Earth Usually Has Second Tiny, Temporary 'Moon'

By John Matson
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The moon has been with us for billions of years, almost since the formation of Earth. From then on, Earth has also had countless other satellite companions. In fact, we probably have one right now.

But these objects are not full-fledged moons. They're more like temporary, extremely small moons. Called irregular natural satellites, they’re boulders from the large population of near-Earth asteroids that get snagged by our gravity. They orbit the Earth for a few months, then escape and move on.

A new study on the Web site arXiv.org says that such satellites should be very common. [Mikael Granvik, Jeremie Vaubaillon and Robert Jedicke, The population of natural Earth satellites] At any given time we probably have one temporarily captured orbiter of about a meter in size.

These space rocks are awfully hard to see. But astronomers seem to have spotted one such transient satellite in 2006. The asteroid, dubbed 2006 RH120, was a few meters in diameter. It was captured by Earth for about a year and then broke away back to interplanetary space.

But not all temporary orbiters escape. About one percent of them actually impact Earth during their captivity, according to the study. Fortunately, the real moon keeps a respectful distance.

[The above text is a transcript of this podcast]