Cruithne

3753 Cruithne is an asteroid which accompanies Earth in its orbit. 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, it was not until 1997 that its unusual orbit was determined by Paul Wiegert and Kimmo Innanen, working at York University in Canada, and Seppo Mikkola, working at the University of Turku in Finland.

Cruithne was named after the first Celtic racio-tribal group to inhabit the British Isles. The Cruithne emigrated from the European continent and appeared in Britain between about 800 and 500 B.C. They were also known as the Picts.[1] The correct pronunciation for 'Cruithne' is 'creen-ya', with the emphasis on the first syllable.

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 Earth years to complete one such horseshoe orbit. This orbital path appears extremely complex and non-intuitive when viewed from Earth's frame of reference. It is much easier to understand by recognizing that from the Sun's frame of reference Cruithne follows a relatively conventional orbit which takes almost exactly the same time to complete as Earth's (one year), but which is slightly more elliptical. The gravitational influence of Earth modifies this elliptical orbit only slightly, just enough to modify Cruithne's precession and prevent it from coming too close.

Cruithne is approximately 5 km in diameter, and its closest approach to Earth is 15 million kilometres (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.

Two other near-Earth asteroids (NEAs), 1998 UPI and 2000 PHS, which exist in resonant orbits similar to Cruithne's, have been discovered.

Other examples of natural bodies known to be in horseshoe orbits at the time of writing include Janus and Epimetheus, natural satellites of Saturn. The orbits these two moons follow around Saturn are much simpler than the one Cruithne follows, but operate along the same general principles.

Mars has one co-orbital asteroid (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.

External Links

Paul Weigart's homepage, with movies

The Dalriada Celtic Heritage Trust

Java-applet based animations showing Cruithne's orbit

Referenced By