Centaur (planetoid)

The centaurs are an unstable orbital class of minor planets named after the mythological race of centaurs. The name was chosen because they behave as half asteroid and half comet. Centaurs have transient orbits that cross or have crossed the orbits of one or more of the giant planets, and have dynamical lifetimes of a few million years.[1]

The first centaur-like object to be discovered was 944 Hidalgo in 1920. However, they were not recognized as a distinct population until the discovery of 2060 Chiron in 1977. The largest known centaur is 10199 Chariklo, discovered in 1997, which at 260 km in diameter is as big as a mid-sized main-belt asteroid.

No centaur has been photographed up close, although there is evidence that Saturn's moon Phoebe, imaged by the Cassini probe in 2004, may be a captured centaur. In addition, the Hubble Space Telescope has gleaned some information about the surface features of 8405 Asbolus.

As of 2008, three centaurs have been found to display cometary comas: Chiron, 60558 Echeclus, and 166P/NEAT. Chiron and Echeclus are therefore classified as both asteroids and comets. Other centaurs such as 52872 Okyrhoe are suspected of showing cometary activity. Any centaur that is perturbed close enough to the Sun is expected to become a comet.

Classification
The generic definition of a centaur is a small body that orbits the Sun between Jupiter and Neptune and crosses the orbits of one or more of the giant planets. Due to the inherent long-term instability of orbits in this region, even centaurs such as 2000 GM137 and 2001 XZ255, which do not currently cross the orbit of any planet, are in gradually changing orbits that will be perturbed until they start to cross the orbit of one or more of the giant planets.

However, different institutions have different criteria for classifying borderline objects, based on particular values of their orbital elements:


 * The Minor Planet Center (MPC) defines centaurs as having a perihelion beyond the orbit of Jupiter and a semi-major axis less than that of Neptune.
 * The Jet Propulsion Laboratory (JPL) similarly defines centaurs as having a semi-major axis, a, between those of Jupiter and Neptune (5.5 AU < a < 30.1 AU).
 * In contrast, the Deep Ecliptic Survey (DES) defines centaurs using a dynamical classification scheme. These classifications are based on the simulated change in behavior of the present orbit when extended over 10 million years. The DES defines centaurs as non-resonant objects whose instantaneous (osculating) perihelia are less than the osculating semi-major axis of Neptune at any time during the simulation. This definition is intended to be synonymous with planet-crossing orbits and to suggest comparatively short lifetimes in the current orbit.

The collection The Solar System Beyond Neptune (2008) uses the traditional definition of Centaurs, limited to semi-major axes smaller than that of Neptune, classifying the objects on unstable orbits beyond this limit as members of the scattered disk. Yet, other astronomers still prefer to define centaurs as objects that are non-resonant with a perihelion inside the orbit of Neptune that can be shown to likely cross the hill sphere of a gas giant within the next 10 million years. Thus centaurs can be thought of as inward scattered objects that interact more aggressively and scatter more quickly than typical scattered disc objects.

These differences in classification methods make it difficult to classify objects like (44594) 1999 OX3, which has a semi-major axis of 32 AU but crosses the orbits of both Uranus and Neptune. Among the inner centaurs, 2005 VD, with a perihelion distance very near Jupiter, is listed as a centaur by both JPL and DES.

Distribution
The diagram at right illustrates the orbits of all known centaurs in relation to the orbits of the planets. For selected objects, the eccentricity of the orbits is represented by red segments (extending from perihelion to aphelion).

Centaurs' orbits are characterised by a wide range of eccentricity, from highly eccentric (Pholus, Asbolus, Amicus, Nessus) to more circular (Chariklo and the Saturn-crossers: Thereus, Okyrhoe).

To illustrate the range of the orbits' parameters, a few objects with very unusual orbits are plotted in yellow on the diagram:

A dozen known centaurs, including Dioretsa ("asteroid" spelled backwards), follow retrograde orbits.
 * 1999 XS35 (Apollo asteroid) follows an extremely eccentric orbit (e=0.947), leading it from inside of the Earth's orbit (0.94 AU) to well beyond Neptune (>34 AU)
 * 2007 TB434 follows a quasi-circular orbit (e<0.026)
 * 2001 XZ255 has the lowest inclination (i<3°).
 * Damocles is among a few centaurs on orbits with extreme inclination (prograde i>70°, e.g. 2007 DA61, 2004 YH32, retrograde i<120° e.g. 2005 JT50; not shown)
 * 2004 YH32 follows such a highly inclined orbit (nearly 80°) that, while it crosses from the distance of the Main belt from the Sun to past the distance of Saturn, it does not even cross Jupiter relative to the plane of Jupiter's orbit.