Binary asteroids -- two rocky objects orbiting about one another -- appear to be common in Earth-crossing orbits, astronomers report today in the journal Science. This makes them an important new asteroid class to study in case future generations find one coming near Earth.

Relative size and distance of binary asteroid 2000 DP107. Credit: NASA/JPL

"If you see two bodies orbiting each other, you can tell how far away from each other they are and how fast they go around each other," said Dr. Lance Benner, an asteroid researcher and an author of the paper from NASA's Jet Propulsion Laboratory, Pasadena, Calif. "This helps us to determine the asteroids' mass, volume, internal structure and what they're made of."

Using the world's two most powerful astronomical radar telescopes, Benner and his colleagues, led by Jean-Luc Margot of the California Institute of Technology, Pasadena, estimate that about 16 percent of near-Earth asteroids larger than 200 meters (219 yards) across are likely to be binary systems. These systems may have been formed by the pull of gravity during close encounters with our planet, Mercury, Venus or Mars.

The first near-Earth binary asteroid ever detected, 2000 DP107, was found by radar in September 2000 at NASA's Goldstone, Calif., tracking telescope facility. Subsequent observations were made at the National Science Foundation's Arecibo Observatory in Puerto Rico, operated by Cornell University. Like Earth's Moon, the smaller (300-meter or 1,000-foot diameter) body always presents the same face to the larger (800 meters, or about a half-mile diameter) asteroid body as it orbits. To date, five near-Earth binary systems have been identified by radar. But none of them, adds radar astronomer Jon Giorgini, have orbits that could threaten Earth, at least through this century.

Near-Earth asteroids may become binaries when the planets' much larger gravities pull on their rubble-clustered bodies, distorting them and sometimes breaking off a satellite. Theoretical and modeling results show that binary asteroids most likely form when the asteroids closely encounter the inner planets Earth or Mars, sometimes just 10,000 miles from a planet's surface.

"Of course, the most important thing to know about any asteroid is whether it is two objects or one, and this is why we want to observe these binaries with radar whenever possible," said Dr. Steve Ostro, a senior research scientist at JPL. "Radar is the best way to identify interesting and potentially hazardous asteroids. Radar observations provide information that can be later used by spacecraft to do more detailed studies efficiently and at lower cost."

Previous evidence that near-Earth binary asteroids were common came from craters on the Earth and Moon that formed in pairs and were exactly the same age. Astronomers also have noted the changes in brightness of reflected sunlight for some near-Earth asteroids, suggesting that a double system was causing an eclipse or occultation of one by the other.

Jean-Luc Margot, of the California Institute of Technology, led the research. The article is also co-authored by Michael Nolan, research associate at Arecibo; Raymond Jurgens, Jon Giorgini and Martin Slade at the Jet Propulsion Laboratory; and Donald Campbell, professor of astronomy at Cornell University, Ithaca, New York. The observations were made at the 70-meter Goldstone NASA tracking telescope in California and at Arecibo Observatory, which is operated by the National Astronomy and Ionosphere Center at Cornell under a cooperative agreement with the National Science Foundation. JPL, a division of the California Institute of Technology, manages many missions for NASA's Office of Space Science, Washington, D.C.