By Robert Roy Britt
Senior Science Writer
and Robin Lloyd
Science Editor

The tricky photo and science opportunity, billed by NASA as a "risky bonus mission" to see the heart of the comet, is slated for 6:30 p.m. EDT Saturday, Sept. 22.

The effort may well fail.

"This is a high-risk encounter and the encounter simply may not work," said Marc Rayman, project manager for Deep Space 1. "We're of course trying hard to make it work but we've got an aged and wounded bird up there. We're pushing it far, far, far beyond what it was meant to do."

What the probe was meant to do, and has done, was to test a next-generation propulsion method, a navigation system that makes its own decisions, and other technologies for use on future missions. Since its launch in 1998, the craft has been battered by solar storms, suffered long stretches of silence and disorientation, and failed to achieve on the extra tasks it was asked to performed and never designed to do.

Rayman, part of a team of about dozen people who now monitor the craft during this extended mission period, says it's as though Deep Space 1 "is kept flying with duct tape and good wishes."

The Saturday flyby attempt hearkens back to two previous NASA longshots recently tacked onto the end of already successful missions the triumphant landing of the Near Earth Asteroid Rendezvous (NEAR) probe on asteroid Eros in March 2001, and an attempt in 1999 to crash land the Lunar Prospector in a crater on the Moon.

While NEAR succeeded in obtaining close-up pictures, Lunar Prospector did not create a hoped-for plume of material that researchers had planned to study with ground-based telescopes.

The good wishes may make the difference between success and failure this weekend, when the plucky probe attempts a close pass of Comet Borrelly.

"The craft will try to smell, see and hear the comet with its instruments," Rayman says. "And if it survives, it will describe its spine-tingling adventures to its anxious human colleagues elsewhere in the solar system."

Those colleagues would be scientists here on Earth who are starved for information on comets, mysterious and sometimes glowing wanderers of the solar system. Deep Space 1 will endeavor to plunge into the comet's coma, a shroud of material that has long kept comets innermost secrets hidden from the view of telescopes.

The coma is created when a comet nears the Sun and encounters an increasingly strong and super hot stream of charged particles, known as the solar wind. This ubiquitous river of energy burns off the outer layers of a comet's nucleus, which is made mostly of ice but thought to also contain rocky minerals.

Before Deep Space 1 zooms beyond the comet, it will use its automatic navigation software, called AutoNav, to try to pass within 1,250 miles (2,000 kilometers) of the comet while zooming along at 36,900 mph (16.5 kilometers per second).

If the maneuver succeeds, it will be one of the few time humans have reached out to a comet. Twin Russian spacecraft, Vega 1 and Vega 2, imaged comet Halley in March 1986. The snapshots helped direct a Halley flyby later that same month by the European Space Agency's Giotto mission, which buzzed its target at just 373 miles (600 km) away.

But, as Rayman told SPACE.com, scientists don't even have good information on where their target is.

"We don't know where the comet is exactly," he said. And once that's determined, then the spacecraft will be charged with peeking through the shroud of gas and dust to find the nucleus, whose location can only be estimated.

A "fusillade of high-speed debris from the comet" will pummel the craft, which was not designed for such an encounter, Rayman said. The budget was spent on the futuristic technologies, not common things like shielding.

"When a single particle of dust just the thickness of a human hair strikes the spacecraft, it will deliver as much energy as a bowling ball does when it crashes into the pins," Rayman says. "We don't know exactly what the dust environment of the comet is, but it is probable that the spacecraft will be hit by a few hundred pieces of dust of that size and still larger."

And Deep Space 1 is already a wreck. Saturday's flyby, if it succeeds, won't be the first coma the craft has experienced.

AutoNav was designed to depend on a star tracker, using the distant points of light to orient the spacecraft and then make decisions about speed, attitude and trajectory.

But in November 1999 the probe's star tracker stopped working. The craft had a brain, but no eyes. It was for all intents and purposes dead. Or at least in a coma.

But in a nifty fix more than four months later, engineers beamed newly written software it to the craft with instructions to employ its science camera as substitute star tracker.

By June of 2000, Deep Space 1 had a new lease on life. Still, twice since then, intense solar storms have generated blizzards of radiation that confused the camera, which lost its lock on guide stars. In each case, engineers managed to help the craft recover.

So on Saturday, the same camera that will be looking for the comet's nucleus must also keep an eye on the stars, so that the probe does not become disoriented. Any delay or misidentification of the nucleus or of a guide star will doom the flyby.

And all this tricky navigation will happen within about two hours of the closest approach. So Rayman and his colleagues won't be able to help much. The encounter will occur more than 143 million miles (230 million km) from Earth -- 600 times as far away as the Moon. Messages takes more than 25 minute to make a round trip.

Even if the craft performs to its top potential, mission planners know that the task is near the limit of its capabilities, if not beyond. Another flyby, of asteroid 9969 Braille, was completed on July 28, 1999. The pictures resulting from the encounter were disappointing, however, illustrating the limitations that the spacecraft will face this week.

Deep Space 1's camera was supposed to take pictures of that asteroid upon approach, then use the new images to make its own navigational decisions in order to get even closer and snap more pictures.

But while the camera produced the intended images, they were too dim for the onboard computer to recognize.

"The pictures would be taken, delivered to the computer, and AutoNav said, 'No asteroid here, I'll keep pointing this way,'" Rayman explained a few months after the flyby.

Rayman himself asks the obvious question about this week's attempt to photograph a comet: "Against such odds, why do we even bother at all? Well, as members of a self-respecting space-faring species, how can we not try to do our best?"

If all goes well Saturday, Deep Space 1 will take black-and-white pictures of the comet. The primary goal is a picture when the nucleus is about 50 pixels across (a pixel is the smallest element of the digital camera's view).

The probe will also make infrared images that would help researchers explore the comet's surface, gases that boil off of it as it nears the Sun. And sensors that monitor the ion propulsion system will serve as the ears, having been reprogrammed to listen for magnetic fields and plasma waves in and around the comet.

But regardless of what happens, Deep Space 1 is considered a wild success by mission managers.

An ion engine uses xenon gas for fuel and electricity generated from solar panels. An electric current runs through the gas, creating charged particles, called ions. A voltage is applied to the engine, to charge it positively, and the ions are pushed out the back at up to 78,000 mph (35,000 meters per second).

Because the resulting thrust is gentle -- equivalent to the force required to hold a sheet of paper in the palm of your hand -- it takes months or years to get a spacecraft to full speed. But the engines are reliable and relatively inexpensive to operate compared to conventional rockets.

The ion engine was required to complete 200 hours of operation to be considered a success, which it did.

In March of this year, the engine passed the 10,000-hour mark. By the time it reaches Comet Borrelly, it will have logged roughly 14,000 hours.

Ion propulsions greatest benefit is in powering spacecraft over long distances where time is not a crucial factor. Compared to chemical propulsion, it takes a long time for an ion engine to get a spacecraft up to speed. But because the engines are durable and require very little fuel to operate, great speeds can eventually be obtained.

But you won't see humans riding any ion-powered spacecraft anytime soon.

"You're not going to be flying people around on ion propulsion because it takes too long to get where you're going," says Jerry Grey, head of aerospace and science policy at the American Institute of Aeronautics and Astronautics.

Advocates of human missions to Mars say, however, that if people can be transported to the Red Planet by more conventional rocket-powered craft, then ion engines could support a fleet of slower-moving cargo vehicles that would arrive at regular intervals. Transit time would not be important, so long as enough cargo ships are in the fleet to provide frequent deliveries.