles as being so small you can't do anything with them," Brownlee said. "But the fact is they're too big for anything we want to do."

Sliced up even further and examined by electron microscopy, their composition and structure should reveal details of the environments they formed in, Brownlee said. That includes "the record of their radiation history, the temperatures they formed in, whether they were sputtered [bombarded] by gas ... ."

"We also expect to see particles that contain the isotopic records of other stars," he said - variants of atomic elements in proportions alien to our solar system.

"The question is what fraction of comets formed from materials around other stars?" he said.

The comet dust will be captured in panels of aerogel, a meringue-like foam, as the spacecraft flies through Wild 2's coma. The biggest risk this week will be the chance - rated at 2 percent - that Stardust will smack into a really big chunk of Wild 2.

"A marble-sized particle would penetrate our meteoroid bumper on the front end of the spacecraft." Brownlee said. "That wouldn't be a happy situation."

On the other hand, he said, "compared to landing on Mars, this is easy."

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