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.

“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

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. More information on asteroid radar research
is available at
and .