About one in six of all near-Earth asteroids are
binaries – in other words, two bodies that travel in close
companionship as they orbit the sun. A new study reveals that
binaries most likely form when a single asteroid swings close to
Earth, is ripped apart by the planet’s tidal attraction, and
eventually reforms into separate bodies.

In a refereed article to be released Thursday, April 11 on the
Science Express Web site of the journal Science, California Institute
of Technology astronomer Jean-Luc Margot and his co-authors report
detailed information on the near-Earth asteroid currently assigned
the rather unpoetic name 2000 DP107, and also on four other binary
asteroids. 2000 DP107 comprises two bodies that are about three
kilometers apart, the larger of the two being about 800 meters in
diameter and the other about 300 meters. Using particularly detailed
radar data, the study is a description of the system and explains how
both these particular bodies and near-Earth binaries in general can
be formed.

Near-Earth asteroids were formed between Mars and Jupiter, like all
other asteroids, but are kicked into elliptical orbits by the
gravitational influence of Jupiter and occasionally pass near Earth.
An Earth-crossing orbit is one in which the asteroid actually crosses
the path that Earth follows around the sun, which means the two
bodies could eventually collide.

Margot, a postdoctoral researcher in the Division of Geology and
Planetary Science at Caltech, led the observations in October 2000
that uncovered 2000 DP107’s binary nature, just months after the
asteroid was first discovered by MIT researchers. The current study,
of which Margot is lead author, employs data obtained from the
70-meter Goldstone NASA tracking telescope and the Arecibo
Observatory’s radio telescope in Puerto Rico, which is funded by the
National Science Foundation with additional support from NASA and
operated by Cornell University, to yield a much more detailed picture
of the two orbiting bodies and their dynamics.

Other details from the radar data show that the two bodies are
probably in a tidal lock, which means that a person standing on the
larger body would always see the same face of the smaller body, but a
person on the smaller body would see the larger body spinning. This
is exactly like the tidal lock of the Earth-moon system.

Further, the research suggests that the tidal force applied to an
asteroid by a larger planet can be the cause of its breaking apart.
The process, known as “spin and fission,” means that a body
approaching Earth is made to change its spin rate. Specifically, the
tidal force tends to make an asteroid passing nearby spin at the
orbital rate, which can increase rather substantially in a close
approach to a planet. This increase in spin rate, coupled with the
tidal pull itself, can cause a loosely-bound, gravel-like
accumulation such as the near-Earth asteroids, to sling off material.
Later, the weak gravitational attraction of the particles allows the
material to reform in a second body.

But the most important issue raised by the paper is that near-Earth
binaries are so common, says Jet Propulsion Laboratory researcher
Steve Ostro, one of the authors. “The discovery of the existence and
substantial abundance of binary asteroids in Earth-crossing orbits is
a major one,” says Ostro, an expert on the radar characterization of
asteroids. “Presumably, binary asteroids have hit Earth in the past,
and will do so in the future.”

“Of course, the most important thing to know about any (potentially
hazardous asteroid) is whether it is two objects or one, and this is
why we want to observe these binaries with radar whenever possible.”

“The use of radar allows precise measurements of asteroid densities,
a very important indicator of their composition and internal
structure,” says Margot.

“Getting (near-Earth asteroid) densities from radar is dirt-cheap
compared with getting a density with a spacecraft,” Ostro explains.

In addition to Margot and Ostro, the other authors are Michael Nolan
of the Arecibo Observatory; Lance Benner, Raymond Jurgens, Jon
Giorgini, and Martin Slade, all of JPL; and Donald Campbell of
Cornell University.

Note to editors: Additional information and images are available at
the Web site http://www.gps.Caltech.edu/~margot/2000DP107