Cruithne

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An asteroid (catalogue number 3753) currently in an unusual orbit near Earth. It was officially discovered on October 10, 1986 by D. Waldron, working with R. McNaught, M. Hartley and M. Hawkins at Siding Spring Observatory, Coonabarabran, Australia. However, its unusual orbit was not determined until 1997 by Paul Wiegert and Kimmo Innanen, working at York University in Canada, and Seppo Mikkola, working at the University of Turku in Finland. Cruithne is pronounced 'croo-EEN-ya'.

Cruithne shares Earth's orbit, but does not actually orbit the Earth. Instead, it follows a spiralling path that moves along the Earth's orbit in a horseshoe shape, the two ends of the horseshoe approaching either side of Earth but not quite reaching it. It takes Cruithne 385 years to complete one such horseshoe orbit.

Cruithne is approximately 5 kilometers in diameter and its closest approach to Earth is 15 million kilometers (approximately 40 times the separation between Earth and Luna). Although Cruithne's orbit is not thought to be stable over the long term, there is no danger of it colliding with the Earth in any forseeable future. Cruithne is not visible to the naked eye at any point in its orbit.

There is only one other known example of natural bodies in a horseshoe orbit at the time of writing, the moons of Saturn named Janus and Epimetheus. The orbit these two moons follow around Saturn is much simpler than the one Cruithne follows, but operates along the same general principles.

Mars has one co-orbital asteroid (its name is 5261 Eureka), and Jupiter has many (about 400 objects, the Trojan asteroids); there are also other small co-orbital moons in the Saturnian system: Telesto and Calypso with Tethys, and Helene with Dione. However, none of these follow horseshoe orbits.

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