Earth Has Just One Moon, Right? Think Again | WAMU 88.5 - American University Radio
Filed Under:

Earth Has Just One Moon, Right? Think Again

Play associated audio

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.

"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.

Copyright 2012 National Public Radio. To see more, visit http://www.npr.org/.

NPR

After The Presents, A Buttery Tea Cake Tradition

For one family in Overland Park, Kan., it's not Christmas without Mrs. Lawrence. The tea cake, rich with butter and spices, is named for the neighbor who would hand deliver it every holiday season.
NPR

Why Bury Fig Trees? A Curious Tradition Preserves A Taste Of Italy

For generations, Italian-American fig growers in the Northeast have buried their trees in trenches for the winter. It's a tradition that preserves both flavor and ancestral ties to southern Italy.
NPR

What To Expect In The 2016 Presidential Announcement Season

With Jeb Bush signaling he's likely to run for president in 2016, it's another sign that the presidential announcement season is underway. Here's a look at who has jumped in the race early and what to expect in the coming months.
NPR

Online Sellers Pop Up In Real Life, For A Limited Time Only

One-click shopping is changing the ways people shop and retailers sell their wares. But some online retailers are opening physical stores — some of which last as short as a day.

Leave a Comment

Help keep the conversation civil. Please refer to our Terms of Use and Code of Conduct before posting your comments.