Probe’s comet encounter yields close-ups

Operating on a makeshift navigation system and performing an extra mission assigned on the fly, NASA’s Deep Space 1 probe (DS1) has executed a stunning rendezvous with a comet.

The probe passed within 2,200 kilometers of Comet Borrelly’s frozen nucleus and through the comet’s coma of dust and gas. During the fly-by, DS1 captured black-and-white and infrared images of the nucleus as well as data about ions and other particles that radiate from it.

The Sept. 22 encounter occurred between the orbital paths of Earth and Mars, just days after Borrelly reached the point in its 6.9-year orbit closest to the sun. The only other probe that’s photographed a comet’s nucleus is the European Space Agency’s probe Giotto, which encountered Comet Halley in 1986.

Data from DS1’s encounter hint at some surprises. The center of Borrelly’s envelope of ions is offset from the nucleus by about 7,000 km. This unexpected finding, supported by visual images, indicates that the fissures spewing jets of ionized gas are clustered on one side of the comet’s nucleus, says David T. Young, a mission scientist.

The encounter also confirms some assumptions. Harold A. Weaver, a comet specialist at Johns Hopkins University in Baltimore, is pleased by images showing that the nucleus is 8 km long and 4 km wide. Using data from the Hubble Space Telescope of Borrelly’s brightness, he had predicted those dimensions, but nailing them down “takes a spacecraft flying through the coma,” he says.

Comet Borrelly wasn’t on the itinerary when scientists at NASA’s Jet Propulsion Laboratory in Pasadena, Calif., launched DS1 in October 1998. The probe completed its primary mission–testing its payload of a dozen experimental technologies, including an ion-propulsion engine–in September 1999. That’s when the team decided to go after Borrelly.

Just months into the 2-year journey to the comet, however, DS1’s main navigational camera broke. Scientists managed to jury-rig another on-board camera to track stars, enabling the crippled probe to keep navigating. Still, they weren’t sure that DS1, built without a protective coating, could withstand the whizzing dust particles in the comet’s coma.

The probe’s success bodes well for several upcoming missions to comets. Further data analysis will also help scientists know what to expect when navigating probes within the comas, says Joseph Veverka, an astronomer at Cornell University. Veverka and his team plan to usher a NASA probe called Contour past at least two comet nuclei after its launch in July 2002.