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 one unlikely setup, a asteroid 90 miles (145 kilometers) wide harbors a "moonlet" about one-tenth that size, just 500 miles away and orbiting once every 4 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 strange things in space are understood.

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 strange 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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