SPACE.com -- Probe Attempting Comet Fly-By Doomed to Death

The well-travelled Deep Space 1 spacecraft is holding out for one more scientific mission -- a flyby of a comet.

A 1994 image of Comet Borrelly on one of its swings around the Sun.

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	Probe Attempting Comet Fly-By Doomed to Death By Robert Roy Britt Senior Science Writer posted: 08:00 am ET 20 September 2001

You are certainly correct that dust might destroy the spacecraft

If its encounter with a comet doesn't kill NASA's Deep Space 1 probe this Saturday, a finite budget will. That is if mission managers don't purposely destroy the spacecraft first.

Deep Space 1, designed to test futuristic technologies, has been a smart, studious, obedient and somewhat autonomous robot for engineers who dream of one day propelling humans vicariously beyond the solar system.

It has shown that a craft can navigate the cosmos on its own, making decisions with no input from back home. Good news for future missions in which the transmission of an instruction would take days instead of hours.

And Deep Space 1's ion engine has proven that a cheap and abundant gas found in our atmosphere -- xenon -- can power a spacecraft to great speeds over long distances, cracking opening new exploratory doors to places that were once too expensive to consider visiting.

But now it is doomed.

No matter what happens this Saturday, when Deep Space 1 makes a risky attempt to snap close-up pictures of comet Borrelly, the spacecraft is very near the end of its life, destined to become no more than a cold hunk of steel wandering around the Sun, dark, alone and no longer needed.

Smack!

The worn-out space probe is about to play a game of odds with cosmic dust. Particles of debris boiling off comet Borrelly could hit the spacecraft with the force of bowling balls during an hours-long flyby this weekend. The craft was not fully designed for such an encounter and has no shielding. NASA has therefore billed the opportunity as a bonus event.

But the dust particles around Borrelly will probably be spread widely, with nothing but harmless atoms of other material in between, said Bob Farquhar, a mission planner at Johns Hopkins University's Applied Physics Laboratory.

Farquhar, who is not involved in the Deep Space 1 mission, does have some experience in getting spacecraft to go beyond the primary call of duty. He engineered the landing of the NEAR spacecraft on asteroid Eros this past February. While not an official part of the mission, the landing was something Farquhar had thought about and planned for well before NEAR launched, he said.

Likewise, while the encounter with comet Borrelly was not part of Deep Space 1's primary mission, it was in the minds of mission planners all along, Farquhar said.

And so despite NASA's caution, Farquhar thinks Deep Space 1 has an excellent chance of surviving the encounter. And while the spacecraft does not have shielding, it was in fact designed to handle flybys. A previous encounter with asteroid Braille was part of the probe's main mission.

Myriad threats

Either way, Deep Space 1's clock is ticking, and a little dust may turn out to be the least of its worries.

"Even if dust does not destroy it, there are myriad other threats," said Marc Rayman, project manager for Deep Space 1.

At any moment, Rayman said, the craft could run out of hydrazine, a conventional rocket propellant that is fired through small thrusters to maintain or change orientation. Deep Space 1 is alive today only because of an unconventional last-minute decision about this fuel.

"The night before the hydrazine was loaded onto the spacecraft, we decided to add a little extra and take a bit of a chance with the launch, just in case the additional hydrazine might come in handy," Rayman says.

Even if the fuel holds out, however, the probe could also simply lose its way. Failure of a star-tracking device has forced the probe's imaging camera into double duty. So while focusing its camera on the comet, the probe will be forced to lose sight of the stars it uses as guideposts to maintain its direction, its attitude, its very senses.

Farquhar thinks the fuel and directional problems are the greater dangers. And he cautions that any resulting images may be blurry and not too scientifically useful.

The comet's nucleus is thought to be potato-shaped and only about 5 miles (8 km) long. Scientists don't know exactly where it is, how bright it will be, or if it will appear cross-ways or end-on when photographed.

For now, no news is good news. As of Wednesday afternoon, the craft had not reported any problems.

Destroyed in double-overtime

Meanwhile, Rayman told SPACE.com that even if the flyby is successful, the end is near for Deep Space 1. The extended mission has operated for two years now with an annual budget of $5 million that is soon to expire.

With just weeks to go, however, there is a slight chance for a little more work.

"In the unlikely event that the spacecraft not only survives the encounter but is healthy enough to continue flying beyond the end of this extended mission, we are going to begin what I like to call the hyperextended mission," Rayman said.

This highly doubtful double-overtime period would take the 1,071-pound (486-kilogram) probe back to what it was designed for -- technology testing. But this time, the testing would be a little more rigorous. Possibly even destructive.

"For about six or eight weeks, we will undertake tests of the ion propulsion system and other technologies that were too risky, too complex, or otherwise inappropriate even for the extended mission," he said.

Among the things engineers would like to do is to run the craft's futuristic engine in modes that might destroy it, thereby possibly learning something about its limitations.

Twin ion engine keeps going

Back on Earth, in a lab at NASA's Jet Propulsion Laboratory (JPL), a second and identical ion engine has been chugging along since before Deep Space 1 launched. That engine is running at a minimum thrust level through the end of this year, says John Brophy, a JPL engineer who has worked since 1984 to prove the worth of ion engines.

Brophy figures the test engine will be switched to full power next year -- an attempt to learn its limitations. In an interview earlier this year in his laboratory, Brophy said he expects the engine to fail sometime in 2003.

"It will be a very uninspiring failure," Brophy said. "Nothing dramatic will happen."

But that lack of drama -- possibly a short between the electrodes that accelerate the ions out the back of the engine -- would help engineers design more robust ion engines in the future.

But Deep Space 1 will not see 2003, at least not alive. If it survives the comet, and then endures the stress tests on the engine, its reward will be a rather unceremonious disconnection.

"By late November, in the very unlikely case the spacecraft is still working we will say goodbye to it then and have no further contact," Rayman said.

Here's his view of what would happen to the deaf and mute craft, if things get that far:

"The spacecraft eventually will slowly begin to rotate so that its solar arrays are not pointed to the Sun and the battery will drain. Then Deep Space 1 will just be another piece of cosmic flotsam. It will continue in orbit around the Sun just as any other member of the solar system family."

SPECIAL REPORT: Full coverage of the Deep Space 1 Flyby of Comet Borrelly