Radar reveals an asteroid's strange shape.

Radar reveals an asteroid's strange shape

Radar images of a small, recently discovered asteroid depict not a single sphere or even a roundish lump, but two such lumps side by side. Steven J. Ostro of Jet Propulsion Laboratory in Pasadena, Calif., describes the object -- designated 1989 PB -- as "two-lobed" or "bifurcated."

Less than 2 kilometers long and rotating on its axis about every four hours, the stony or "S-type" asteroid orbits the sun about every 400 days. Its elliptical path crosses Earth's orbit and carries it from between the orbits of Venus and Mercury out beyond the orbit of Mars. When discovered, the asteroid was as close to Earth as it comes -- about 4 million km away, Ostro says. It will not pass that close again for several decades.

Eleanor F. Helin of Jet Propulsion Laboratory made the discovery on Aug. 9 using an optical telescope on Palomar Mountain. Ten days later, a team headed by Ostro observed the asteroid with the 300-meter Arecibo radiotelescope in Puerto Rico. The group included John F. Chandler and Irwin I. Shapiro of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass., and Alice Hine of Arecibo.

The Arecibo radar images of the rotating asteroid, made at approximately 9-minute intervals, show it froma distance of about 5.7 million km. Like frames of a motion picture, the images form a sequence, offering views of each side. Though they show no details smaller than about 300 meter across, Ostro says computer analysis over the next few months will make it possible to reconstruct the asteroid's three-dimensional shape. The individual images in the sequence show only one view of 1989 PB at a time, but Ostro expects to create a computer graphic that scientiests can rotate to examine it from different perspectives. In addition, scientiests could translate the images into a solid model of the asteroid, revealing in crude detail any craters, depression or high elevations.

Analysis of the asteroid's shape should "let us make some statements about how tightly gravity is holding the two lobes together and begin to formulate theories about how it was formed," Ostro says.

Astronomers know little about such basic asteroid questions and have hardly more than hypotheses at this point. "Probably sometime within the last 100 million years," says Ostro, noting that the orbits of most "Earth-crosser" asteroids probably evolved no longer ago than that, "there was a big, violent collision between objects in the main asteroid belt [lying between the orbits of Mars and Jupiter]. A lot of stuff was thrown out at very high velocity, some at low velocity; some chunks were fractured, some unfractured."

Among all those chunks, he says, some may have ended up close to one another and moving at similar speeds, until mutual gravitational attraction drew them together. This process could have produced multilobed shapes like that of 1989 PB. And the collisions that formed these asteroids in the main belt may well have thrown some of them into orbits nearer the sun, where Earth-based astronomers can observe them more easily and spacecraft can visit them more cheaply.

Ostro says improvements to Arecibo, as well as spacecraft missions such as the planned Comet Rendezvous Asteroid Flyby (SN: 12/3/88, p. 358), may reveal a variety of odd shapes among small asteroids, because chunks less than a few tens of kilometers across probably would not have been compressed into spheres by their own gravity. Of the 56 asteroids he has studied by radar during the past decade, 23 are Earth-crossers--including three or four whose radar signatures, though not as detailed as those of 1989 PB, at least suggest concavities that could signify large craters or other low spots, he says.