3753 Cruithne

3753 Cruithne is an asteroid in orbit around the Sun. Its orbit has unusual properties, however, when viewed in relation to the orbit of the Earth (see below). It was 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.

3753 Cruithne seen through the 0.75 m telescope of the Astronomical Society of Kansas City's Powell Observatory

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] (http://www.electricscotland.com/webclans/cairney/51.htm). The correct pronunciation for 'Cruithne' is croo-een-ya, with the emphasis on the -een-.

Cruithne is approximately 5 km in diameter, and its closest approach to Earth is 15 Gm (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 a collision with Earth for millions of years, if ever. Cruithne is not visible to the naked eye at any point in its orbit.

Cruithne has been featured in the novel Manifold: Time by science-fiction author Stephen Baxter, perhaps due to its unconventional orbit in relation to Earth.

Cruithne's horseshoe orbit and the orbit of the Earth

Cruithne is in a normal elliptic orbit around the Sun. However, because its period of revolution around the Sun is almost exactly equal that of the Earth, they appear to "follow" each other in their paths around the Sun. Cruithne's distance from the Sun and orbital speed vary a lot more than the Earth's, so from our point of view Cruithne actually follows a kidney bean shaped path ahead of us, taking slightly less than one year to complete a circuit of the bean. Because it takes slightly less than a year, the Earth "falls behind" the bean a little bit more each year, and so from our point of view, the kidney bean circuit is not quite closed (being more like a kidney bean shaped spiral loop) and moves slowly away from the Earth.

After many years, the kidney bean has fallen behind far enough that it is now actually "catching up" on the Earth from "behind". When it eventually does catch up, something very interesting happens. Cruithne makes a close approach to the Earth, and gravitationally exchanges orbital energy with Earth; this alters Cruithne's orbit by a little over half a million kilometres (whilst Earth's orbit is altered by about 1.3 centimetres) so that its period of revolution around the Sun is now slightly more than a year (this will next happen in the year 2285). The kidney bean now starts to migrate away from the Earth again in the opposite direction —instead of the Earth "falling behind" the bean, the Earth is now "pulling away from" the bean.

After 385 years or so, the kidney bean is now approaching Earth again from the other side, and the Earth, once more, alters the orbit of Cruithne so that its period of revolution around the Sun is again slightly less than a year. The pattern then repeats itself.

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.

External links

The Asteroid

• Paul Wiegert's page about Cruithne, with movies (http://www.astro.uwo.ca/~wiegert/3753/3753.html)
• Java-applet based animations showing Cruithne's orbit (http://burtleburtle.net/bob/physics/cruithne.html)

The Celtic Tribe

• The Dalriada Celtic Heritage Trust (http://www.dalriada.co.uk)

fr:3753 Cruithne nl:Cruithne ja:クルイシン pl:3753 Cruithne zh:小行星3753