Stardust
Pinpoints Where the Wild Thing Is
Forty-nine days before its historic rendezvous
with a comet, NASA's Stardust spacecraft successfully
photographed its quarry, comet Wild 2 (pronounced
Vilt-2), from 25 million kilometers (15.5 million
miles) away. The image, the first of many comet
portraits it will take over the next four weeks,
will aid Stardust's navigators and scientists
as they plot their final trajectory toward a
January 2, 2004 flyby and collection of samples
from Wild 2.
"Christmas came early this year,"
said Project Manager Tom Duxbury of NASA's Jet
Propulsion Laboratory, Pasadena, California.
"Our job is to aim a 5 meter (16 foot)
long spacecraft at a 5.4 kilometer (3.3 mile)
wide comet that is closing on it at six times
the speed of a bullet. We plan to miss the comet
by all of 300 kilometers (188 miles), and all
this will be happening 389 million kilometers
(242 million miles) away from home. By finding
the comet as early and as far away as we did,
the complexity of our operations leading up
to encounter just dropped drastically."
The ball of dirty ice and rock, about as big
as three Brooklyn Bridges laid end-to-end, was
detected November 13 by the spacecraft's optical
navigation camera on the very first attempt.
The set of images was stored in Stardust's onboard
computer and downloaded the next day where mission
navigator Dr. Shyam Bhaskaran processed them
and noticed a white blob of light bisecting
the base of a triangle made by three stars Stardust
uses for deep space navigation.
"When I first looked at the picture
I didn't believe it," Bhaskaran said. "We
were not expecting to observe the comet for
at least another two weeks. But there it was,
very close to where we thought it would be."
The Wild 2 sighting was verified on November
18 using the second set of optical navigation
images downloaded from Stardust. To make this
detection, the spacecraft's camera saw stars
as dim as 11th visual magnitude, more than 1,500
times dimmer than a human can see on a clear
night.
The early detection of Wild 2 provides mission
navigators critical information on the comet's
position and orbital path. Future optical navigation
images will allow them to do more fine-tuning.
In turn, these new orbital plots will be used
to plan the spacecraft's approach trajectory
correction maneuver. Stardust's first such maneuver
is planned for December 3.
Unlike other orbiting bodies, the paths of
comets cannot be precisely predicted because
their orbits about the Sun are not solely determined
by gravity. The escape of gas, dust and rock
from comets provides a "rocket effect"
that causes them to stray from a predictable
orbital path. The actual orbital path cannot
be precisely determined from Earth-based telescopes
because the comet is shrouded in a cloud of
escaping gas and dust. What is seen from Earth
is not the actual 5.4 kilometer (3.3 mile) wide
body composed of rock and ice, but the cloud
of debris and gas that envelops it.
"With these images, we anticipate we will
fly by comet Wild 2 at an altitude of 300 kilometers,
give or take about 16 kilometers," Bhaskaran
said. "Without them, we wouldn't be able
to safely get any closer to the comet than several
thousand kilometers."
Stardust will return to Earth in January 2006
to make a soft landing at the U.S. Air Force
Utah Test and Training Range. Its sample return
capsule, holding microscopic particles of comet
and interstellar dust, will be taken to the
planetary material curatorial facility at NASA's
Johnson Space Center in Houston, where the samples
will be carefully stored and examined. Stardust's
cometary and interstellar dust samples will
help provide answers to fundamental questions
about the origins of the solar system.
Stardust, a part of NASA's Discovery Program
of low-cost, highly focused science missions,
was built by Lockheed Martin Astronautics and
Operations in Denver, Colo., and is managed
by JPL for NASA's Office of Space Science in
Washington, D.C. JPL is a division of the California
Institute of Technology in Pasadena. The principal
investigator is astronomy professor Donald E.
Brownlee of the University of Washington in
Seattle.
