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Galileo snaps first close-up of an asteroid.

Asteroids have intrigued skywatchers since astronomers first detected these rocky bodies nearly 200 years ago. From Earth, they appear only as faint streaks of light, leaving many unanswered questions about their origin, shape and chemistry. Last week, that state of knowledge changed dramatically as the Jupiter-bound Galileo craft radioed back the first close-up images ever taken of an asteroid.

On Oct. 29, Galileo's camera snapped 150 pictures as it passed within 1,600 kilometers of a tiny asteroid called 951 Gaspra. The images--only a few of which actually show the asteroid--were stored in an on-board tape recorder because researchers have been unable to open the craft's main antenna (SN: 8/3/91, p.79).

As recently as mid-October, researchers doubted they could retrieve any Gaspra images before December 1992, when the craft swings around Earth again to gain speed on its way to Jupiter. The problem: Relying on a small antenna that transmits at the painfully slow rate of 80 hours per picture, Galileo would be unable to ratio more than a few images during the two weeks following the Gaspra flyby. Scientists decided the transmission effort wouldn't even be able bother, since they wouldn't even be able to determine which stored images contained the asteroid.

As Galileo closed in on Gaspra, however, an unusually successful navigation enabled engineers to identify four likely asteroid images among a mosaic of 36 photographs. In fact, the two visible-light and two near-infrared pictures relayed to an Australian radio receiver show Gaspra at dead center, scientists reported at a briefing last week at the Jet Propulsion Laboratory in Pasadena, Calif.

These images offer graphic testimony to Gaspra's violent history, says Cornell University astronomer Joseph Veverka. The irregularly shaped asteroid, which he likens to a dented football pitted with craters, measures about 12 by 20 by 11 kilometers, he says. The craters that pepper its surface range in diameter from 2 kilometers to 160 meters--the smallest features visible in the images.

Researchers believe Gaspra is a fragment chipped from a larger object. "We suspect [Gaspra] is a survivor of a series of catastrophic collisions . . ., in which a succession of parent bodies got broken down into smaller and smaller pieces," Veverka says. On the basis of the degree of cratering discerned in the photographs and the estimated frequency of collisions that Gaspra would suffer at its current location -- about 411 million kilometers from Earth, near the inner edge of the asteroid belt -- astronomers calculate that the asteroid may have taken its present shape a relatively scant 300 million years ago. Before the flyby, Richard P. Binzel and Noriyuki Namiki of the Massachusetts Institute of Technology reported a similar estimate in the June GEOPHYSICAL RESEARCH LETTERS.

Gaspra's brush with violence didn't end there, Veverka notes. A series of apparent ridges or grooves on its surface suggests that a massive asteroid or some other energetic object has since slammed into Gaspra with nearly enough force to smash it into bits, he says. Astronomers have identified similar grooves on Phobos, a Martian moon roughly the same size as Gaspra.

Even Galileo's highest-resolution images and spectroscopic data, which the craft can't transmit until sometime next year, may not reveal Gaspra's interior composition, several astronomers note. The softened features of the asteroid's surface suggest that a layer of dust or soil 3 to 10 meters deep may blanket Gaspra, making it difficult to probe the interior, Veverka says.

Further analysis, he adds, may settle a debate over whether Gaspra and similar asteroids contain material that has remained basically unchanged since the solar system formed. Some scientists contend that blasts of heat may have altered the composition of these asteroids so that iron and other dense materials concentrate in the core white lower-density minerals lie closer to their surfaces.

A first look at a composite of the four Galileo images reveals subtle color differences that suggests at least some variation in Gaspra's chemical composition. And Robert W. Carlson of the Jet Propulsion Laboratory, who leads studies with Galileo's near-infrared mapping spectrometer, says the low-resolution data he received hint at compositional differences between the asteroid's northern and southern hemispheres. Finding fresh craters that expose subsurface material might offer the best hope of studying the interior, Veverka adds.

If Galileo had flown past a larger asteroid -- as intended before the 1986 Challenger disaster delayed the craft's launch -- it could have measured the object's gravitational tug, enabling researchers to infer its chemical makeup, Veverka says. Such studies must now await future space missions that will rendezvouz with an asteroid rather than whiz past it, says William K. Hartmann of the Planetary Science Institute in Tucson, Ariz.
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Title Annotation:space probe
Author:Cowen, Ron
Publication:Science News
Date:Nov 23, 1991
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