Why go through so much effort to collect comet dust? “Comets like Wild 2 are leftovers from the creation of the solar system,” explains Brownlee, “and are believed to be the solar system’s most unchanged, pristine bodies. This means that their materials, from which all of the planets formed, are the oldest and most basic available for study.”
Like balls in a pinball game, comets tend to get batted around, either hitting planets or being tossed between them. Many are eventually destroyed or catapulted out of the solar system. But Wild 2’s relatively short exposure time in the inner solar system has left it fairly intact. And, importantly, it is within reach.
“We chose this comet because we knew we could get to it with our propulsion capability and get back,” says Brownlee. “But designing and building the spacecraft was a huge challenge.”
The Stardust spacecraft weighs only about 770 pounds and is the size of a desk, except for its solar panels, which extend several feet in the front and back. (The use of solar power was one of the guidelines of the mission.) “The spacecraft needed to operate in harsh environments —extreme heat, extreme cold—using only solar power, farther from the sun than any spacecraft has ever been,” says Brownlee. “It’s really a formidable challenge to foresee all problems that might arise, and it’s very difficult to fix things in space. The simplest things can do you in. That’s why missions are so expensive. The quality control and attention to detail are incredible.”
Beyond the equipment itself, there was the challenge of collecting samples from a comet passing the spacecraft at a speed of 22,000 kilometers per hour. The team had to design an orbit for the spacecraft that brought it as close to the comet as possible without being destroyed—the optimal distance being a source of much debate —while traveling at the same speed as the comet. And, of course, the orbit had to allow for the capsule’s safe return to Earth. To accomplish this, the spacecraft made three loops around the sun, its trajectory intersecting with the comet on the second loop.
That daring encounter took place on January 2, 2004. Brownlee, who watched from the control room of NASA’s Jet Propulsion Laboratory, recalls it being “a real white knuckle experience.”
“We were basically trying to accomplish a miracle, trying to make the orbital paths of the two bodies as similar as possible,” says Brownlee. “There are a lot of things we had some level of control over, but we had no control over the comet. We had to get close enough to collect a significant number of small particles, but we didn’t want the spacecraft to be hit by any rocks larger than one centimeter.” At its closest point, the spacecraft was 243 kilometers from the comet.
The fly-by was a huge success. The spacecraft extended its collector, resembling an oversized tennis racket, to capture thousands of comet particles. The particles became embedded in aerogel, a highly porous material used in the collector. A spacecraft camera also took photos during the encounter, which Brownlee describes as “phenomenal.”
There have been other moments of drama during the seven-year mission. At one point, a solar flare blinded the cameras used for controlling the spacecraft’s navigation, sending Stardust into a spin with the solar panels pointing at the sun. There was no signal from the spacecraft for at least a day, causing great concern. But the signal eventually returned and Stardust suffered no permanent damage. A similar event occurred more recently.
“Disaster is always right around the corner,” says Brownlee. “With a spacecraft, things can go completely wrong at the snap of a finger.”
Brownlee can breathe easier when the mission completes its final hurdle— the capsule’s safe return to Earth—in January. Although far less daunting than the fly-by in space, the return has required considerable planning.
“The capsule will come in from the west at a low angle, aimed at a dry lake area just west of Salt Lake City,” says Brownlee. “We will release the capsule four hours before. It will start spinning and release two parachutes along the way.” Folks in northern Nevada will be able to see the capsule’s return as “a fireball across the sky” if they happen to be awake at 3 a.m., when the capsule reenters the Earth’s atmosphere.
That will be a banner moment for scientists around the world who have
been eager to analyze the comet dust from Wild 2. While the samples from this comet are certainly not the first extraterrestial materials to be collected and studied, they are valuable because their source is known.
“Most other materials are orphans—we don’t know where they came from,” explains Brownlee. “One of our goals for these samples is to provide context for our existing samples.”
But first, Brownlee looks forward to witnessing the completion of a mission that began seven years ago.
“The capsule’s return is the easiest thing we’ve done, but there are still risks,” he says. “I definitely plan to celebrate when this is over.”