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3753 Cruithne

3753 Cruithne
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 ?
Discoverer various, 1986

3753 Cruithne is an asteroid with the catalogue number 3753 that is currently in an unusual orbit near 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. Cruithne is pronounced 'croo-EEN-ya'.

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 (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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