Mini moons may zip around Earth

One found so far, but many more of these huggable-sized asteroids exist, astronomers say

Mini moon trajectory map

JUST VISITING  A mini moon sidles up to Earth (yellow trajectory) and dances along one possible complex trajectory before leaving several months later (red trajectory). Earth and moon are not shown to scale, but the mini moon trajectory is.

K. Teramura/UH IfA 

HONOLULU — Earth most likely has groupies. A revolving door of tiny space rocks, or “mini moons,” might flit around our planet, and Robert Jedicke is determined to find them.

“Only one is known,” Jedicke said August 3 at a meeting of the International Astronomical Union. “It’s not fictional.”

With just one temporary tagalong in hand, though, researchers have relied on computer simulations to learn about these visitors from the asteroid belt between Mars and Jupiter. The one discovered in 2006 —roughly 3 meters wide (enormous by presumed mini moon standards) — orbited Earth for about a year. These elusive satellites are tantalizing targets for scoping out asteroids (SN: 8/23/14, p. 22) without having to go too far from home.

If only researchers could find more.

Typically no larger than a washing machine, mini moons are temporarily caught by Earth’s gravity. Simulations indicate that the average mini moon hangs around for just nine months. Our planet may attract two meter-sized moons at any one time, said Jedicke, a planetary scientist the University of Hawaii in Honolulu. Beach ball-sized mini moons buzz around Earth by the dozen, like a swarm of interplanetary gnats, while dump truck-sized asteroids swing by once every 50 years or so.

These are just theoretical predictions, says Steve Chesley, a planetary scientist at NASA’s Jet Propulsion Laboratory in Pasadena, Calif. “But [the mini moons] have to be there, otherwise it would be quite startling.” Since a steady stream of space rocks drifts from the asteroid belt toward the sun, Earth is bound to snag some of them. These moons are so tiny, however, that without a lot of time on a very large telescope, most remain hidden.

Jedicke has time reserved next year on the Subaru Telescope in Hawaii. With five nights on this 8-meter-wide telescope, there’s an 80 percent chance of finding one mini moon, he said.

A more dedicated search will have to wait until the Large Synoptic Survey Telescope comes online in Chile early next decade. The LSST will scan the entire southern sky once every three days. It could find dozens of mini moons as small as half a meter across every year, Jedicke said.

Mini moons will make great targets for space missions, Chesley notes. There’s no need to spend years observing and planning. “Anytime you want to launch your rocket and go to one of these things,” he says, “you can do it because they’re always going to be there.” 

Christopher Crockett is an Associate News Editor. He was formerly the astronomy writer from 2014 to 2017, and he has a Ph.D. in astronomy from the University of California, Los Angeles.

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