Imagine driving through heavy fog to a place you’ve never been, guided
only by a faint taillight in the distance. The challenge is similar to
one NASA will take in January 2004 by flying its Stardust mission
through the halo of dust that surrounds the nucleus of a comet.

"With Mars and other planets, we know relatively well where the
planets are," said Dr. Shyam Bhaskaran, a Stardust navigation
specialist at NASA’s Jet Propulsion Laboratory in Pasadena, Calif.
"This is not the case with comets, which are not easily observed
because they are small objects with gas jets. It is much harder to
predict their orbits, which is why we have a little extra help from a
camera onboard the spacecraft."

One of three methods the Stardust navigation team is using to find
their way, optical navigation involves placing a 200mm focal length
camera onboard the spacecraft as it flies to its target, a comet
called Wild 2 (pronounced Vilt). The camera photographs the view from
the spacecraft about twice a week until 10 days before its encounter
with the comet. It then photographs the view three times daily until
72 hours before encounter, when it begins taking one image per hour.
These images continuously help engineers on the ground figure out
where the spacecraft is in relation to the comet. Based on those
images and other data, engineers can plan maneuvers accordingly and
document the mission. This method is especially necessary since the
comet flew behind the sun as viewed from Earth in May 2003, thus
making Earth-based observations impossible.

"Trying to view the comet from Earth at this point would be almost
like trying to find a firefly behind a fire," Bhaskaran said.

The Wild 2 comet is not expected to emerge until several weeks before
its encounter with the spacecraft. Its fiery path aside, the comet is
the mission’s best available target. The decision to fly by this comet
was based on a number of factors including fuel constraints and
mission launch date, as well as a database of information established
by ground-based astronomers.

Along with optical navigation, Stardust engineers employ standard
Doppler and range tracking techniques during the mission’s cruise
phase. The techniques, used by all interplanetary missions, involve
relaying radio signals from the spacecraft to Earth via the Deep Space
Network’s three worldwide tracking stations in Australia, Spain and
California. These signals reveal details about the spacecraft’s
orbital path when compared with a mathematical model of the solar
system, and allow engineers to pinpoint the spacecraft’s position
relative to Earth. Engineers are also able to adjust the path of the
spacecraft based on this information.

As of December 15, the spacecraft will be about 9.5 million kilometers
(6 million miles) from the comet, and closing in at a rate of about
530,000 kilometers (330,000 miles) every day. The comet was first
spotted by the spacecraft’s camera on November 17, and more images
have been coming in every few days, allowing the navigators to more
accurately determine where the spacecraft is with respect to the
comet. Comets, however, are unpredictable objects.

"We don’t anticipate any surprises, but we have to be prepared, and
that’s what makes this job anything but routine," Bhaskaran said.
"With a little luck and a lot of skill, we should be able to meet the
mission’s goal of flying by Wild 2 at a distance of 300 kilometers
(186 miles)."

Stardust’s cometary and interstellar dust samples will help provide
answers to fundamental questions about the origins of the solar
system. More information on the Stardust mission is available at