Going Comet Wild
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Going Comet Wild
The Stardust spacecraft is set to blast off for an
historic rendezvous with Comet Wild-2
Feb.
5, 1999: 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 the European Giotto spacecraft's
fly-by of Comet Halley (1986) and Comet 26P/Grigg-Skjellerup (1992),
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.
Right: Comet P/Wild-2 photographed
by K. Meech on Dec. 17, 1990 using an 88 inch reflector telescope
on Mauna Kea, Hawaii.
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.
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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.
Above: Where is comet Wild-2 right
now? This view, looking down on the sun shows the orbit of Wild-2
and its current location. This picture from JPL's
Solar System Simulator is updated every 5 minutes. Hit reload
for the latest image. See comet
Wild's orbit in 3D, from Liftoff! (requires java)
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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.
Stardust
Mission home page
-- from JPL
The Science
of Stardust -- from JPL
Stardust
Education web page -- from JPL
Why
comet Wild-2? -- from the JPL Stardust team
The Solar System -- including comet Wild, from
Liftoff! to Space
Exploration
Where
is comet Wild-2 right now? -- from the JPL Stardust team,
updated every 5 minutes
Orbital
elements of comet Wild-2 -- from the JPL Stardust team
More about aerogel -- from JPL
Leonids Sample Return Mission -- an attempt to capture samples of comet Tempel-Tuttle during the 1998 Leonid meteor shower, Nov. 16, 1998, from Science@NASA
Leonids Sample Return Payload Recovered -- Nov. 23, 1998, from Science@NASA
More space science headlines -- from Science@NASA
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For more information, please contact: Dr. John M. Horack , Director of Science Communications |
Author: Dr.
Tony Phillips Curator: Bryan Walls NASA Official: John M. Horack |