A spacecraft's serendipitous rendezvous with a comet

A NASA probe is scheduled for a rare encounter with a comet's coma that may yield new insights.

By , Staff writer of The Christian Science Monitor

NASA's Deep Space 1 could be called the little spacecraft that might.

If all goes well tomorrow, the tiny craft will burrow into comet Borrely's coma, a cloak of dust and gas enshrouding its nucleus. The goal is to deliver the most detailed, close-up images of a comet's core in more than a decade.

The rare encounter is likely be a nail-biter. The craft was designed to test futuristic technology, not endure bombardment by a comet's rocky cast-offs. A balky navigation camera will probably force the craft to rely on less-precise gyros as it tries to find and photograph the comet's nucleus. (Camera problems nearly terminated the mission two years ago.)

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Despite the high risk of failure, mission planners say the "bonus" science and engineering data they stand to gain from the flyby is worth the effort.

"This is an adventure too exciting to pass up," says Marc Rayman, Deep Space 1 project manager at Cal Tech's Jet Propulsion Laboratory in Pasadena, Calif. But, he adds, "the real science return will come from future missions we have enabled" with this project.

If successful, tomorrow's rendezvous would represent the first in a set of early 21st-century encounters with comets. During the next few years, three US missions and one European project are slated to meet with comets and perform a range of tasks - from returning samples from a comet's coma to shooting probes into a comet's nucleus in order to map its size and structure. The European Space Agency's Rosetta mission, set for launch in 2003, will try to "soft land" a suite of instruments on a comet's nucleus.

A meeting with 19P/Borrelly is a fitting close, mission supporters say, to a successful project that NASA hopes will move advanced space technology from the pages of science fiction to the blueprints of spacecraft designers.

During its three-year cruise, Deep Space 1 has tested a dozen new technologies, including souped-up solar panels, a navigation system that uses artificial intelligence, and a motor that generates a flow of ionized gas that gently pushes the craft.

Although an ion motor delivers a weaker punch than a conventional rocket motor, an ion-powered craft can reach higher velocities because the motor can run continuously.

Deep Space 1's ion power plant has propelled the craft to comet Borrelly's neighborhood, currently some 126.5 million miles from the sun. The craft will close on the comet at 36,900 miles per hour and pass within about 1,250 miles of the nucleus. Deep Space 1, or what's left of it, is expected to clear the 31,000-mile-wide coma in about an hour.

Assaults on these "dirty snowballs" are motivated by planetary scientists' belief that comets are actually construction debris from the dawn of the solar system some 4.6 billion years ago. Because this debris is thought to be relatively pristine, researchers hope studies of the material will provide a window on conditions that prevailed early in the solar system's history.

"We suspect that comet nuclei are quite diverse, and probably no two are alike," says Joseph Veverka, a planetary scientist and chairman of Cornell University's astronomy department. "A good image of Borrelly's nucleus would certainly be useful as a comparison with more extensive data expected from missions such as Contour, Rosetta, and others."

Even a distant image will be valuable, Mr. Veverka adds, explaining that the success of future missions depends, in part, on knowing how starkly a comet's nucleus stands out from the brightness of the surrounding coma. The information is vital to optical navigation and remote-sensing measurements. Theoretical models have posited answers, he says, but those need to be tested.

In addition to images from a new camera prototype, other instruments will gather data on the composition of dust and gas the craft passes through and try to measure the material's interaction with the solar wind.

Comet Borrelly is a relative newcomer to the inner solar system. Scientists calculate that, sometime in the 19th century, Jupiter's gravity pulled the comet into an orbit that sends it streaking through the inner solar system once every 6.9 years.

The $152 million Deep Space 1 was launched on Oct. 25, 1998. On its way to completing its test program on Sept. 18, 1999, the craft flitted past the asteroid Braille. At the time, the craft registered the closest approach ever to an asteroid, passing some 17 miles from its surface.

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