"We have dust," Team Stardust announced triumphantly on Friday.
The Stardust spacecraft successfully flew into the coma of comet Wild 2, gathering dust and taking pictures. The first image was received within an hour of the flyby, and showed a round snowball pocketed with holes and deep caverns. These may be craters caused by other objects impacting the comet, or they could be sinkholes caused by solids turning directly into a gas (a process known as "sublimation"). There also appear to be at least five jets of gas and dust spewing out from the comet’s nucleus.
|During the comet flyby, the closest view yet of a comet at about 149 miles. |
Scientists knew about cometary jets from images taken of the comets Halley and Borrelly. They wondered if the Stardust spacecraft would run into similar jets, but predicted it was not likely to happen.
"We were proved wrong, quite wrong today," says Tom Duxbury, project manager for the Stardust mission. "There were two significant jets that we barreled right through. But as you can tell from our smiles today, we did survive."
Comets are composed of ice, dust, and gas – primitive materials that were the building blocks of the solar system. Comet particles therefore may be able to tell us something about how the planets formed. In addition, comets may have played a major role in the origin of life on Earth, delivering a significant share of the Earth’s water as well as carbon-rich organic compounds.
The comet Wild 2 formed in the Kuiper belt region, which is located past the orbit of Neptune, roughly 30 to 100 AU from the sun. Before 1974, the comet orbited from Jupiter to just past Uranus. But then Wild 2 had a close encounter with Jupiter, and this altered the comet’s orbit. It now swings between the orbits of Mars and Jupiter.
|Icy-rock core of Halley’s Comet, with cometary jets similar to the five jets observed on Wild-2. The jets were not predicted for Wild-2. |
Credit: ESA/Giotto spacecraft flyby
By staying in the colder regions of the solar system until recently, Wild 2 probably has preserved most of its dust and gases. Comets that travel closer to the sun lose their volatile materials over time. The sun’s heat causes the material to boil off into space, producing the coma and long tail we typically associate with comets.
"In this mission, we are going to a comet, but we’re also traveling back in time because we’re collecting things that are very ancient, and even older than the solar system," says Don Brownlee, a University of Washington astronomer and principle investigator for the Stardust mission. "We are truly collecting stardust, we are literally reaching for the stars. The components in comets are very small, and many of them were formed on other stars in the galaxy."
The Stardust spacecraft flew within 240 kilometers (150 miles) of Wild 2′s nucleus. To collect dust grains streaming off the comet, Stardust used a round, ice cube tray-like instrument filled with a silicon-dioxide material called aerogel.
The aerogel dust collector "collected thousands and thousands of particles to be analyzed by our laboratories here on Earth, and will give a hint of the major role that comets have played in the history of Earth and of life on Earth, including ourselves," says Duxbury.
Such dust particles are immensely small, but they can contain a wealth of information. For instance, a 5 micron-diameter particle (about 5 percent the diameter of a human hair) is composed of many smaller particles, each which has its own story to tell. The scientists anticipate that much of the dust will be composed of elements like iron, magnesium, silicon, oxygen, carbon, and nitrogen.
Although the dust grains from the coma are miniscule, the comet’s icy nucleus is about 5 kilometers (3 miles) in diameter – half the size of the city of San Francisco. This enormous chunk of ice is small for a comet, and so scientists expected it would have an irregular shape. Usually planetary bodies need a certain amount of mass, and therefore gravity, to become spherical. Duxbury admitted he was surprised to discover the comet’s nucleus appears to be round.
|Micron-sized dust particles to be returned to Earth trapped in a silica aerogel in two years time. |
The coma of comet Wild-2 is a tremendously severe environment. The spacecraft encountered a storm of dust particles traveling at over 6 times the speed of a bullet. The spacecraft shields protected it from the hundreds of thousands of particles that impacted each second.
If you had been riding on the Stardust spacecraft as an astronaut, says Duxbury, "you wouldn’t exist anymore. You would’ve been sandblasted to particles as fine as the dust."
Through this particle bombardment, the onboard camera took high-resolution images of the comet nucleus. The mass spectrometer analyzed the composition of particles. The dust flux monitor measured the particle impact rate by recording electric pulses each time it was hit. The spacecraft was also equipped with a sound detector, and the scientists are eager to learn whether any acoustic signals were recorded from impacts on the spacecraft’s shields.
The spacecraft’s survival in the coma of dust was uncertain, because a cometary particle larger than a centimeter could have caused irreparable damage.
"We were really worried about the impact of things larger than a grape or a marble, because those could go through and hit our propellant tank, our electronics, our solar cells, and all kinds of nasty things," says Brownlee. "This is kind of a Russian roulette, with a barrel with 100 empty slots in it and one bullet."
|Artist rendering of pickup scene, Utah Test Range|
Credit: NASA/JPL, U. Wash
During the comet flyby, the spacecraft was under its own control, turning itself to point the camera at the nucleus. A few minutes after the flyby, the spacecraft turned again so the antenna was pointed back at Earth. This allowed the spacecraft to transmit all the recorded data, a process that took about 30 hours.
The spacecraft will return to Earth in two years. On January 15, 2006, it will drop the Sample Return capsule into the Earth’s atmosphere so that it lands at the U.S. Air Force Utah Test and Training Range. The Sample Return capsule then will be sent to NASA’s Johnson Space Center in Houston for analysis.
The last sample return mission was in 1972, when Apollo 17 brought samples of the moon back to Earth.
"Apollo was a phenomenally important mission because it provided samples of the moon for us to analyze," says Brownlee. "Once the samples were back we realized we weren’t even asking the right questions. We’ve been surprised today, by what we’ve seen in the pictures, I’m sure we’ll be surprised when we get the (comet) samples back."
Launched on February 7, 1999, Stardust is the fourth in NASA’s series of Discovery missions. During its nearly five years in space, it has captured interstellar dust using the opposite side of the aerogel collector.
There are two more comet missions currently planned. NASA’s Deep Impact mission will visit the comet Tempel 1 on July 4, 2005. The European Space Agency’s Rosetta mission will launch this year and reach the comet Churyumov-Gerasimenko in November 2014.
"In the progression of things, flyby is the first step, and orbiting is the next step, and then ultimately landing," says Brownlee. "There are also studies of comet nucleus sample return where you would land on a comet, dig up a sample of frozen volatiles, and bring it back to the Earth at very low temperatures. We would’ve liked to have done all that, but we were a first step."
Related Web Pages
The Anatomy of a Comet
NASA Discovery Missions
Washington University, Seattle
Live Webcam of Stardust Mission
Early Wild Success for Stardust
Telescopes for Stardust
Harpooning a Comet
Two-Way Asteroid Trip Takes Off
Tale of a Comet
We Are All Made of Stars
Winter Boon From Deep Space