Researchers have long suspected wandering asteroids might occasionally approach Earth close enough to be captured by gravity and become temporary moons.
Mikael Granvik of the University of Helsinki in Finland and colleagues ran computer simulations looking at the frequency of asteroids of various sizes approaching Earth's neighborhood 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, and the team estimated, on average, one asteroid about 1 yard across is in Earth's orbit at any given time, and 1,000 or so smaller space rocks down to 4 inches across should be in orbit too.
"There's a lot more of these than people may have been thinking," Granvik told NewScientist.com.
Most stay in orbit less than a year, the simulations suggest, although some may stay much longer.