by Mark Perew
Posted January 15, 2001
PASADENA, Calif. - Earth had a small visitor in the early hours of Monday morning. At about 1115 GMT (3:15am PST), an object 5 metres long and weighing roughly 385 kg hurtled 6,000 kms above the Indian Ocean off the tip of Cape Horn. This wasn't a near miss by an Earth crossing asteroid. It was the Stardust probe executing a flyby planned by the Jet Propulsion Laboratory.
Launched just under two years ago, Stardust is NASA's Discovery mission to bring miniscule bits of comet debris back to planet Earth. But, that event won't happen for another five years. This morning's visit was just a quick visit home to pick up more speed and put it on a better trajectory for the comet it will sample in three years.
Comet Wild-2 is named for Swiss discoverer Paul Wild (pronounced 'vilt') who located the comet in 1978. Wild-2 is currently out beyond the orbit of Jupiter, just passed aphelion, the farthest point from the sun in its eccentric orbit. Tracking backwards, astronomers determined that Wild-2 was a newcomer to the inner solar system. Prior to 1974, Wild-2's orbit would have been between Jupiter and Uranus, but a close encounter with Jupiter severely altered its path and brought it inside the asteroid belt for the first time.
This makes Wild-2 nearly pristine and of prime interest to scientists wanting to know not only what comets are made of, but also what the solar system was like in its early days. By the time Stardust arrives at Wild-2 on January 2, 2004, the comet will have made only a handful of trips into the inner solar system. Only a tiny fraction of its volatiles, such as water and carbon dioxide, will have been lost to the interplanetary environment.
When Stardust and Wild-2 meet, the comet will be just slightly past its perihelion where solar heating will cause gasses and particles to be ejected from the comet's nucleus. Onboard Stardust an array of super lightweight foam, called aerogel, will be deployed to catch microscopic particles without damaging them. The aerogel has already been used once on this mission to collect dust from the interstellar wind and will be used once more for that purpose before the encounter with Wild-2.
All of this aerogel will be returned to Earth in almost exactly five years. On its next swing by Earth, Stardust will eject its return canister which will reenter the atmosphere. That .6 metre long, 45 kg canister will parachute to a landing in Utah.
Stardust's return has been free of the protests, demonstrations and lawsuits seen during the previous flybys of Galileo and Cassini. Since Stardust is powered by large solar arrays, and not by plutonium based radioisotope thermal generators, this flyby just hasn't created much of a sensation.
That doesn't mean that Stardust has been a perfect mission. A massive solar flare last November hit Stardust with enough energy to make it see stars, or in this case, stars that weren't really there. That caused the onboard computer to become confused. Eventually the bogus stars faded from Stardust's electronic eyes and the system is now in good shape.
You can track the position of Stardust at http://stardust.jpl.nasa.gov/mission/scnow.html.
Mark Perew is a freelance writer, a member of the National Association of Science Writers and a JPL Solar System Ambassador.