Catching Comet Dust
On Friday, January 2, the Stardust spacecraft will fly into the dusty, gaseous coma of the comet Wild 2.
To collect dust grains streaming off the comet, Stardust has a round, ice cube tray-like instrument filled with a silicon-dioxide material called aerogel. The ultra-light aerogel will act as a dust catcher, snaring tiny cometary particles. The aerogel-encased samples will eventually be brought back to Earth so scientists can study them.
Comets are composed of ice, dust, and gas – primitive materials that date from the earliest days 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.
|Stardust launch, artist rendering|
"A mantra of our mission is the statement, ‘We are stardust,’" says Don Brownlee, a University of Washington astronomer and principal investigator for the Stardust mission. "The atoms inside our bodies, before our solar system formed, were in stardust or interstellar grains before the sun formed, and we believe these are still preserved within comets."
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) actually is composed of many smaller particles, each which has its own story to tell. By analyzing the chemical, elemental, and isotopic composition of a particle, scientists can determine where it came from.
"Did it come from a supernova?" suggests Brownlee. "Did it come from a red giant star? Did it come from our own solar system? Did it carry the kind of organic materials to Earth that we needed for forming life? It’s a wonderful story, and we expect tremendous complexity at a very fine scale, because these particles are made out of components that are less than a wavelength of light in size. Just because they are small doesn’t mean we can’t get information – even a tenth of a micron particle contains billions of atoms."
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. The scientists expect the comet will contain silicates, sulfides, and organic materials – similar to the extraterrestrial materials that constantly rain down on Earth as meteorites or interplanetary dust. The scientists don’t expect to find any new types of atomic structures or elements, although Brownlee jokes that if they did find some new minerals, they would call them "Stardust-ite, J.P.L.-ite (for the Jet Propulsion Laboratory), or Brownlee-ite."
|Stardust close encounter with Wild-2 |
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 because it is so small it has an irregular shape. Planetary bodies need a certain amount of mass, and therefore gravity, to become spherical.
The coma of comet Wild-2 is a tremendously severe environment. The spacecraft will encounter a storm of dust particles traveling at over 6 times the speed of a bullet. The spacecraft is equipped with shields to protect it from the hundreds of thousands of particles that will impact each second.
Through it all, the onboard camera will take high-resolution images of the comet nucleus. The mass spectrometer will analyze the composition of particles, and the dust flux monitor will measure the particle impact rate. This information will be sent to scientists at JPL and Lockheed Martin Space Systems. Since the spacecraft is 242 million miles away, it will take about 20 minutes for the information to travel to Earth.
During the comet flyby, the spacecraft will be under its own control. It will turn itself to point the camera at the nucleus during the flyby.
"This will probably be the most tense period of the mission, where the spacecraft controls itself," says Tom Duxbury, project manager for the Stardust mission. "After it turns itself to point the camera, we will just be getting a signal to tell us everything is o.k. But a few minutes past the flyby, the spacecraft will automatically turn itself back to Earth, point the high-gain antenna to Earth and start dumping data again. That’s when our white knuckles turn back to a normal color and we start breathing normally."
The spacecraft will record data while it is turned away from the Earth. After the flyby, when the antenna turns back to Earth, it will take about 30 hours to download all the recorded data. After that, the scientists will have to wait two years for the spacecraft to bring back the aerogel sample. In January 2006, the Stardust spacecraft will pass by Earth and release the Sample Return capsule into the Earth’s atmosphere. After parachuting into the Utah desert, the Sample Return capsule will be sent to NASA’s Johnson Space Center in Houston.
Wild 2 became the target of the Stardust mission because scientists needed a comet that would allow a trajectory for the spacecraft to get to the comet and then get back to Earth. The target also had to be approached at low speed (and low cost).
|Parachute landing of Stardust near Utah Testing Range|
"As a phenomenal bonus – this is one hell of a comet – this comet has only been in its present orbit for 30 years," says Brownlee.
In 1974, Wild 2 had a close encounter with Jupiter, and this altered its orbit. Its present orbit is from the orbit of Mars to the orbit of Jupiter. Before 1974, it had an orbit that looped from Jupiter to just past Uranus. Long before that, Wild 2 orbited in the Kuiper belt.
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 long tail we typically associate with comets.
Launched on February 7, 1999, Stardust is the fourth in NASA’s series of Discovery missions. In November 2002, the spacecraft tested its instruments during a flyby of Asteroid 5535 Annefrank. During its nearly five years in space, it has captured interstellar dust using the opposite side of the aerogel collector.
Stardust is the second mission to visit a comet. The Deep Space 1 spacecraft visited the comet Borrelly, traveling within 2,000 kilometers of the comet’s nucleus. Stardust will get much closer than that, traveling within 300 kilometers of Wild 2′s nucleus.
There are two more comet missions planned. NASA’s Deep Impact mission will visit the comet Tempel 1 on July 4, 2005. The European Space Agency’s Rosetta mission will travel to comet Churyumov-Gerasimenko in November 2014.