Like a slugger trying to pile up extra home runs after
breaking the world record, a venerable NASA spacecraft
already famed for bringing science fiction’s ion-engine
technology to life is preparing to fly daringly close to a
comet on Saturday, Sept. 22.

Deep Space 1, which has already completed a highly successful
mission testing a number of advanced spacecraft technologies,
will attempt to pass inside the mostly unknown environment
just 2,000 kilometers (about 1,200 miles) from the nucleus of
comet Borrelly at 6:30 p.m. EDT (3:30 p.m. PDT) on Sept. 22.

“It has been a tremendously rewarding effort for the small
Deep Space 1 team to keep this aged and wounded bird aloft,”
said Dr. Marc Rayman, project manager of Deep Space 1 at
NASA’s Jet Propulsion Laboratory (JPL), Pasadena, Calif. “Its
mission to test new technologies is already highly successful
and any science we get at the comet will be a terrific

By the time of the flyby Deep Space 1 will have completed
three times its intended lifetime in space and its primary
mission to test ion propulsion and 11 other high-risk,
advanced technologies in September 1999. NASA extended the
mission, taking advantage of the ion propulsion and other
systems to target a chancy but exciting encounter with

The spacecraft may tell us more about comets and their place
in the solar system. The robotic explorer will attempt to
investigate the comet’s environment when it tries to fly
through the cloud of gas and dust surrounding the comet’s
nucleus, known as the coma.

The risks involved in gathering science data are very high,
so results of this latest venture are unpredictable. The
spacecraft will be traveling through a cloud of gas, dust and
comet pieces to collect its data. Since Deep Space 1 wasn’t
built to go to a comet, it does not carry a protective
shield. “We expect to be hit by debris from the comet, and at
16.5 kilometers per second (about 36,900 mph), even a tiny
particle might prove fatal,” said Rayman. “But this is an
adventure too exciting to pass up.”

If all goes well, scientists will use the comet chaser’s
measurements to find out the nature of Borrelly’s surface and
to measure and identify the gases coming from the comet. The
spacecraft will also attempt to measure the interaction of
solar wind with the comet, a process that leads to formation
of the beautiful tail.

Borrelly makes a good target for study now, as it is just
1.34 astronomical units (about 200 million kilometers or 125
million miles) from the Sun — the closest it will get for
another seven years. The Sun’s heat will make the gases
escaping from the nucleus flow faster and more thickly, so
they will be easier to study. The icy nucleus and the
spacecraft will flash past each other at 16.5 kilometers per
second (more than 36,900 miles per hour).

The flight team is also hoping that Deep Space 1 will have
enough gas to get to the comet. The long-lived spacecraft
keeps itself pointed correctly by firing small thrusters
fueled by hydrazine gas. When the hydrazine runs out, Deep
Space 1 will be unable to keep itself pointed correctly and
the spacecraft will die. The flight team has an estimate of
how much gas is left, but a few hours’ worth of gas could
make all the difference in the comet encounter.

As it approaches the center of the coma, the spacecraft will
face its greatest challenge: to obtain pictures and infrared
measurements of the nucleus. Deep Space 1 can’t tell exactly
where the nucleus is or what it will look like. The craft
will have to locate the nucleus on its own and try to point
the camera toward it as it streaks by.

In late 1999, Deep Space 1 lost its star tracker, which helps
determine the spacecraft’s orientation. Faced with what could
have been a mission-terminating injury, the controllers
performed a spectacular ultra-long-distance rescue. They
reconfigured the spacecraft to use the photographic camera to
orient itself by the stars around it.

The camera cannot align the spacecraft and snap photos of
Borrelly at the same time. Instead, Deep Space 1 will have to
rely on its fiber-optic gyroscopes to help maintain its
orientation. But the gyros are not accurate enough by
themselves, so engineers designed complex new software to
help the camera stay pointed at the comet’s nucleus during
the critical few minutes that the probe will be close enough
to try to get a view of it.

Deep Space 1 was launched in October 1998 as part of NASA’s
New Millennium Program, which is managed by JPL for NASA’s
Office of Space Science, Washington. The California Institute
of Technology in Pasadena manages JPL for NASA. More
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