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

JOE PALCA, BYLINE: Twenty years ago, a young astronomer named Robert Jedicke had a job working at a telescope on Kitt Peak in Arizona. Six nights a month he would sit at the telescope and watch the night sky go by.

ROBERT JEDICKE: As I'm watching the sky go by, I would often see these little trails.

PALCA: These little trails are usually made by objects traveling fairly close to the Earth. Every once in a while, one of these would look like it was actually orbiting the Earth, so Jedicke would report it to the center that keeps tabs on small planets and asteroids. And they'd always tell him he was seeing some space junk from an orbiting satellite.

JEDICKE: And I would say, well, what if it's a natural object in orbit around the Earth? And they say, no, there are no natural objects in orbit around the Earth.

PALCA: How do they know that, Jedicke wondered. It's a question that's been on his mind ever since. Now, he's an astronomer at the University of Hawaii, and with colleagues in Finland and France, he decided to look for a better answer. Astronomers know that asteroids pass by the Earth all the time.

JEDICKE: We know that there is a population of asteroids that are in orbit around the sun, that can come close to the Earth at some point in their orbit.

PALCA: Mostly they go whizzing by, not even slowing down to wave.

JEDICKE: But there's a very small sub-component of that population that are on orbits that are very much like the Earth's.

PALCA: And so these objects, when they go by the Earth, they come by very slowly, kind of like when you're jogging on a circular track and a slightly faster runner passes you.

JEDICKE: And by coming by so slowly, there's a small chance that they can be captured by the Earth's gravity and go into orbit around the Earth.

PALCA: The best estimates say about a million or more small objects pass close to the Earth each year. So what Jedicke and his colleagues did was write a computer program to calculate how many would be caught by Earth's gravity and go into orbit. The results were clear.

JEDICKE: At any time, so right now as we're speaking, there are one or two one-meter diameter asteroids in orbit around the Earth.

PALCA: Really? Really. Jedicke's simulation predicts that most of these objects are fairly small. But about once every 50 years or so there's one the size of a garbage truck.

JEDICKE: And maybe once every hundred thousand years, there'd be an asteroid that's about the size of a football field that's in orbit around the Earth.

PALCA: That's big enough to be seen with the naked eye. No one of these mini moons sticks around for very long. That's because while they are captured by Earth's gravity, it's a loose capture.

JEDICKE: So loose that the 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.

PALCA: Jedicke's calculations appear in the journal Icarus. And there's reason to think his conclusions are correct. Paul Chodas is with the Near-Earth Object program at NASA's Jet Propulsion Laboratory in Pasadena.

PAUL CHODAS: We keep track of all the small asteroids in the Earth's vicinity, and there was one, a very small asteroid, which was discovered to be in orbit around the Earth. We didn't know that right away, and we kind of thought it might be an old rocket stage or something, some left over junk from the space program. But the trajectory indicated it was an asteroid and not a rocket body.

PALCA: 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 mini moons.

Joe Palca, NPR News, Washington.

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