Meet Comet Borrelly

Comet Borrelly
From a mere 3,400 kilometers away, Deep Space 1 took this picture of Comet Borrelly's nucleus — the highest resolution view ever seen of a comet (45 meters per pixel). At the time the spacecraft was 160 seconds from its closest point to the nucleus. The bowling-pin-shaped body is about 8 km long and features a rocky, rough terrain with dark and light patches.
Courtesy NASA/JPL.
In November 1999, when the Deep Space 1 (DS1) spacecraft was flying blind after losing its navigation camera, Marc Rayman and his engineering team surely despaired that their craft would never make its date with Comet Borrelly nearly two years later. But survive it did , and on the night of September 22nd the struggling spacecraft swept just 2,170 kilometers (1,350 miles) from the comet's icy heart. "I have to tell you that the encounter didn't go as anticipated," Rayman deadpanned at a press conference today. "In fact, it went perfectly."

The results, seen here and on the mission's web site , are the most detailed images yet of a cometary nucleus, surpassing views of Halley's Comet taken 15 years ago by the Giotto spacecraft. Measuring about 8 km long, Borrelly's nucleus has an elongated bowling-pin shape, and its jumbled surface displays a surprising range of light and dark markings, which in reality are dark and extremely black. Cosmochemists believe the nucleus is likely coated with a patchy veneer of carbon — and organic-rich slag. "These pictures have told us that comet nuclei are far more complex than we ever imagined," says Laurence Soderblom, who heads DS1's camera team. "They have rugged terrain, smooth rolling plains, deep fractures, and very, very dark material." Another team member more pointedly likened the nucleus to "a Dove Bar the size of Mount Everest."

Images from the spacecraft (and from the 5-meter telescope on Palomar Mountain as well) reveal that this interplanetary iceberg sports a narrow, Sunward-pointing jet of vaporized ice
and dust that looks like it was shot from a trio of side-by-side cannons. And, in fact, one cometary model suggests that over time such ice-fueled jets should eat their way down into the nucleus,hollowing out "wells" from which gas and dust escape. However, Soderblom notes that the main jet-producing region appears to be a flat, relatively bright "plain."

Comet Borrelly
This view of Comet Borrelly has been enhanced to show the jets of dust coming off the comet's icy heart. The main jet (visible from Earth) actually consists of three smaller outbursts (seen on the nucleus' bottom-left limb).
Courtesy NASA/JPL.

Known officially as 19P (for "periodic"), Borrelly circles the Sun every 6.8 years and is thought to have formed in a different part of the primordial solar nebula (and thus have a different composition) than Halley, which has a 76-year orbit.
However, aside from the obvious signature of water in DS1's ion data, and a second feature that suggests carbon monoxide, Borrelly's composition remains uncertain. "We've only scratched the surface," notes project scientist Robert M. Nelson. Luckily, the nucleus passed directly through the entrance slit of the craft's infrared spectrometer, and these observations may show what coats the icy surface.

Another surprise was found in the cloud of ionized gases streaming away from the nucleus. This million-degree plasma wasn't concentrated directly in front of the nucleus, as had been expected, but was instead offset from it by some 2,000 km. "This is just like a jet fighter flying along with its shock wave off to one side," explains experimenter David Young. "It's in the wrong place, period, and we have to figure out why that is." Young, Soderblom, and the rest of Deep Space 1's science team hope to present much more detailed findings at a scientific meeting in late November. However, observes comet expert Donald Yeomans, what's been learned from DS1's results in the last few days already represents "a giant step forward in our understanding of the cometary nucleus."

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Kelly Beatty

About Kelly Beatty

J. Kelly Beatty, S&T's Senior Editor, joined the staff of Sky Publishing in 1974 and specializes in planetary science and space exploration. Learn more about him here.