Flying on a wing and a prayer last Saturday, NASA's "aged and wounded" Deep Space 1 spacecraft returned pictures of the dirty snowball buried within comet Borrelly, revealing its geology for the first time. At a press conference today at the Jet Propulsion Laboratory (JPL) in Pasadena, California, scientists described diverse terrain of the comet's nucleus, only the second to be seen this close. Deep fractures and billowing jets of vaporized ice hint at a potentially catastrophic demise for the 8-kilometer-long, bowling-pin-shaped object.
Launched in 1998, Deep Space 1 was designed as a test bed for a dozen advanced technologies, including its exotic ion propulsion. A 16.5-kilometer-per-second dash through the gas and dust continually blown off a comet nucleus was a risky bonus. Complicating matters, its star tracker, the spacecraft's only means of orienting itself, failed in 1999. With its camera juryrigged as a replacement, "the encounter did not go the way we expected," said project manager Marc Rayman of JPL, "it went perfectly." By sheer luck, the spacecraft dodged a massive jet of dust and gas to return analyses of ions in the comet's coma of dust and gas, infrared spectra of the nucleus, and remarkably crisp black-and-white pictures.
These detailed pictures revealed a rugged terrain. Each end of the nucleus has plateaus. A smooth, brighter plain at the center is emitting at least three columnar jets where the sun's heat is excavating a saddle-shaped depression. In addition, fractures crisscross the comet, a number right in the thin neck of the bowling pin, according to planetary geologist Laurence Soderblom of the U.S. Geological Survey in Flagstaff, Arizona. "It's quite possible" Borrelly could break in two, either at the center or at the neck, he says. Erosion would be hastened by Borrelly's orientation that keeps the jetting saddle permanently in the sun, adds comet specialist Donald Yeomans of JPL. Eventually, it might even break in many pieces and vanish, just like comet LINEAR did in July 2000 (ScienceNOW, 28 July 2000).
Deep Space 1 will meet a less spectacular end: In November, after more strenuous testing of its ion engine, its controllers will simply stop talking to it.