3753 Cruithne is a 5-kilometer-wide near-Earth asteroid that is co-orbital (i.e. shares the same orbit) with our planet. Its path is highly inclined and horseshoe-shaped with respect to Earth, causing Cruithne to alternatively move closer to and then much further away from us.

 

At its closest approach, which happens every 100,000 years, Cruithne comes within about 15 million km of Earth. It was discovered in 1986, by D. Waldron at Siding Spring Observatory, Coonabarabran, Australia, and named for the first Celtic tribal group that settled in the British Isles.

 

Two other near-Earth asteroids are known to be currently in resonant states similar to that of Cruithne: 1998 UP1 and 2000 PH5.