One hundred and thirty-seven million miles from Earth, NASA's revolutionary probe, Deep Space 1, is hurtling towards its almost-certain doom.
Launched in 1998 to test a series of technologies straight out of Star Trek -- including an ion propulsion engine and self-directed guidance system -- Deep Space 1 will attempt one last science-fictionesque feat at 3:30 p.m. PDT Saturday: piercing a comet's gaseous outer layer to examine its nucleus.
It would be only the second time that a comet's core has been observed: The European Space Agency's look at Halley's comet was the first. And with debris from Comet Borrelly likely to smash into Deep Space 1 at 36,900 miles per hour, it's doubtful the probe will survive the encounter intact.
Scientists at NASA's Jet Propulsion Laboratory hope to get a series of infrared and black-and-white pictures of Comet Borrelly's nucleus before Deep Space 1's demise.
Although the approximately $150 million probe is the cheapest interplanetary mission ever conducted by NASA, its new operation is the equivalent of an arcade game's extra ball. Deep Space 1 has already concluded its primary mission, testing advanced navigation, propulsion and communications systems -- many of these for the first time.
The craft's ion propulsion engine, which is 1,000 pounds and 38.6 feet long, is the most important of these technology demonstrations.
Like a standard spacecraft engine, the ion engine accelerates gas to thrust the probe forward. But the ion engine consumes xenon, instead of hydrazine -- a blend of nitrogen and hydrogen, as its fuel. And it uses electrical charges and attraction, instead of heat and pressure, to make the gas go fast.
"The first time I ever heard of (ion propulsion) was on Star Trek," said Dr. Marc Rayman, Deep Space 1's project manager at the Jet Propulsion Lab in Pasadena, California.
The blue-glow thrust from such a system is slight -- only 20 thousandths of a pound -- but it's enough in the vacuum of space, after weeks of constant use, to propel the craft at thousands of miles per hour.
This is the first time any spacecraft has used such a system to go anywhere -- although there were some limited tests of ion propulsion in the 1960s.
Deep Space 1's engine has now worked longer than any propulsion system on any spacecraft ever, with 591 days of operating time. It's been able to last this long because the ion system consumes only a tenth of the propellant used by a standard engine.
"Deep Space 1 has demonstrated that electrical propulsion is a practical method to get to objects in the solar system," said Joseph Veverka, chairman of Cornell University's astronomy department.
The craft employed a largely autonomous navigational system to get to these objects.
Once a week, the craft would point its camera at one of the 40 to 60 asteroids whose orbits the craft has on file. By comparing the position of the asteroid to Deep Space 1's catalog of 250,000 stars, the probe could determine its location.
"It was the first craft to figure out on its own where it was in the solar system," Rayman said.
Armed with this knowledge, and an understanding of its own speed, the planets' gravitational pulls and the basic flight plan, Deep Space 1 knew where it was going and could make adjustments on its own to meet the flight plan's objectives.
This autonomous system is critically important, Rayman said, because the means for communicating with probes -- the series of antennas known as the Deep Space Network -- is already at near-maximum capacity. Self-directed probes can reduce the load on the Network.
In July 1999, Deep Space 1 used the independent navigation system in a close flyby of the asteroid Braille, where it took a limited number of pictures, but not as many as mission planners had hoped.
Four months later, Deep Space 1's navigation program failed. But rather than leaving the craft for dead, NASA scientists spent the next nine months retooling the system. The craft is now using a traditional, ground-based scheme to meet Comet Borrelly.
The "Deep Impact" comet exploration mission, set to launch in January 2004, will use a version of the autonomous navigation that Deep Space 1 pioneered.
