More and more supposedly single asteroids are turning out to be pairs traveling through space together. Only a few weeks ago, astronomers reported the third and fourth of such twins, orbiting out in the main asteroid belt between Mars and Jupiter (ScienceNOW, 21 September ). Now planetary scientists have for the first time found twin asteroids near Earth. Although no immediate threat to our planet, this is the first pair close enough to have created the double craters found around the world.
DP107, as the single asteroid was known, was just 7 million kilometers from Earth--a 40th the distance to the asteroid belt--when its true nature was found. That nature emerged during a long-running survey of near-Earth objects conducted by Steven Ostro of the Jet Propulsion Laboratory in Pasadena, California, and his colleagues. By bouncing radar pulses off a near-Earth object and using the Doppler effect induced by its rotation, they can make a picture of sorts. Observations using NASA's Goldstone radar in Southern California on 22 and 23 September revealed that DP107 was two separate objects traveling at least a kilometer apart at times, according to the International Astronomical Union Circular announcing the discovery.
Theoreticians already have a plausible explanation for binaries like DP107. In 1996, planetary dynamicists William Bottke of the Southwest Research Institute in Boulder, Colorado, and Jay Melosh of the University of Arizona in Tucson proposed that Earth's gravity could split an asteroid in two if it passed nearby, assuming that numerous collisions with other asteroids had already reduced it to a flying pile of rubble.
How often does this happen? Astronomer Petr Prevac of Ondrejov Observatory near Prague recently estimated that about one-sixth of near-Earth objects may actually be twins. Based on the abundance of double craters pocking the planets, Bottke and Melosh came to the same estimate for asteroids crossing Earth's path. That could make it more difficult to send future spacecraft to orbit them, notes Ostro; and diverting a pair on a collision course with Earth would make Bruce Willis's job in the movie Armageddon look like child's play.
The IAU circular announcing the discovery