3753 Cruithne

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**3753 Cruithne**
Discovery

Discovered by:	Duncan Waldron
Discovery date:	October 10, 1986
Alternative names:	1983 UH; 1986 TO
Minor planet category:	Near-Earth asteroid, Venus-crosser asteroid, Mars-crosser asteroid
Orbital characteristics
Epoch July 14, 2004 (JD 2453200.5)
Aphelion distance:	226.105 Gm (1.511 AU)
Perihelion distance:	72.415 Gm (0.484 AU)
Semi-major axis:	149.260 G m (0.998 AU)
Eccentricity:	0.515
Orbital period:	364.019 d (1.00 a)
Avg. orbital speed:	27.73 km/s
Mean anomaly:	190.171°
Inclination:	19.810°
Longitude of ascending node:	126.311°
Argument of perihelion:	43.719°
Physical characteristics
Dimensions:	~5 km
Mass:	1.3×10¹⁴ kg
Mean density:	2 ? g/c m³
Equatorial surface gravity:	0.0014 m/s²
Escape velocity:	0.0026 km/s
Rotation period:	? d
Albedo:	0.15 ?
Temperature:	~275 K
Spectral type:	?
Absolute magnitude:	15.1

3753 Cruithne (pronounced /ˈkrihnʲə/) is an asteroid in orbit around the Sun. Due to its unusual orbit relative to that of the Earth, it is a periodic inclusion planetoid. It is sometimes called "Earth's second moon",^[1] although it is not a satellite of the Earth.

1 Discovery
2 Dimensions and orbit
3 Similar minor planets
4 See also
5 External links
6 References

[edit] Discovery

Cruithne was discovered on October 10, 1986, by Duncan Waldron on a photographic 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 Toronto, and Seppo Mikkola, working at the University of Turku in Finland.

The asteroid is named after the Cruithne people (also known as the Priteni or the Picts) who inhabited Scotland between 800 BC and 1000 AD; the name may specifically refer to their legendary first leader, also called Cruithne.

[edit] Dimensions and orbit

Cruithne is approximately 5 km in diameter, and its closest approach to Earth is approximately 30 times the separation between Earth and the Moon (12 Gm or million kilometres). 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. Because it does not orbit the Earth, it is not considered a moon in the normal astronomical meaning of the word. However, because its period of revolution around the Sun is almost exactly equal to 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 more each year, and so from our point of view, the circuit is not quite closed, but rather like a spiral loop that 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 centered on 2676). The pattern then repeats itself.

[edit] 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 (more than 1000 known 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.