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  • Dick writes about Earth and planetary science for Science magazine.
 

Black Sheep Asteroid Is Back in Fold

3 August 1999 5:30 pm
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Asteroids tend to wander far from home, but researchers can now reunite a wayward offspring with its "parent" in the main asteroid belt between Mars and Jupiter. At a press conference today at the Jet Propulsion Laboratory (JPL) in Pasadena, California, researchers announced that observations of an asteroid called Braille, returned by the Deep Space 1 spacecraft last week, shows that the 2-kilometer-long rock is probably a chip blasted off the 500-kilometer asteroid Vesta, the third largest in the solar system.

First reports had last Wednesday's encounter with Braille a bust, with Deep Space 1's camera missing its target and pointing into empty space. However, some of the last data beamed back did provide images of a distant, lumpy, elongated body and "colors" of the asteroid in the infrared range. Apparently, Deep Space 1 lost sight of Braille as it approached the asteroid from its dark side, but reacquired its target soon after passing to the asteroid's daylit side, according to mission engineer Marc Rayman of JPL.

Team members regretted losing what would have been the closest look at an asteroid by far, but the infrared spectra proved some consolation. Braille's distinctive absorption pattern is "a remarkably close match" both to the asteroid Vesta in the main belt and to a type of meteorite known as a eucrite, said team member Laurence Soderblom of the U.S. Geological Survey in Flagstaff, Arizona. The close resemblance of eucrite spectra to that of Vesta had persuaded most astronomers and meteoriticists that eucrite meteorites come from Vesta, the only strong meteorite-asteroid link anyone has been able to make. Now asteroid Braille looks to be a bigger chip off Vesta.

Given the match with Vesta, planetary scientists have a plausible story of how Braille, as well as eucrite meteorites, were born. As impact specialist Eileen Ryan of New Mexico Highlands University in Las Vega, New Mexico, explained, Braille and the eucrites could have been blasted off Vesta in the huge impact that left a 460-kilometer crater visible in Hubble Space Telescope images of the asteroid. The likely debris can be seen as small, Vesta-colored asteroids near their parent and strewn across the asteroid belt to a point where Jupiter's gravity could fling it toward Earth. Braille hasn't yet gotten as far as the eucrites, but Soderblom noted that it will be drifting across Earth's path in the next few thousand years, possibly making it the planet's "Y6K" problem.

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