THE moon may look lonely, but it is far from alone. Small asteroids too dim to detect seem to stray into Earth's orbit quite frequently and stay for short periods of time. We may even be able to bring one of these moonlets back to Earth for study.

Researchers have long suspected that wandering asteroids might occasionally get snagged by Earth's gravity and become temporary moons, and a few years ago one of these was spotted. Called 2006 RH120, it is a few metres across and wandered into orbit around Earth in July 2006 before drifting off again a year later.

Some initially thought it might be a discarded rocket stage. But Paul Chodas of NASA's Jet Propulsion Laboratory in Pasadena, California, analysed its motion and showed that it was too heavy to be a rocket stage, making the asteroid explanation more likely. But it wasn't clear how common such captured asteroids might be. Now, new calculations suggest many other temporary moons are orbiting Earth, but are too small and dim to be easily spotted by surveys of the sky.

Mikael Granvik of the University of Helsinki in Finland and colleagues ran computer simulations of the abundance of asteroids of various sizes in Earth's neighbourhood and the likelihood of their capture in a close encounter.

To be captured, an object must start out in an orbit nearly identical to Earth's. That means it is travelling at roughly the same speed as our planet, making it easy for it to get snagged by Earth's gravity, helped by gravitational tugs from the sun and moon. Similar perturbations eventually shake them loose.

The team found that, on average, one asteroid about 1 metre across is in Earth's orbit at any given time, and 1000 or so smaller space rocks down to 10 centimetres across should be in orbit too (Icarus, DOI: 10.1016/j.icarus.2011.12.003). "There's a lot more of these than people may have been thinking," Granvik says.

They orbit at distances between five and 10 times as far from Earth as the moon. Most stay in orbit less than a year, although some stay much longer. One object in the team's simulations stayed in orbit for almost 900 years.

Given the huge numbers of small space rocks out there, it makes sense that Earth captures some of them from time to time, says Richard Binzel of the Massachusetts Institute of Technology, who was not involved in the study. "It would be a little more surprising if they weren't there at all," he says.

It is possible that some have been seen in the past by asteroid surveys, but not followed up because they were assumed to be artificial satellites or bits of space junk, Granvik says. Future surveys by the Pan-Starrs telescope and the planned Large Synoptic Survey Telescope might be able to spot the small, dim objects more easily.

If we could find one, we might be able to grab it and bring the whole rock back to a lab on Earth, says Granvik. This would give us a better picture of what pristine asteroids are like as opposed to the roasted, shattered meteorites we sometimes find on Earth, he says. Since asteroids are leftover building blocks from the early solar system, studying them could provide insights into how the planets formed.

NASA, the European Space Agency and the Japanese space agency JAXA are each planning missions to return samples from the asteroid belt, but these little moons would be far easier to get to, says Granvik. "The price of the mission would actually be pretty small."

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