3753 Cruithne
Template:Minor Planet 3753 Cruithne is an asteroid which accompanies Earth in its orbit. It was officially discovered on October 10, 1986, by J. Duncan Waldron, working with Robert H. McNaught, Malcolm Hartley and Michael R. S Hawkins at Siding Spring Observatory, Coonabarabran, Australia. The 1983 apparition (1983 UH) is credited to Giovanni de Sanctis and Richard M. West of the European Southern Observatory in Chile. 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. [1] The correct pronunciation for 'Cruithne' is 'croo-een-ya', with the emphasis on the -een-.
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. This orbit sometimes takes it closer to the Sun than Venus, and sometimes halfway to Mars' orbit. 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 the Moon). Although Cruithne's orbit is not thought to be stable over the long term, calculations by Wiegart and Innanen showed that it has likely been in sync with Earth's orbit for a long time. There is no danger of it colliding with Earth for millions of years, if ever. Cruithne is not visible to the naked eye at any point in its orbit.
Three other near-Earth asteroids (NEAs), (54509) 2000 PH5, (85770) 1998 UP1 and 2002 AA29, which exist in resonant orbits similar to Cruithne's, have since 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 known 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.
Cruithne has been featured in the novel Manifold: Time by science-fiction author Stephen Baxter, perhaps due to its unconventional orbit of Earth.
External links
The Asteroid
- Paul Wiegert's page about Cruithne, with movies
- Java-applet based animations showing Cruithne's orbit
The Celtic Tribe