| Orbital characteristics | |
|---|---|
| Orbit type | Near-Earth |
| Semimajor axis | 0.99778 AU |
| Eccentricity | 0.514784 |
| Orbital period | 365d 6h |
| Inclination | 19.8122° |
| Physical characteristics | |
| Diameter | 5 km |
| Mass | ? kg |
| Density | ? g/cm3 |
| Rotation period | ? |
| Spectral class | ? |
| Albedo | ? |
| History | |
| Discoverer | various, 1986 |
3753 Cruithne is an asteroid with the catalogue number 3753 that is currently in an unusual orbit related to that of 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.
The Dalriada Celtic Heritage Trust inform us the Cruithne were the first Celtic racio-tribal group to come to the British Isles, appearing between about 800 and 500 B.C., and coming from the European continent. They were also known as the Picts.[1] The correct pronunciation for 'Cruithne' is 'croo-ee-nya'. The emphasis should be on the 'croo'.
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 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 natural satellites 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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