12 Jan 2006 USA: Stardust of Yesterday
Spacecraft bringing comet dust back to Earth
By David E Williams
CNN – NASA's Stardust space probe is racing toward Earth, carrying a tiny payload of cometary and interstellar dust – particles that scientists believe are leftovers from the creation of our solar system.
If all goes as planned, Stardust will release the 100lb capsule carrying the samples at 1 am ET on Sunday. It would enter Earth's atmosphere about four hours later and parachute to the ground in Utah at 5:12 am ET.
If there is a delay, scientists will settle in and wait for Stardust to make another orbit around the sun – a trip that would take about another four years.
Stardust has been in space for just shy of seven years and made three laps around the sun, traveling 2.88 billion miles (4.63 billion kms).
The highlight of the mission came on January 2, 2004, when Stardust flew within 149 miles (240 kilometers) of the comet Wild 2.
Stardust captured particles from the comet's tail in a tennis-racket sized collection unit made with blocks of aerogel, a strong, lightweight silica glass that is 99.8 percent air and looks like frozen smoke.
"The fundamental reason for this mission is that we are collecting what we believe are the best preserved samples of the formation of our solar system and they are preserved because they formed these comet bodies beyond the major planets out beyond Neptune," said principal investigator Don Brownlee of the University of Washington.
"So we're just using the comet as a storage device," he added.
Scientists believe comets are icy, rocky debris left over from the beginning of the solar system 4.5 billion years ago. Wild 2 is thought to come from the Kuiper Belt, a band of debris that orbits the sun beyond Neptune. They expect that analysis of the Stardust samples will help them better understand how the planets formed and evolved.
Stardust has already transmitted photographs and other measurements that provided some valuable data.
"The appearance of the nucleus of this comet was kind of surprising, and different from other comet nuclei that have been recording in spacecraft type detail, so that was quite exciting," said David Jewitt, an astronomer at the University of Hawaii. Jewitt and astronomer Jane Luu discovered the Kuiper Belt in 1992.
Jewitt said that Kuiper Belt objects appear as faint, unresolved points of light, even in the world's largest telescopes. He said astronomers can count them and track their orbits and even learn some things about their surface composition, but they are too far away to learn about their geography.
"The dust that it hopefully picked up in the aerogel collectors in some sense is a sample of materials from the Kuiper Belt so that's very exciting, too, because we don't have the technology to go out to the Kuiper Belt grab some stuff and then come back with it. It's just too hard to do," Jewitt said.
Once Stardust ejects the capsule, there will be no turning back. Brownlee said the capsule would spin towards Earth on a ballistic trajectory with no power and no controls.
The capsule will enter the atmosphere over the Pacific Ocean, traveling at a speed of nearly 29,000 miles per hour.
At 105,000 feet, a small parachute will deploy to begin to slow the craft, and at 10,000 feet the main parachute will open to bring it in for a soft landing at the Air Force's Utah Test and Training Range southwest of Salt Lake City. The training range is a little bit bigger than the state of Delaware.
"This is the fastest return vehicle that has ever been brought back to Earth," said Mission Systems Manager Ed Hirst. "So bringing it home for the first time is the only way to test a system like this. You do testing on the ground to the extent you can so there is some residual risk that something could happen on return. We think the probability of that is very low at this point."
NASA's last sample return mission did not go as planned. In 2004, the parachutes on the Genesis spacecraft didn't open, and it crashed into the ground at full speed. Stardust mission managers say they're confident that won't happen this time.
Three helicopters will be waiting outside of the 27-by-47mile (44-by-76km) landing zone to recover the capsule within 10 minutes of the touchdown, NASA says. If bad weather grounds the helicopters, the search will be conducted in ground vehicles.
Once the capsule is found, it will be transported to a "clean room" at the nearby Dugway Proving Grounds, where it will be opened and the canister containing the samples will be removed.
Eventually, the canister will be moved to a special repository at NASA's Johnson Space Center in Houston, where the samples will be housed on a long-term basis.
Fast results, long-term study
Scientists expect to find a little less than a thimble-full of material in the aerogel.
"The biggest particles that hit the collector should be quite large, should be somewhere pushing a millimeter in size, which may have gone all the way through the collector," Brownlee said. "So anyway, these small particles, if they do not fragment, they produce long carrot-shaped tracks in the aerogel and you can see those."
He said that those tracks should be visible to the naked eye. Brownlee said scientists would have some results, such as the number of particles recovered, within days and more detailed findings within weeks.
"We'll be able to say some things almost instantly. One of the big questions is do comets contain minerals that contain water, called hydrated silicates and under an electron microscope you can identify those practically instantly," Brownlee said.
But he said it would take years to fully study the samples. Brownlee likened the process to the ongoing study of rocks brought back from the Moon.
"The last Apollo mission was 1972. And people are still discovering very exciting things on the Apollo samples," he said. "Samples are a resource that are unending. And so unless we consume all the samples, they will certainly be studied decades from now."
He said that some of the procedures that will be used on the samples had not even been invented when the Stardust mission was proposed.
CNN's Kate Tobin contributed to this report.