This image of Comet Borrelly was taken on Sept. 22, 2001, using the National Science Foundation's telescope at Kitt Peak National Observatory. CREDIT: NOAO/UA

A 1994 image of Comet Borrelly on one of its swings around the Sun.
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Comet Borrelly Puzzle: Darkest Object in the Solar System
By Robert Roy Britt
Senior Science Writer
posted: 02:30 pm ET
29 November 2001


Covered in a crust of blackness likened to the toner in a copy machine, a 5-mile-long potato-shaped comet called Borrelly has been found to be the darkest object in the solar system, scientists announced today.

The determination should help researchers learn what comets are made of, though one scientist said he can't figure how anything could be so dark.

Comet Borrelly reflects less than 3 percent of all the sunlight that hits it, about 1 percent less than NASA scientists initially suggested in the days following the Deep Space 1 spacecraft's Sept. 22 flyby when the first pictures of Borrelly were released.

Previously, the darkest object known was comet Halley, which reflects 4 percent of the sunlight it receives.

Comets are seen as pristine messengers from the formation of the solar system, but scientists rarely get close-up looks at them. Their interiors remain unexplored. Even their surfaces are largely enigmatic, because when comets are close enough to study -- anywhere near Earth -- they are also close to the Sun, which boils away the comet surface and hides it in a cloud of dust and vapor, called a coma.

Sunlight bounces off the particles in the coma and in a comet's tail, giving the objects their popular bright appearance that on occasion grace the night sky. But inside those bright halos, the story is altogether different.

Dark as asphalt

Borrelly's dark nature was revealed when Deep Space 1 took the best close-up images ever obtained of a comet's core, or nucleus, as it zoomed through Borrelly's coma.

"We have known for years that the surface of the earths Moon is dark -- about as reflective as an asphalt parking lot," said Robert Nelson, a project scientist on the Deep Space 1 mission at NASA's Jet Propulsion Laboratory. "The nucleus of Borrelly is about half as reflective as the Moon."

Scientists call this reflectivity -- the sunlight that is not absorbed by an object -- albedo.

"I'm not sure how you get an albedo that low," said Donald Yeomans, an expert on comets and asteroids at JPL. "It must have to do with texture -- it can't all be color."

Yeomans suggested that fine-grained surface structures might create shadows that can't be individually detected in the Deep Space 1 images. He and others are amazed at the complexity of Borrelly.

"The surface looks to be fairly bizarre," Yeomans said. It is riddled with bright and dark patches, smooth areas, jumbled hills and bright jets that emit material from its insides.

The overall darkness indicates what materials likely make up the comet.

Carbon and iron are eligible candidates, Nelson said in a telephone interview. But there is no way to know what lies underneath the crust of the nucleus, he said.

Nelson presented the findings today at a meeting of the American Astronomical Society's Division for Planetary Sciences in New Orleans. They are part of a trickle of discoveries that promises to soon turn into a flood as Borrelly becomes the most highly scrutinized comet ever.

"We've just scratched the surface with regard to analyzing this data set," Nelson said.

Separate view from Hubble

During the same week that Deep Space 1 imaged Borrelly from up close, the Hubble Space Telescope examined the comet's entire coma. The Hubble observations, which have not been released but will be published next year, were led by Alan Stern of the Southwest Research Institute.

Stern told that Hubble showed the coma was loaded with red dust, which he said is consistent with the dark appearance of the comet. His team also calculated the rate at which water evaporated off the surface of the comet, which he said was in line with predictions made in recent months of several tons every second.

Scientists had once described comets as dirty snowballs. Now they believe comets have less water than was previously calculated. "Icy dirtballs" is a more apt description, they say.

Stern and his colleagues made other findings, including a confirmation of the length of a day on Borrelly -- how long it takes to rotate on its axis.

"We measured the rotation rate, and we're still refining the number," he said. "It's about a day."

Scientists expect another NASA mission, called Deep Impact, will answer many of their questions about what's going on inside comets. Deep Impact's mission plan calls for shooting a camera-packing copper probe headlong into Comet Tempel 1 on Independence Day in 2005. The collision is expected carve out material and provide the first ever peek inside a comet.

SPECIAL REPORT: All the Pictures and Stories from Deep Space 1's Flyby of Borrelly

More About Comets: Astronomy News by Topic


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