Discovery
Discovered by	Duncan Waldron
Discovery date	October 10, 1986
Designations
Named after	Cruthin
Alternate name(s)	1983 UH; 1986 TO
Minor planet category	Near-Earth asteroid, Venus-crosser asteroid, Mars-crosser asteroid
Orbital characteristics[1]
Epoch June 18, 2009 (JD 2455000.5)
Aphelion	1.51 AU (226.105 Gm)
Perihelion	0.484 AU (72.415 Gm)
Semi-major axis	0.998 AU (149.260 Gm)
Eccentricity	0.515
Orbital period	1.00 a (364.01 d)
Average orbital speed	27.73 km/s
Mean anomaly	170.36°
Inclination	19.81°
Longitude of ascending node	126.28°
Argument of perihelion	43.74°
Proper orbital elements
Physical characteristics
Dimensions	~5 km
Mass	1.3×1014 kg
Mean density	2 ? g/cm³
Equatorial surface gravity	0.0014 m/s²
Escape velocity	2.6 metres per second (8.5 ft/s)
Rotation period	27.44 hours[1]
Temperature	~ 273 K
Spectral type	?
Absolute magnitude (H)	15.1[1]

3753 Cruithne ( /ˈkrɪnjə/, from Old Irish [ˈkrɪθnɛ]; Modern [ˈkrɪhnʲə] or [ˈkrɪnʲə]) is an asteroid in orbit around the Sun in approximate 1:1 orbital resonance with the Earth. It is a periodic inclusion planetoid^{[What does this mean?]} orbiting the Sun in an apparent horseshoe orbit.^[2] It has been incorrectly called "Earth's second moon", but it is a quasi-satellite^[2], not a moon. Cruithne does not orbit Earth, and at times it is on the other side of the Sun.^[3] Its orbit takes it inwards towards the orbit of Mercury, and outside the orbit of Mars.^[3] Cruithne orbits the Sun in about 1 year, but it takes 770 years for the series to complete a horseshoe-shaped movement, with the Earth in the gap of the horseshoe.^[3]

The name Cruithne is that of an early medieval people of Ireland.

[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 or Cruthin, a people of early medieval Ireland mentioned in the Annals of Ulster.^[3]

[edit] Dimensions and orbit

Cruithne and Earth seem to follow each other because of a 1:1 orbital resonance.

Cruithne appears to make a bean-shaped orbit from the perspective of Earth.

Cruithne's distance to Earth and the Sun plotted over 500 years (top) and 10 years (bottom).

Cruithne is approximately 5 kilometres (3.1 mi) in diameter, and its closest approach to Earth is approximately thirty times the separation between Earth and the Moon (12 Gm or twelve million kilometres). From 1994 through 2015, Cruithne makes its annual closest approach to Earth every November.^[4] Although Cruithne's orbit is not thought to be stable over the long term, calculations by Wiegert and Innanen showed that it has probably been synchronized with Earth's orbit for a long time. There is no danger of a collision with Earth for millions of years, if ever. Its orbital path and Earth's do not cross, and its orbital plane is currently tilted to that of the Earth by 19.8°. Cruithne, having a maximum near-Earth magnitude of +15.8, is fainter than Pluto and would require at least a 12.5-inch (320 mm) reflecting telescope to be seen.^[5][6]

Cruithne is in a normal elliptic orbit around the Sun. Its period of revolution around the Sun, approximately 364 days at present, is almost equal to that of the Earth. Because of this, Cruithne and Earth appear to "follow" each other in their paths around the Sun. This is why Cruithne is sometimes called "Earth's second moon".^[2] However, it does not orbit the Earth and is not a moon.^[7] In 2058, Cruithne will come within 0.09 AU (13.6 million kilometres) of Mars.^[4] Due to a high orbital eccentricity, Cruithne's distance from the Sun and orbital speed vary a lot more than the Earth's, so from the Earth's 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 will have fallen so far behind that Cruithne will then actually be "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—while Earth's orbit is altered by about 1.3 centimetres (0.51 in)—so that its period of revolution around the Sun will then become slightly more than a year. The kidney bean will then start to migrate away from the Earth again in the opposite direction – instead of the Earth "falling behind" the bean, the Earth is "pulling away from" the bean. The next such series of close approaches will be centred on the year 2292 – in July of that year, Cruithne will approach Earth to about 12,500,000 kilometres (7,800,000 mi).

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

Figure 1. Plan showing possible orbits along gravitational contours (Not to scale)

More resonant near-Earth objects (NEOs) have since been discovered. These include 54509 YORP, (85770) 1998 UP₁, 2002 AA₂₉, and 2009 BD which exist in resonant orbits similar to Cruithne's. 2010 TK₇ is the first and so far only identified Earth trojan.

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 four known co-orbital asteroids (5261 Eureka, 1999 UJ₇, 1998 VF₃₁, and 2007 NS₂, all at the Lagrangian points), 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.

[edit] Cruithne in popular culture

Cruithne plays a major role in Steven Baxter's novel Manifold: Time, which was nominated for the Arthur C. Clarke science fiction award in 2000.

[edit] See also