Deep encounter reveals asteroid's ancestry

Science News,  August 14, 1999  by R. Cowen

• E-mail
• Print
• Link

A tiny, wayward asteroid called 9969 Braille left home a long time ago, but astronomers may have just identified its parent. Infrared spectra taken by the Deep Space 1 spacecraft in late July suggest that Braille, only 2 kilometers long, is a chip off the old block 4 Vesta, the third largest asteroid in the solar system.

At 12:46 a.m. EDT on July 29, Deep Space 1 passed within 26 km of Braille. That's the closest a spacecraft has ever flown by an asteroid. At the crucial moment, however, the craft's camera did not point in the right direction and failed to capture close-up images of the rock.

Planetary scientists got a consolation prize, however. Infrared light reflected from Braille revealed the unmistakable fingerprint of the mineral pyroxene. Vesta is one of the few objects in the main asteroid belt, located between Mars and Jupiter, with a high concentration of this mineral.

"It's a perfect match," says Deep Space 1 researcher Daniel Britt of the University of Tennessee at Knoxville. "If I would have made up the data, I would have put in more noise to make it believable."

NASA released the Deep Space 1 findings last week at a press briefing at the Jet Propulsion Laboratory in Pasadena, Calif.

Recent images from the Hubble Space Telescope show that Vesta has a giant crater (SN: 9/20/97, p. 184). Braille, which had exited the main belt and crossed Mars' orbit, probably represents a fragment gouged from Vesta during the collision that made the crater. Alternatively, Vesta could be Braille's big brother rather than its parent, with both rocks ejected from an even larger body.

The link between Braille and Vesta is especially intriguing because NASA chose Braille at random to study, and few asteroids have shown a composition similar to that of Vesta. About 500 km in diameter, potato-shaped Vesta is the presumed source of a small class of meteorites that fall to Earth. Known as basaltic achondrites, the group accounts for only 6 percent of terrestrial meteorites.

Its placement in the asteroid belt prevents Vesta from directly delivering its fragments to Earth. In the early 1990s, however, researchers found a string of small asteroids with Vesta-like composition that lie at the right place in the belt to do so (SN: 10/24/92, p. 286). Like a breadcrumb trail, the discovery of several small asteroids, including Braille, that lie nearer Earth strengthens the association between Vesta and the meteorites, says Richard P. Binzel of the Massachusetts Institute of Technology. Braille is expected to cross Earth's orbit in a few thousand years.

Even if its camera had been properly pointed, the craft's photo opportunity wouldn't have been ideal. Deep Space 1 was hurtling past Braille at 40,000 km per hour. In contrast, the Near Earth Asteroid Rendezvous mission is expected to cozy up to the asteroid 433 Eros early next year and remain close for 12 months.

COPYRIGHT 1999 Science Service, Inc.
COPYRIGHT 2008 Gale, Cengage Learning