These comets dive inward on cigar-shaped orbits that take millions of years to complete. But as they pass closer and closer to Jupiter, the planet can fling them out of the solar system entirely or jostle their orbits into smaller loops. Comet Wild 2, the quarry of Stardust, became a Jupiter-family comet in 1974 when the gas giant's gravity shrank its orbit and corralled it inward, toward Mars and Earth. Because Wild 2 has ventured close to the sun only five times since then, Brownlee thinks it is a fairly fresh comet, with less of the black crust that insulates older bodies that have been cooked repeatedly by the sun.
Stardust will study Wild 2 with a camera and other instruments, but its main goal is to snatch thousands of the motes streaming off the comet's nucleus. "The other missions are studying the paintings," Brownlee says. "We're studying the paint. It's like taking the Mona Lisa and obliterating it with sandpaper."
Peter Tsou has spent most of the 29 years
he has worked for NASA in search of comet pieces. As
the deputy principal investigator on the Stardust
mission, he personally developed the form
of aerogel used in the tennis-racket-like collector that should
pass through the tail of comet Wild 2 in January
Photograph by Tom Tavee
Getting a peek inside a comet is every bit as challenging as deciphering Mona Lisa's enigmatic smile. Peter Tsou, a planetary scientist at JPL, has labored since 1982 to collect dust from a comet; Stardust was his 13th proposal. In the beginning, the idea of landing on a comet, digging up a sample, and then flying it back was outrageously expensive, potentially costing billions of dollars. Tsou realized that "a comet is a self-excavating body. You don't have to land on one to get a sample and bring it back to Earth." Unfortunately, no one knew exactly how to trap the particles whizzing by. Comet dust particles would hit a canister or containment device with such force that they would burn up or smash themselves into molecule-size bits.
"How do I catch something moving many times faster than a speeding bullet without melting or vaporizing it?" Tsou asked colleagues. "I talked to a lot of scientists. They just laughed at me." Eventually, he figured out that he would need a shock-absorbing material to slow the particles down gradually—perhaps a load of fibers that could absorb the particle whole, just as a bale of hay can stop a bullet without flattening it.
First he tested Styrofoam, using a high-powered gun at NASA's Ames Research Center in Moffett Field, California, to slam tiny aluminum balls into it. "The first time it worked, I ran up to my boss and said, 'I caught one!'" Tsou remembers. But he decided Styrofoam would not work for comet missions, because it would disintegrate under the sun's UV radiation in about a week. And because Styrofoam is opaque, comet dust would be impossible to find. To find bits of embedded dust far smaller than the thickness of a hair, scientists would need a collecting material that was transparent.
Then in the late 1980s, while on a visit to Los Alamos National Laboratory in New Mexico, Tsou saw a piece of aerogel sitting on a researcher's windowsill. Aerogel was first created by chemist Samuel Kistler in the 1930s and offered by the Monsanto Company for paints, cosmetics, and toothpaste. Decades later, physicists at Los Alamos discovered that it worked well as insulation for fusion research and for a classified weapons program. When Tsou tried it on small high-speed particles, it did the trick. The particles formed obvious tracks as they plowed to a stop in the clear gel. The tracks were 100 to 1,000 times as long as the widths of the particles, and they looked like arrows pointing to the flecks at the end.
Tsou worked with scientists at Lawrence Livermore National Laboratory in California to make the aerogel even lighter to improve its particle-catching ability. After years of testing—aerogel flew to the space station Mir and in the open cargo bay of the space shuttle—Tsou was ready to propose the Stardust mission. The aerogel on Stardust is as light as two ounces per cubic foot—so light that a block the size of a king-size mattress weighs five pounds. "It's truly a miracle material," says Brownlee. "It's beautiful. Sometimes it's so clear that it's hard to find it on your desk."
When Stardust approaches Wild 2 in a few months, the spacecraft will raise a tray of ice-cube-size chunks of aerogel on an arm shaped like a tennis racket. Grains of comet dust will slice into the gel and briefly heat up tiny streaks to hundreds of degrees as they screech to a stop. Earlier, in 2000 and 2002, the craft positioned aerogel collectors on the opposite side of the arm to snare particles of interstellar dust, suspected to be as small as one-tenth the size of comet grains. That dust should be younger than comet dust, and Brownlee hopes to compare the two types.
When the particles come back to Earth, researchers will probe them with laser light and extract some with tungsten fibers or microtweezers that will ease particles out of the aerogel along their impact tracks. Labs around the world will then set about analyzing the samples. If they look anything like the comet dust that Brownlee thinks he sees drifting into Earth's atmosphere, the particles should resemble exploded kernels of popcorn.
Anytime scientists talk about bringing samples from space back to Earth, they worry about the danger of contamination. Michael Zolensky, curator of the Houston facility where NASA will store Stardust's aerogel, says the dust poses no danger of unleashing a real-life Andromeda Strain. "Comet grains enter our atmosphere day and night, less altered than the ones we'll get from the aerogel," Zolensky says. Any viruses, spores, or other biological material should be sterilized by the heat of their entry into the aerogel.
The bigger unknown is whether Stardust's comet samples will make it back to Earth at all. When the spacecraft swings by our planet in January 2006, nearly seven years after launch, it will have to release the protective capsule containing the aerogel on precisely the right trajectory. If it comes in too shallowly, it will skip off the atmosphere; if it arrives too steeply, it will burn up. An error greater than eight-hundredths of an angular degree would most likely spell doom. That's the equivalent of aiming a golf ball accurately enough to make a 250-foot-long putt. "We will have had seven years of experience and practice in pointing the spacecraft," says Tom Duxbury, the Stardust mission manager at JPL. "We can meet this level of accuracy."
Still, Brownlee's mouth will be as dry as the dust of the Utah desert on January 15, 2006. "It's either going to be a great success or a total failure," he says, putting his forehead on his desk. "We'll know when we open the box."
At the University of Maryland in College Park, a huge football stadium looms over the brick building that houses the astronomy department. That seems appropriate, because some of the scientists there intend to gouge a stadium-size crater out of comet Tempel 1. They're taking the brute-force approach to understanding comets.
Almost entirely air (99.8 percent), aerogel begins
life as a silica dioxide gel similar to Jell-O. The gel
is dried so that the structure becomes
spongelike and porous.
Both aerogel and glass are made
from the same compound, but glass is 1,000
times as dense.
Aerogel has a smoky blue appearance
and extremely high insulating properties that can
withstand temperatures up to 3,600° F.
Photograph by Tom Tavee
The mission, called Deep Impact (the name was chosen before the movie came out), could not be more different from the delicate catch-and-return strategy of Stardust. "Deep Impact is a boy mission. None of this namby-pamby go in, rendezvous, and have a relationship with the comet. It's going to be really cool, really fast. We get to put a nice big hole in this thing, go kablooey, and see what the heck is inside it. So it has a little aspect of being 10 years old, throwing a rock at something, and breaking it up," says Carey Lisse, one of the mission's lead scientists.
This gung-ho attitude makes some researchers cringe. "I realize the public identifies with it as a Buck Rogers-type thing," says one scientist who asked that his name be withheld. "But I'm not sure it will tell us more than other missions are already telling us, and I'm not sure it will work." But the Deep Impact team is convinced an aggressive approach is justified, because scientists simply do not know what lies beneath the surface of a comet. The inner material might be as hard as concrete or as soft as a new snowbank. The black, sunbaked crust covering the comet could be a quarter-inch thick, or even a dozen feet. Smaller "cometesimals" might loosely join to make a nucleus riddled with weak seams, or the whole thing could be a coherent icy dirt ball. "It's like we're trying to see under a car cover, and we don't know whether it's a beat-up old car or a brand-new Jaguar," says JPL planetary scientist Jacklyn Green.
