Close Encounter with a Comet

For all its spectral magnificence, the tail of a comet is about as close as you can get to nothing at all, a banner of dust so tenuous that a cubic mile's worth wouldn't fill a shoebox. Yet that near nothingness holds many secrets. Comets are leftovers from the creation of the solar system 4.5 billion years ago; they also delivered organic chemicals and water--the crucial building blocks of life--to the young Earth. Scientists would love, therefore, to get a bit of comet into the lab for analysis.

Now it looks as though they will. On Feb. 6 a remarkable space probe called Stardust is scheduled to take off for a January 2004 encounter with Comet Wild (pronounced Vilt) 2. With a tennis racquet-size collector, Stardust will snatch dust particles from the comet's tail as it flies by and then, in an audacious interplanetary maneuver, return to parachute its precious cargo to Earth two years later.

What's more, the entire mission is coming in at an absurdly cheap (for NASA) $165.6 million. Stardust is part of the space agency's "fast and cheap" Discovery series (Mars Pathfinder with its robot rover was another). Like Pathfinder (and unlike the instrument-packed billion-dollar probes of the 1970s and '80s), Stardust is as stripped down as it can be. The 848-lb. spacecraft carries just solar panels, a camera, a radio, a spectrometer to analyze sunlight bouncing off the comet, a few sensors and the all-important sample-collecting system.

The other instruments had to be light, but the collector had to be ingenious. Although the probe will loop two times around the sun to help it match orbits with the comet, Stardust and Wild 2 will still shoot past each other at nearly 4 miles per second, 10 times as fast as a speeding bullet. In order to catch dust particles without disintegrating them, Jet Propulsion Lab engineer Peter Tsou first thought of making a trap out of Styrofoam; he figured dust would bury itself harmlessly inside. Unfortunately, says Tsou, "cosmic dust particles are so small that on Styrofoam, I wouldn't be able to see them."

So he switched to aerogel, an ultra-lightweight glass foam that's 99.8% air. It resembles nothing so much as solidified smoke. The aerogel is packed into a collector that resembles a circular ice-cube tray about a foot across. En route to Wild 2, one side will trap dust that's wafting in from beyond the solar system--another item of great interest to astronomers--and once there, it will flip to scoop up comet dust.

If Stardust makes it back safely, the collector, nestled in a sample-return capsule, will drop into the atmosphere and parachute to ground on the salt flats of central Utah. The scientists haven't quite perfected their techniques for extracting the particles, some of which will be smaller than the width of a human hair, but they expect to have it down by 2006. They may not have the luxury of a pure sample: a perfect seal would have been too expensive, which leaves a remote chance that some earthly dust could contaminate the aerogel on re-entry, making analysis more complicated.

It's also conceivable that a tiny visitor could hitch a ride in the aerogel. But while a few astronomers have suggested that bacteria and viruses may exist in or on comets, Stardust scientists aren't worried about a real-life Andromeda strain. Some 400,000 tons of extraterrestrial particles drift to Earth every year. They're not ideal for research because you can't be sure which ones come from comets and which don't. But we've been exposed to them for millions of years, and any harm has long since been done. Observes Stardust team leader Don Brownlee of the University of Washington: "If you eat lettuce tonight, you're probably eating several comet particles."