3753 Cruithne

Cruithne was discovered on 10 October 1986 by Duncan Waldron on a photographic plate taken with the UK Schmidt Telescope at Siding Spring Observatory, Coonabarabran, Australia. A 1983 precovery (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 Toronto, and Seppo Mikkola, working at the University of Turku in Finland. == Dimensions and orbit ==

. Cruithne is approximately in diameter, and its closest approach to Earth is , approximately thirty times the separation between Earth and the Moon. From 1994 through 2015, Cruithne made its annual closest approach to Earth every November. Although Cruithne's orbit is not thought to be stable over the long term, calculations by Wiegert and Innanen showed that it has probably been synchronized with Earth's orbit for a long time. There is no danger of a collision with Earth for millions of years, if ever. Its orbital path and Earth's do not cross, and its orbital plane is currently tilted relative to that of Earth by 19.8°. Cruithne, having a maximum near-Earth magnitude of +15.8, is fainter than Pluto and would require at least a reflecting telescope in order to be seen. Cruithne is in a normal elliptic orbit around the Sun. Its period of revolution around the Sun, approximately 364 days in the early 21st century, is almost equal to that of Earth. Because of this, Cruithne and Earth appear to "follow" each other in their paths around the Sun. This is why Cruithne is sometimes called "Earth's second moon". However, it does not orbit Earth and is not a moon. In 2058, Cruithne will come within 0.09 AU () of Mars. == Similar minor planets ==

More near-resonant near-Earth objects (NEOs) have since been discovered. These include 54509 YORP, , , and which exist in resonant orbits similar to Cruithne's. is the first identified Earth trojan (out of only two known ). Other examples of natural bodies known to be in horseshoe orbits (with respect to each other) 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 four known co-orbital asteroids (5261 Eureka, , , and , all at the Lagrangian points), and Jupiter has many (an estimated one million greater than 1 km in diameter, the Jovian trojans); there are also other small co-orbital moons in the Saturnian system: Telesto and Calypso with Tethys, and Helene and Polydeuces with Dione. However, none of these follow horseshoe orbits. == In popular culture ==

Cruithne plays a major role in Stephen Baxter's novel Manifold: Time, which was nominated for the Arthur C. Clarke Award for best science fiction in 2000. Cruithne is mentioned on the QI series A episode "Astronomy", in which it is described as a second moon of Earth. In a later episode ("Knowledge" in series K), it was added that, if we use the same definition, Earth has over 18,000 "mini-moons". In Astonishing X-Men, Cruithne is the site of a secret lab assaulted by Abigail Brand and her S.W.O.R.D. team. It contains many Brood before Brand destroys it. In the Insignia trilogy, 3753 Cruithne has been moved into an orbit around Earth to serve as a training ground for the Intrasolar Forces. In the third novel, Catalyst, it is intentionally directed at Earth. While it is destroyed before impact, its fragments rain down on Earth's surface, killing nearly 800 million people across the world. ==Gallery==

Cruithnes distance to Earth and Sun.jpg|Cruithne's distance to Earth (blue) and the Sun (yellow) plotted over 500 years (top) and 10 years (bottom) Lagrange Horseshoe Orbit.jpg|Plan showing possible orbits along gravitational contours (not to scale) Animation of (419624) 2010 SO16 orbit.gif|An example of a horseshoe orbit == See also ==