Our moon is not alone: Scores of unseen mini-moons are now in orbit around Earth, new computer models predict.
What's more, these tiny moons occasionally plummet through our planet's atmosphere, creating brilliant fireballs, the researchers say.
The findings are based on supercomputer simulations of ten million asteroids known to fly through the Earth-moon system. The models show that objects that circle the sun in orbits similar to Earth's are likely to be captured as mini-moons.
"We accurately tracked their motion—including the gravitational tugs from the sun and all the other planets and big asteroids in the solar system—and found that 18,000 of [these asteroids] were captured and briefly went into orbit around the Earth," said study co-author Robert Jedicke, an astronomer at the University of Hawaii.
"We estimate that there are one or two washing machine-size mini-moons and about a thousand larger than a softball [orbiting Earth] at any time," he said.
(Related: "Earth Had Two Moons, New Model Suggests.")
The captured moons would orbit Earth in twisted, convoluted paths. In fact, the simulations show that most mini-moons hang around for less than a year before they're either spit back out to orbit the sun or end up on a collision course with Earth, Jedicke said.
"The moon perturbs the orbit of about one in a thousand, so they hit the Earth—some of the meteors that you see at night are actually mini-moons falling to Earth."
Prehistoric Double Moons?
In addition to small space rocks, the models predict that once in awhile Earth captures something even larger.
The team's estimates show that every half century an object the size of a large dump truck—about 33 feet (10 meters) across—joins our roughly 2,100-mile-wide (3,400-kilometer-wide) moon.
And even larger objects—each the size of a football field, or about 328 feet (100 meters) across—can be captured by Earth's gravity every hundred thousand years.
At that size, Jedicke speculates, the extra moons might even be visible to the naked eye.
"A hundred thousand years is about the time frame that human beings have been doing things like leaving their handprints on cave walls, so maybe in that time frame somebody once actually looked into the sky and saw a mini-moon moving across the sky," he added.
Jedicke and his team are the first to make predictions about mini-moon sizes and distribution, and it appears their predictions are fairly accurate.
The only known mini-moon was a 9.8-foot-wide (3-meter-wide) asteroid dubbed 2006 RH120, which orbited Earth less than a year before resuming its previous life orbiting the sun.
"The size and orbital properties of 2006 RH120 are perfectly consistent with our models," Jedicke said. "Had we done our study ten years ago, we could have predicted that an object like 2006 RH120 would be detected soon."
Mini-Moons Still Hard to Spot
Even with the new simulations, the researchers caution that actually seeing more mini-moons will be challenging, because the objects are relatively small and thus faint.
In addition, the gravitational effects that draw in Earth's extra moons tend to set them whipping around the planet at high speeds, making them even harder to pinpoint.
(Find out about an asteroid that recently crossed between Earth and the moon.)
"We are currently trying to figure out how to use astronomical surveys to spot them regularly," Jedicke said.
For instance, "the largest ones could be detectable by the advanced amateur astronomer with a 50-centimeter-diameter [20-inch-diameter] telescope," he said.
"But discovering new mini-moons will require an asteroid survey that covers much of the sky in a single night and detects objects that are very faint."
The study of Earth's mini-moons was published in March in the journal Icarus.