NUMBER 5
Asteroid Companions
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With these two asteroids, both about the same size, which
one is the moon?
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Our
conception of what a moon is has taken on a whole new meaning in recent years,
as scientists have discovered space
rocks orbiting other space rocks in
our solar system.
In one unlikely setup, an asteroid 90 miles (145 kilometers) wide
harbors a "moonlet" about one-tenth that size, just 500 miles away
and orbiting once every four days. In another, two asteroids of roughly equal size orbit around
a common midpoint.
Scientists are stumped. The math says these asteroids should not be able
to hold a moon in orbit.
"These objects exist despite the weak gravity field of the primary
asteroid," says William Merline of the Southwest Research Institute.
"The gravity field of the often odd-shaped asteroids is also irregular and
uneven, further making it difficult to keep a moon in orbit."
And how these small moons can avoid being destroyed by other space rocks
is a mystery to researchers. Merline says exploration by a spacecraft might be
needed to figure out how asteroids manage
to pair up,
and it could be a decade or two before these weird things in space are
understood.
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Weird Fact
Our own Moon is thought to have formed when
a Mars-sized object smashed into Earth.
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More About Asteroid Companions
The asteroid
companion systems
that have been found exist in the main
asteroid belt,
between Mars and Jupiter. Merline says the duos may have
been together for billions of years. But their small mass generates so little
gravity, even the tug
of the Sun
should be enough to disturb the delicate balance needed to sustain an orbit.
There are several theories about how asteroids gain a companion. The
most common is that they formed after getting whacked by another object. But the
systems so far discovered are each very different, indicating that several
processes might be at work, Merline says.
The rocky nature of space travel presents another problem. Asteroids in the main belt undergo many
collisions -- just look at the many craters on their surfaces. Large asteroids
might survive collisions, but smaller ones -- the moonlets -- should be turned
to rubble or knocked out of orbit.
"We know that the large primary asteroids may have avoided breakup
for most of the age of the solar system," Merline says. "But this is
not always true for smaller objects, like moons. So why are they still there?
Were they only formed recently? Or have they been broken up and reassembled
several times?"
Answering such questions about these weird things in space will help
researchers understand the formation of the solar system. After all, it was a
giant impact that carved a pretty well known Moon out of the third rock from
the Sun. Or at least that's how the
thinking
goes.
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