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Turns out the moon's not the Earth's only traveling companion. Space scientists have discovered an asteroid that's been following our fair planet for thousands of years, at least — and there may be many more where it came from, according to a recent study.

If other so-called Trojan asteroids are found, they could turn out to be ideal candidates for a visit from astronauts, something NASA hopes will be possible within the next 15 years.

Most of the asteroids in the solar system populate the belt of rocky debris between Mars and Jupiter. But planets can pull asteroids into their orbits, too. More than 4,000 Trojan asteroids have been discovered around the gas giant Jupiter, along with a few around Neptune and Mars.