This weekend a NASA spacecraft will blast off from the Kennedy Space Center for an historic rendezvous with periodic comet Wild-2. Its ambitious goal is to intercept Wild-2 in 2004, to capture tiny bits of comet dust and debris, and then return them to Earth for analysis in 2006. Stardust is the first comet rendezvous mission since since the European Giotto spacecraft's fly-by of Halley in 1986, and the first ever to attempt to return a comet sample to Earth. It's a long, 7-year mission, but one most scientists feel is worth the wait.
Scientists are curious about comets because they are thought to be the oldest, most primitive bodies in the solar system. Comets are made up of the same stuff as the early Solar Nebula that collapsed to form the sun and planets. It is now known that comets contain significant amounts of water ice, dust, and carbon based compounds. They may have been an important source of water and organic molecules for Earth when many comets collided with our planet during a period of heavy bombardment over 4 billion years ago. Modern-day comets are like a time machine. They offer a window into the past when the Solar System was young and life on Earth was just beginning.
Why Comet Wild-2?
History is filled with famous comets. Halley's comet, Hale-Bopp, Hyakutake and
others have dazzled observers with their brilliant nuclei and dramatic tails.
Recent comets like Hale-Bopp have been viewed by hundreds of millions of people,
and Halley's comet has had a real impact on history, as in 1066 when it was so
bright that it terrified millions of Europeans and was widely credited with the
Norman victory at the Battle of Hastings.
Unlike its famous cousins, comet Wild-2 is a relatively dim, new arrival to the inner solar system. Until recently it circled the sun in an orbit between Jupiter and Uranus, but everything changed in September 1974 when Wild-2 passed within 0.006 AU of Jupiter. That encounter with the giant planet, at only 10 times the distance which fragmented P/Shoemaker-Levy 9 in 1994, altered Wild-2's orbit so that its closest approach to the sun now lies just inside the orbit of Mars.
During its first passage relatively near to Earth (1.21 AU) on January 6, 1978, the comet was discovered by Paul Wild. Since then, the best apparition of Wild-2 was in March 1997 when it passed within 0.85 AU of our planet, brightening to an unimpressive 10th magnitude. That's too faint to be seen with the naked eye, but bright enough for modest amateur telescopes.
So, why visit an obscure, hard-to-see object like Wild-2, when there are so many more notorious comets to choose from? There are two important reasons:
- It's fresh. Before its near miss with Jupiter in 1974 comet Wild-2 was
well-preserved in the frigid outer solar system. With its new orbit, Wild-2 now
comes much closer to the sun. When a comet passes close enough to the sun, some
of its material is boiled off into interplanetary space. After about a thousand
trips past the sun, it loses most of its volatile materials and no longer
generates a coma or tail. Since Wild-2 has passed the sun only a few times, it
still has most of its dust and gases - it is "pristine." By the time Stardust
encounters the comet, Wild-2 will have made only five trips around the sun. By
contrast, Comet Halley has passed the sun more than 100 times.
- It's in the right place at the right time. Wild-2 presents a unique opportunity -- it is in the right place at the right time. Scientists have found a flight path that allows the spacecraft to fly by the comet at a relatively low speed, only 13,600 mph. Because of this "low velocity" flyby, comet dust can be captured by collectors on the spacecraft, rather than blowing right through the collectors and out the back side! This comet dust can then be brought back to the Earth to be analyzed.
When Stardust catches up with comet Wild-2 in January 2004, both the comet and the spacecraft will be beyond the orbit of Mars. Although they will be far from the sun, solar heating will still be sufficient to cause particles to bubble off the surface of the comet's nucleus. The spacecraft will pass within 100 km of Wild-2. Cometary debris will hit the dust catcher at up to six times the speed of a bullet fired from a high-powered rifle.
A unique substance called aerogel is the medium that will be used to catch and preserve the high speed dust samples. Aerogel is the lightest known solid, and is considered the best substance available for capturing fragile particles from a comet without damaging them. When a high-velocity dust particle hits the aerogel, it buries itself in the material, creating a carrot-shaped track up to 200 times its own length. Since aerogel is translucent scientists can use these tracks to find the tiny particles. The track is largest at the point of entry, and the particle can be collected intact at the point of the cone.
After the flyby is done, Stardust will return to Earth. In 2006 the craft's aerogel sample collectors will descend by parachute toward the United States Air Force Test and Training range in Utah, about 100 miles southwest of Salt Lake City in the desert.
By the time the Stardust mission is over, comet Wild-2 - dim, obscure, and little-known - will take its rightful place in the pantheon of historic comets.