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:
#1 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.
#2 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.
Catching comet fluff
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.
Above: This photo from a laboratory
experiment shows the cone-shaped track made by a tiny high-velocity
particle in aerogel. The captured particle is located just beyond
the narrow end of the cone. Credit NASA/JPL.
After the flyby is done, Stardust will return to Earth. In 2006
the craft's aerogel sample collectors will descend by parachute
toward the U.S. 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.
|