Much larger cubes of aerogel are 205 million miles from Earth, flying aboard a remarkable NASA spacecraft called Stardust. The probe, which Brownlee oversees from the University of Washington in Seattle, is hurtling toward a meeting with comet Wild 2 (pronounced in the German fashion as "vilt" 2). Aerogel and Wild 2 should meet each other this January. When Stardust flashes past, dust shed from the comet will strike the aerogel at a speed some six times that of a rifle bullet. The aerogel will snare the particles, and two years later Stardust will return to Earth. The precious payload—comet flecks embedded in aerogel and protected by a heat-resistant casing—will parachute to the desert floor in Utah.
Stardust is one of as many as four missions that promise to finally unveil the mysteries of what comets are made of and how they work. Each probe will bring unusual, often untested technologies to the task. One spacecraft, called Deep Impact, will attempt to blast a huge hole in a comet; another is intended to land on a comet and scoop up parts of its surface. Why spend all this time, money, and thought to study objects that astronomers often describe as little more than "dirty snowballs"? Because of what lies frozen inside. Comets are the most primitive objects in the solar system, preserved samples of the raw ingredients that formed Earth and the other planets. "We wouldn't be here without them," says Don Yeomans of NASA's Jet Propulsion Laboratory, in Pasadena, California. "We may well owe our existence to these things."
Comets crashing into Earth more than 4 billion years ago may have delivered much of the water that makes life here possible, Yeomans says. Those impacts may also have seeded Earth with carbon-rich components, possibly creating the chemical conditions that led to the origin of life. Comets even allow us to retrace our history to the time before the sun and the planets were born. "We think comets are made of interstellar grains, perhaps formed around millions of stars in the Milky Way," Brownlee says as he handles a piece of aerogel. "If that's true, our mission will return a nanobucket of star dust to Earth. . . . This is a treasure hidden in our own backyard. It's like going out and getting a piece of the galaxy."
Stardust and Deep Impact will be the first two probes to test these sweeping ideas. As Stardust heads back to Earth with its flecks of Wild 2, Deep Impact will be getting ready to launch an 820-pound copper bullet into comet Tempel 1. The results of those fireworks—scheduled, appropriately enough, for the Fourth of July in 2005—should expose the interior of a comet for the first time. Toward the end of the decade, another NASA mission may swing past as many as three comets. The more encounters the better, because no two comets are alike. "Comets all have different personalities," says Robert Farquhar of the Johns Hopkins University Applied Physics Laboratory. But in order to see those differences, he says, "you've really got to get up close." Finally, in 2014, Rosetta, a European mission, will attempt a landing on the comet Churyumov-Gerasimenko.
Our history of trying to get close to comets has been dismal: It has happened only four times, and fleetingly. A flurry of spacecraft, led by a European craft called Giotto, flew past Giacobini-Zinner in 1985 and Halley's comet in 1986. Then, except for a minor encounter by Giotto with comet Grigg-Skjellerup, 15 years passed. "For a long time, comet missions did not have pizzazz," says Benton Clark, chief scientist for space exploration systems at Lockheed Martin, near Denver. "NASA was still in the business of conquering planets and taking grand tours. Comets were strange little objects, and there were only a handful of comet experts in the world."
Then in 1996 and 1997 two bright comets passed spectacularly close to Earth, and comets became instant celebrities. "Hale-Bopp and Hyakutake really raised the interest level," Clark says. Comet probes also fit neatly into NASA's new budget commands of faster, better, and cheaper. Comet researchers got a bonus in 2001 when NASA's Deep Space 1 probe zipped by comet Borrelly. Although the mission was flown to test new types of propulsion and navigation systems, the technology worked well enough to get Deep Space 1 to Borrelly.
Photos of Borrelly and Halley exposed a startling truth about comets: They're darker than charcoal. On the outside at least, the so-called dirty snowballs look like all dirt and no snow. Our handful of decent images show rugged black lumps spouting jets of gas and dust that erupt from their interiors. Close to the sun, the material stretches into a long, tenuous tail that reflects sunlight. Only then does a comet shimmer serenely in the sky. "What we see in a picture of a comet is not a comet at all," says Brownlee. "It's the comet coming apart. The real comet is the black rock in the middle."
Astronomers suspect the black coatings are full of carbon-rich compounds. It's anyone's guess, however, whether the stuff is powdery or rock solid. Without enough data to draw on, scientists have to rely on their imaginations when describing the surfaces of comets. Astrophysicist Carey Lisse of the University of Maryland compares them to "smoking parking lots" tortured by exposure to sunlight. "Their surfaces get baked and roasted by the sun until they are darker than asphalt. Every once in a while they crack, and flakes and chunks come off."
Brownlee has been trying to extract grand stories from tiny space particles since the mid-1960s. A highlight of his graduate student days at the University of Washington was working on collectors that flew on Gemini 10 and Gemini 12. Designed to detect micrometeorites, the metal-coated glass plates registered no impacts during six hours outside the capsules. "We saw lots of astronaut urine," says Brownlee, glancing at his mischievous moon painting. "It crystallized into flakes when they dumped it and formed soap-ring structures on the plates. People thought the flux of particles from space would be a million times greater than it is and that the first satellites would erode away within a year. Obviously, they didn't."
Still, there is plenty of stuff to run into up there—about 40,000 tons of extraterrestrial dust wafts down onto Earth every year. Brownlee has analyzed hundreds of such particles. He estimates that about one-quarter of them were shed by comets. The rest came from asteroids, which collide and spray dust through the inner solar system. The asteroid fragments are interesting in their own right, but only the comet bits contain intact, unaltered samples of the cold, dark nebula that eventually evolved into our solar system.
The particles that make up comets began life as the dying breaths of previous generations of stars. When a star runs out of hydrogen fuel in its core, it expands into a bloated red giant. Its atmosphere cools off drastically, allowing minuscule grains of carbon, silicon, and oxides to form. The grains are the size and color of black smoke particles, about four-millionths of an inch wide. Born in red giant stars or supernovas, they drift through the galaxy and eventually mingle with interstellar clouds of gas and dust, the places where new stars and planets arise.
On the outskirts of our young solar system, such dust grains stuck together with tiny grains of ice—now known to be common throughout space. These granules gradually accumulated into blobs of icy, dusty fluff as large as big cities. Trillions of these infant comets formed in the region where Jupiter, Saturn, Uranus, and Neptune now orbit. When the comets drifted near one of the giant planets, gravitational slingshot interactions propelled many of them to the Oort cloud, a zone of comets far beyond Pluto's orbit. Most are still there, orbiting extremely slowly until a disturbance—from a passing star, for instance—nudges them back toward the sun.