Earth Has Just One Moon, Right? Think Again

Frederic J. Brown/AFP/Getty Images

The last lunar eclipse of 2011 as seen from the San Gabriel Valley east of Los Angeles on Dec 10, 2011.

Everybody knows that there's just one moon orbiting the Earth. But a new study by an international team of astronomers concludes that everybody is dead wrong about that.

"At any time, there are one or two 1-meter diameter asteroids in orbit around the Earth," says Robert Jedicke, an astronomer at the Institute for Astronomy at the University of Hawaii.

Since most of these objects are too small to see, Jedicke had to use indirect methods to reach his conclusions. He started with a few well-known facts.

The minimoons that orbit Earth don't follow nice, smooth paths ? they seem to dance around the planet in the shape below.

Last week, Joe Palca told the story of the Messenger probe's journey to Mercury. It didn't take a smooth path, either ? watch the video below to see its trek to the innermost planet in our solar system.

Institute for Astronomy/University of Hawaii

The squiggly lines on this image show the path of a simulated minimoon that is temporarily captured by Earth. The asteroid in the corner of the image, 1999 JM8, is nearly 2 miles across and more than 1,000 times larger than the minimoons.

A Long, Twisted Path To Mercury

It took the Messenger spacecraft nearly eight years to reach orbit around Mercury. Follow the topsy-turvy path the probe took to reach the planet closet to the sun.

"We know that there is a population of asteroids in orbit around the sun, that can come close to the Earth at some point in their orbit," he says. Mostly these go whizzing by, not even slowing down to wave. "But there's a very small subcomponent of that population that are on orbits that are very much like the Earth's."

When these objects go by the Earth, they do so very slowly. It's like when you're jogging on a circular track, and a slightly faster runner passes you.

"And by coming by so slowly, there's a small chance they can be captured by the Earth's gravity and go into orbit around the Earth," says Jedicke. The best estimates say about a million or more small objects pass close to the Earth every year.

So what Jedicke and his colleagues, Mikael Granvik from the University of Finland and Jeremie Vaubaillon of the Observatoire de Paris, did was write a computer program to calculate how many would be caught by Earth's gravity and go into orbit.

For the most part, their calculations indicate that most of these objects are no larger than 3 feet across. But about once every 50 years, there's one the size of a garbage truck, "and maybe once every 100,000 years, there'll be an asteroid that's about the size of a football field in orbit around the Earth," Jedicke says.

That's big enough to be seen with the naked eye.

No one of these minimoons sticks around for very long. That's because while they are captured by Earth's gravity, it's a loose capture. "So loose that little gravitational nudges from the other planets in the solar system or from the moon can eventually just sort of dislodge them from the Earth's gravity and allow them to go back into orbit around the sun," says Jedicke.

The new research appears in the journal Icarus.

There's reason to think Jedicke's conclusions are correct.

"We keep track of all the asteroids in the Earth's vicinity," says Paul Chodas, with the Near-Earth Object Program at NASA's Jet Propulsion Laboratory in Pasadena. "And there was one very small asteroid which was discovered to be in orbit around the Earth. We didn't know that right away. ... We thought it might be an old rocket stage or some other junk left over from the space program. But the trajectory indicated it was an asteroid.

Chodas says now that there's good reason to believe they're there, astronomers will be able to adjust their observations to keep an eye out for these temporary minimoons.

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