NASA’s Deep Space 1 probe is about to begin its greatest adventure yet — a
daring plunge into a comet.

Kirk: “I take it the odds are against us and the situation is grim.”

Picard: “You could say that.”

Kirk: “Sounds like fun!”

— Captains James T. Kirk and Jean-Luc Picard, from Star Trek: Generations

September 19, 2001: On October 24, 1998, a Delta rocket lifted off from Cape
Canaveral. It was a routine launch bearing an ordinary-looking spacecraft.
But NASA insiders knew better. This mission was something special — even
its name was out of the ordinary: Deep Space 1 (DS1).

Most space probes are put together using tried-and-true technology,
minimizing the chances of failure. But DS1 left Earth bristling with the
un-tried. Its mission was to test a dozen new exotic technologies. Among
them: a long-lasting ion propulsion engine, a solar array that focused
sunlight for extra power, and software that endowed the craft with
artificial intelligence to conduct its own affairs.

Ground controllers must have nail-biting nightmares about such missions.
Almost anything on Deep Space 1 might go wrong. And when it did the
spacecraft itself would be in charge!

But as they say, fortune favors the bold. Deep Space 1 spent nearly a year
trying out its uncertain technologies and, improbably, they all worked —
many beyond expectations. DS1’s ion engine, for instance, is now the
longest-operating propulsion system in the history of the space program.

“Deep Space 1 tested these advanced technologies so that other missions
wouldn’t have to bear the costs of being first,” explains Marc Rayman, the
Deep Space 1 Project Manager at the Jet Propulsion Laboratory. “And it was a
big success.”

Rayman leads a resourceful band of JPL scientists and engineers who share
their spacecraft’s trailblazing spirit. Always thinking of new things to
try, they were plotting Deep Space 1’s next adventure even before its
primary mission of technology testing was done. “We wanted to fly by a
comet,” recalls Rayman. It’s something few spacecraft had tried before.
Indeed, comets are virgin territory compared to better-studied planets and
asteroids. Flying into the unknown seemed a natural mission for Deep Space
1.

Rayman continued: “When we were firing the ion engine during tests in 1998
and 1999 we always steered to keep our options open for an encounter.” Their
favorite target was 19P/Borrelly, an intriguing comet that had veered toward
the Sun during the 19th century when it passed too close to Jupiter. It’s
been swinging through the inner solar system every 6.9 years since. Borrelly
would reach perihelion (its closest approach to the Sun at 1.36 AU) on Sept.
14, 2001 — just right for a meeting with Deep Space 1.

But first NASA had to approve the scheme. There were good reasons to go,
says Rayman, but approval was not a foregone conclusion. “NASA has many
important objectives and, of course, limited funding. Still, we were hopeful
because our primary mission had been so successful.”

Only one spacecraft has ever captured close-up pictures of a comet before:
the European Space Agency’s Giotto probe, which flew a scant 596 km from the
core of Comet Halley in 1986. Photographs revealed Halley’s nucleus as one
of the blackest objects in the solar system — one of Giotto’s most
surprising finds! Unfortunately, flying comet dust destroyed Giotto’s
camera, so when the probe flew even closer to comet Grigg-Skjellerup in
1992, the craft was blind.

At least four more spacecraft — CONTOUR, Stardust, Deep Impact, and the
ESA’s Rosetta — will rendezvous with comets in the years ahead. But
scientists aren’t sure if Giotto’s experience will be a reliable guide for
those missions.

“Comets are as individual as people,” explains Rayman. “You couldn’t expect
to learn about the whole human race by studying a single person. Likewise,
Halley can’t reveal all we need to know about comets.”

By visiting Borrelly, Deep Space 1 could examine a second comet at close
range, perhaps as near as 2000 km, and provide valuable intelligence to
comet mission planners.

In late 1999 NASA headquarters agreed: Deep Space 1 could visit Borrelly.
“We were overjoyed,” says Rayman, ever game for a new adventure. There was
little time to spare for celebration, though. When Rayman heard the good
news Borrelly was already plunging toward perihelion. “We needed to leave
soon,” he recalls.

And, of course, that’s when disaster struck.

Just as Deep Space 1 was setting sail for the comet in late 1999, its
guidance system, called the “Star Tracker,” suddenly failed. The Star
Tracker was a computerized camera (ironically not one of the probe’s
experimental devices) that measured DS1’s orientation with respect to the
stars. Without it, the spacecraft was lost.

JPL engineers quickly came up with a solution. They would reconfigure
“MICAS” (an experimental camera that DS1 had just finished testing) to serve
as a makeshift Star Tracker. It was a brilliant plan, but there was one
drawback: Deep Space 1 was 300 million km away. No one could lay their hands
on the spacecraft! Their only option was to transmit radio commands across
interplanetary space, telling the onboard computer how it could transform
MICAS into a star tracking navigator.

It was a spine-tingling operation: Time was short and testing was hurried —
one false command to the navigation system might disorient the craft
forever! But in the end, Deep Space 1 was saved. “It was a thrilling
rescue,” recalls Rayman, “and just in time to catch Borrelly.” (You can read
a more complete account here.)

On June 28, 2000, with MICAS at the helm, Deep Space 1 aimed for the comet
and revved its ion engine — the beginning of a 15-month journey. Twice
since then solar storms have scrambled the makeshift navigation system, and
once MICAS tried to fix on a star that was too dim. “In each case, the DS1
team managed to coax the spacecraft back to normal,” says Rayman. “It was
stressful work. And unforgettable. There’s nothing quite like joining a
half-dozen space experts (powered by Oreo cookies) in the middle of the
night to joystick a spacecraft that’s hundreds of millions of kilometers
away.”

One more such incident, notes Rayman, and the encounter with Borrelly will
likely fail. Fortunately the long journey is almost over. On September 22nd,
Deep Space 1 will finally reach the comet — and begin its greatest
adventure yet.

This month amateur astronomers can easily spot Comet Borrelly through a 10
inch or larger telescope. Glowing at 9th magnitude in the constellation
Gemini, it looks like a fuzzy blob with a short, faint tail.

That blob is the comet’s coma, a huge cloud of gas spanning 50,000 km which
hides Borrelly’s nucleus deep within. The nucleus is an asteroid-sized
mixture of ice, dust and rock. Albeit small, it’s the source of all we see.
Ices in Borrelly’s nucleus, warmed by sunlight, are vaporizing furiously —
spitting dust and blowing gaseous jets. It’s a maelstrom that could be
deadly to a ship like Deep Space 1.

“Unlike Giotto,” says Rayman, “DS1 was not built to encounter a comet. It
has no shielding to protect it from flying debris. A single particle of
comet dust the width of a human hair can deliver as much energy as a bowling
ball does when it crashes into the pins.” Deep Space 1 could be rocked hard
by such particles dozens of times as it transits the coma — about an
hour-long journey at 16.5 km/s (37,000 mph).

That’s not much time to find Borrelly’s nucleus. “Our goal is to capture a
black and white photo of the sunlit side of the nucleus while the spacecraft
is about 8000 km away,” says Rayman. But, he adds, it won’t be easy. As soon
as Deep Space 1 plunges into the haze, the navigation camera will have to
stop tracking stars and begin searching for the core. Onboard gyros will do
their best to hold the spacecraft steady as MICAS snaps a rapid-fire
sequence of photos and analyzes them for hints of the comet’s core. The
procedure is complicated by the fact that no one knows if the nucleus will
be dark (like Halley’s) or bright; or if the irregular 5 by 5 by 8 km core
will appear end-on or edge-on. And, of course, the spacecraft itself might
be listing, buffeted by comet dust.

If MICAS beats the odds and finds the nucleus, scientists will get more than
just a black and white photo. In addition to its optical camera, MICAS is
also equipped with an experimental infrared spectrometer that can sense
minerals from afar. (Even the impressive Giotto probe didn’t carry such a
device.) Not only will scientists see what Borrelly’s nucleus looks like,
they’ll also learn what it’s made of!

While MICAS conducts its feverish search for the nucleus, other instruments
on board DS1 will be calmly collecting valuable measurements of the
surrounding coma. “We’ve reconfigured many of DS1’s sensors to study
Borrelly,” Rayman says. For instance, ion engine diagnostic sensors will
“listen” for plasma waves in the gaseous coma and attempt to detect the
comet’s magnetic field. Also, an experimental mass spectrometer named “PEPE”
will identify atoms and molecules in gases blowing by the spacecraft. “This
is a wonderful opportunity to learn what the coma is made of,” says Rayman.

Whether DS1 will emerge from the coma with stunning new data — or emerge
intact at all — remains to be seen. But no matter, the Borrelly encounter
is a guaranteed success by virtue of its daring. If the spacecraft is
destroyed, other missions will be more wary and perhaps survive thanks to
DS1’s sacrifice. If the spacecraft lives, it will beam back priceless
scientific information.

“This journey has been filled with adventure — much of it different from
what we had planned,” says Rayman. “But that’s OK. You’re not going to win
the big prizes if you don’t take the big risks.”