The clearest radar pictures of a near-Earth double
asteroid system were taken by astronomers last week using
NASA’s Goldstone radar telescope, revealing clues to the
system’s current structure but raising questions about its
origin and future.

A team of astronomers studied images that show the trail
of the smaller component orbiting a larger object, made with
the Goldstone radar, a 70-meter (230-foot) antenna in
California’s Mojave Desert. The asteroid, 1999 KW4, came
within five million kilometers of Earth (over 3 million miles)
on Friday, May 25.

“This system, 1999 KW4, is the third binary near-Earth
asteroid pair revealed by radar, but this is the first time
we’ve been able to image the system over a complete orbit of
one component around the other,” said Dr. Steven Ostro of
NASA’s Jet Propulsion Laboratory, Pasadena, Calif., leader of
the team that made the discovery. “Goldstone was able to track
the asteroid for up to eight hours daily for a week. Then we
made close-up images of each component using the Arecibo
telescope in Puerto Rico, which is not as fully steerable but
is much more powerful.”

The images can be seen at .

The radar team also included Dr. Lance Benner and Jon
Giorgini of JPL, Dr. Jean-Luc Margot of the California
Institute of Technology, Pasadena, and Dr. Michael Nolan of
Arecibo Observatory, Arecibo, Puerto Rico.

“The asteroid pair 1999 KW4 is classified a Potentially
Hazardous Asteroid because eventually its path through space
could intersect Earth. However, the radar measurements, which
are accurate to 15 meters (about 49 feet), indicate there is
no significant chance of 1999 KW4 colliding with Earth for at
least a thousand years,” said Giorgini. He said the larger
component is spheroidal and roughly 1.2 kilometers (three-
quarters of a mile) in average diameter, while the smaller
component is asymmetrical and roughly one-third as large.

“1999 KW4 is one of fewer than two dozen known asteroids
whose orbits cross the orbits of Mercury, Venus and Earth,”
said Benner. “However, the only known solar system bodies that
get closer to the Sun and have a more steeply inclined orbit
than 1999 KW4 are comets, so perhaps this object is an extinct
comet nucleus.”

“Our first look at the images suggests an orbital period
of roughly 16 hours,” said Margot. Later, detailed analysis
of all the radar data will determine very precisely the
period, which is the time it takes the smaller object to orbit
the larger one. Using the laws of celestial mechanics, the
team will measure the objects’ masses and densities, which
will tell what they are made of and how porous they are. For
single asteroids, that kind of information can only be
obtained by sending a spacecraft close to the body, and so
most asteroids’ densities, compositions and meteorite
associations are not well known. “Yet this kind of
information is the key to understanding relationships between
meteorites, near-Earth asteroids, main-belt asteroids and
comets,” said Margot.

“This might be the first discovery of an ex-comet’s
density,” said Dr. Don Yeomans, head of NASA’s Near Earth
Object program office at JPL. Three known objects are
officially designated both an asteroid and a comet.

“The existence of binary near-Earth asteroids raises
perplexing questions about their origins,” said Nolan.
“Nobody understands exactly how binary asteroid systems
formed, or even how stable the current binary systems are,
that is, how they might evolve, with the two components either
separating completely or collapsing onto one another to form a
contact binary. The theoreticians really have their work cut
out for them now.” Nolan said that the near-Earth binary
systems might have formed during certain kinds of collisions.
Or, if they came from loosely bound, unconsolidated piles of
rubble instead of solid rocks, binary asteroids might have
formed during close passages by Earth when gravity pulls them

The first binary asteroid was found in August 1993 when
NASA’s Galileo spacecraft took pictures of asteroid Ida and
revealed its tiny moon Dactyl.

Current statistics suggest that at least several percent
of the near-Earth asteroids are binaries. Ostro said that the
existence of binary asteroids on potentially hazardous orbits
means that we have to start figuring out how to maneuver
spacecraft close to such objects.

“Robotic spacecraft, and eventually people, are destined
to go to such objects someday, either for defense against one
of them, to exploit mineral resources, to satisfy our
curiosity about what they’re like close-up or simply for the
adventure of exploring a diminutive double world,” Ostro said.

JPL is a division of the California Institute of
Technology in Pasadena.