Centaur (planetoid)

See also: Centaur (homonymy)

In Astronomy, the centaurs are Astéroïde S frozen which revolve around the Sun between Jupiter and Neptune. They were named according to the mythical race of the Centaure S.

History

The first centaur was discovered in 1977, it acted of (2060) Chiron. While approaching sound Perihelion, it developed a cometary tail and was reclassified like Comet (under the name of 95/P Chiron); he from now on is regarded at the same time as an asteroid and a comet, though of a size definitely larger than traditional comets.

The two centaurs discovered following (2060) Chiron were (5145) Pholus in 1992 and (7066) Nessus in 1993.

Exploration

No centaur at present has (semi-2007) summer photographed closely by a Space probe, although it seems that Phœbé, one of the moons of Saturn, photographed by the probe Cassini in 2004, is a captured centaur. The Space telescope Hubble also glané some information on the surface of (8405) Asbolus.

In 2010, the probe New Horizons should fly over by far a centaur, (83982) Crantor.

Orbits

The centaurs are not located on stable orbits on the scale of the hundred thousands of years. Studies of their orbit seem to indicate that it is about an orbital state of transition between the Ceinture from Kuiper and comets at short period. It is possible that they are objects of the belt of Kuiper disturbed gravitationally and whose orbit crossed that of Neptune.

The orbit of the centaurs is chaotic and evolves/moves quickly when the object approaches the one of the giant planets of the solar system. Certain centaurs end up crossing the Jupiter orbit; if they show a cometary activity, they are reclassified like comets.

In the long term, it seems that their destiny concluded by a collision with the Sun (or a planet) or by an ejection from the solar system.

The diagram illustrates the orbits of the centaurs known compared to the orbits of planets. For some objects, the eccentricity of the orbits is represented by the red segments (Périhélie with the Aphélie) with the Inclinaison represented on the vertical axis.

To illustrate the fork of the parameters of the orbits, four objects with the extreme orbits were shown in (yellow):

  • on the most tilted orbit (~39°) but also almost circular (the smallest eccentricity < 0.01)

  • Asbolus on the most eccentric orbit (eccentricity = 0,69)

  • on an almost coplanar orbit with the ecliptic (slope < 3°).

The Histogramme S present the distribution of the slope I (interval 5°) and of the eccentricity E (interval 0,05) of the orbits.

Physical characteristics

The size of the centaurs makes impossible the direct observation of their surface but the indices colors and their spectrum S can help to study their composition and to provide indices on the origin of these objects.

Colors

The centaurs have an extraordinary diversity of colors defying any attempt to model in a simple way their composition.

The index of color is the measurement of the differences of Magnitude connect object seen through filters blue (B), neutral (V; green-yellow) and red (R). The graph represents the known indices of the transneptuniens except for largest.

The centaurs can be gathered in two classes:

  • Very red, like Blue Pholus
  • (gray or neutral blue according to the authors) like Chiron

The many theories which try to explain these differences in color can in their turn being classified in two categories

  • Those which explain the difference in color like a consequence of the “true” difference in composition and of origin
  • Those which treat the difference in colors right like a simple result of modifications by radiation and the typical activity with the Comet S.

Like example of the second category, the reddish color of Pholus would have been due to an irradiated organic matter crust while Chiron would have had its layer of ice exposed like result of its comet episodes. According to another explanation, Pholus would have been expelled very recently of the Ceinture of Kuiper and the modification of surface did not take place yet. Another theory suggests several processes in competition, radiation making the object redder and the collisions with the activity of comet making it more blue.

List centaurs

Among the objects considered currently as centaurs, one can quote:

External bonds

  • List of centaurs and scattered objects (MPC)
  • Centaurs from The Encyclopedia off Astrobiology Astronomy and Spaceflight

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