An asteroid once thought to
be on a collision course with Mars passed the Red Planet today without
incident.
Astronomers first estimated
that asteroid 2007 WD5 had as high as a 3.6 percent chance of striking the
planet. Newer observations kept lowering the odds for the 164-foot space rock
until Jan. 9, when NASA's Near-Earth Object (NEO) program office effectively ruled
out chances of an impact.
"Mars sees these kinds
of near-miss encounters every ten or twenty years, but the impact rate for
asteroids this size is about once in a thousand years," said Steve Chesley,
an astronomer at the Jet Propulsion Lab in Pasadena, California.
Astronomers had hoped the
fleet of spacecraft orbiting Mars would get a chance to observe the asteroid
plowing into the Martian surface. The subsequent crater would have roughly
equaled the size of the Meteor Crater that formed in northern Arizona
50,000 years ago, with a 0.5-mile diameter. Such an impact would have also
allowed scientists to study the dust cloud from the impact.
"We were hoping for a
spectacular show to reveal a lot," Chesley said. "We've actually
never seen a significant impact on a terrestrial planet."
Mars is a smaller and harder
target for space rocks to hit when compared with Earth, but about five times as
many asteroids cross the Martian orbit, according to Chesley. 2007 WD5's path
around the sun ranges from just outside Earth's orbit to the outer edge of the
asteroid belt between Mars and Jupiter, but will not impact with either Mars or
Earth in the next century, JPL researchers said.
The asteroid missed Mars by
a distance of approximately 6.5 Mars radii.
Similar near misses occur with
Earth. And similarly, astronomers sometimes give odds on a possible impact and
then, with further observations, reduce the odds to zero.
In fact, the Mars flyby
occurred a day after a 500-foot
asteroid flew by Earth at a distance somewhat greater than from the Earth
to the Moon.
Chesley and other
astronomers considered having one of the Martian rovers eyeball the passing
2007 WD5, but judged the task too difficult for the robotic explorers. None of
the orbiting spacecraft turned their cameras or other equipment on the passing
rock, either.
"After we knew it was
going to miss, it's really a pretty ordinary asteroid cruising around the solar
system," Chesley said.
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