3753 Cruithne
Encyclopedia : 3 : 37 : 375 : 3753 Cruithne
| Orbital elements | |
|---|---|
| Eccentricity (orbit)>e | |
| Inclination>i | ° |
| Perihelion>q |
|
| Argument of perihelion>ω | ° |
| Semi-major axis>a | AU |
| Longitude of the ascending node>ω | ° |
| Aphelion>Q |
|
| Mean anomaly>M | ° |
| Orbital period>P | years |
| Mean motion>n | °/day |
| Time of perihelion passage>TP | |
Discovery
Cruithne was discovered on October 10, 1986, by Duncan Waldron, on a plate taken with the UK Schmidt Telescope 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 (aka Priteni or Picti) emigrated from the European continent and appeared in Britain between about 800 and 500 B.C. [link].
Dimensions and orbit
Cruithne is approximately 5 km in diameter, and its closest approach to Earth is 12 Gm (million kilometres; approximately 30 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 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 horseshoe orbit ahead of the earth, 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 circuit is not quite closed (being more like a spiral loop) and moves slowly away from the Earth.
After many years, the Earth has fallen behind far enough that Cruithne is now actually "catching up" on the Earth from "behind". When it eventually does catch up, Cruithne will make a series of annual close approaches to the Earth, and gravitationally exchange orbital energy with Earth; this will alter 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. 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. The next such series of close approaches will be centred around the year 2292 —in July of that year, Cruithne will approach Earth to about 12.5 million km.
After 380 to 390 years or so, the kidney-bean-shaped orbit approaches 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 (this last happened with a series of close approaches centred on 1902, and will next happen with a series centred on 2676). The pattern then repeats itself.
Similar minor planets
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 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 and Polydeuces with Dione. However, none of these follow horseshoe orbits.
Trivia
- At one point Cruithne was thought to have been artificial, possibly "space junk" left over from the Apollo program.
- Cruithne had a major role in the novel by science-fiction author Stephen Baxter, due to its unconventional orbit in relation to Earth.
External links
- [Paul Wiegert's page about Cruithne, with movies]
- [Java-applet based animations showing Cruithne's orbit]
- [A simulation of Cruithne's orbit with animation]
- [More Moons Around Earth?]
| The minor planets |
|---|
| Vulcanoids | Near-Earth asteroids | Main belt | Jupiter Trojans | Centaurs | Damocloids | Comets | Trans-Neptunians (Kuiper belt · Scattered disc · Oort cloud) |
| For other objects and regions, see: , , asteroid moons and the Solar system For a complete listing, see: List of asteroids. See also Pronunciation of asteroid names and Meanings of asteroid names. |
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