Hard bodies pair off

About one out of every eight asteroids orbiting near Earth travels with a companion. Scientists have come up with that estimate by observing near-Earth asteroids, rocks that cross Earth’s orbit, with the two most sensitive radar telescopes ever built. Some 16 percent of near-Earth asteroids larger than 200 meters across travel with a companion, report Jean-Luc Margot of the California Institute of Technology in Pasadena and his colleagues in the May 24 Science.

During the past 2 years, the astronomers discovered the first five pairings of near-Earth asteroids. To detect those objects, Margot’s team used the Arecibo Observatory in Puerto Rico and NASA’s Goldstone Tracking Facility in California’s Mojave Desert. The astronomers found the pairs by beaming precisely tuned radio waves at the space rocks and then measuring the time it took for the waves to bounce back to radio telescopes on Earth.

By detecting how far apart the partners are and how rapidly they orbit each other, scientists can determine the mass, volume, and composition of the binary asteroids. Astronomers suspect that many asteroids are relatively fragile, composed of rubble that can range in size from sand grains to kilometer-wide boulders (SN: 7/28/01, p. 61: A Rocky Bicentennial). Simulations suggest that binary asteroids form when a single rock passes so close to either Earth or Mars that the planet’s uneven gravitational tug on different parts of the asteroid causes it to crumble.