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Astronomers Jonathan Horner and Wyn Evans, at the Universities of Oxford and Cambridge, and Mark Bailey and David Asher, at the Armagh Observatory, have provided new insight into the links between comets and asteroids in the outer solar system and their dynamical evolution. These bodies, many of which have diameters greater than 100 km, are called "Centaurs" because of their "half-comet, half-asteroid" status. The first Centaur, called Chiron, was discovered in 1977, but since then more than 100 roughly similar objects have been found, circulating on orbits extending from just beyond Jupiter to as far out as Neptune and Pluto.

Up to now, owing to the great diversity of their orbits, it has been difficult to understand the behaviour of Centaurs as a group. Many of their orbits are extremely unstable by astronomical standards, close approaches to the major planets leading to large changes of their orbits, and perhaps even ejection from the solar system, on time-scales of millions to tens of millions of years. Gravitational interactions of Centaurs with planets, especially Jupiter and Saturn, can also cause them to migrate into the inner solar system, even onto Earth-crossing orbits. Owing to their typically large sizes, Centaurs are occasionally described as "giant comets", and the passage of such an object through the inner solar system would produce an enormous, variable amount of interplanetary dust. Such "dustings", and the accretion of larger meteoroids contained within the cometary dust trail, would pose a serious environmental hazard to Earth, possibly affecting the Earth's climate.

The research, which was begun in Armagh and constitutes part of Jonathan Horner's PhD thesis, is to be published in the 21 August issue of the Monthly Notices of the Royal Astronomical Society. It illustrates the close interrelationships between all the currently known Centaurs, and proposes a simple dynamical classification that allows the key characteristics of each body's orbit to be identified and linked into the more elaborate existing schemes for describing comets. The new classification scheme means that it should be possible to make more precise statistical statements as to the likelihood of any object discovered in the Centaur region to be eventually lost from the system, or to become an Earth-crossing object, and on what time-scale.

If Centaurs are lost, either by disintegration - like the wings of Icarus - by entering a region too close to the Sun, or perhaps by leaving the solar system entirely owing to planetary perturbations, where do they come from? This study, as well as much previous work, suggests that the Centaur source orbits are located predominantly in the Edgeworth-Kuiper belt beyond Neptune, with a significant number perhaps originating in the inner core of the Oort cloud. In either case, the Centaurs give clues to the origin of the planetary system. However, these distant regions of the solar system have only recently become accessible to direct observation, and the interpretation of the clues is problematic and remains an active field of research.

FOR FURTHER INFORMATION CONTACT: Mark Bailey, Armagh Observatory, at Tel: 028-3752-2928 or e-mail: meb@arm.ac.uk; or Jonathan Horner, Department of Physics, University of Oxford, at Tel: 018-6527-3977 or e-mail: j.horner1@physics.ox.ac.uk.

See also:
Space.com: Crazy Names
Royal Astronomical Society Press Release
Oxford Astrophysics
Notes on Centaur Names
List of Centaurs and Scattered-Disk Objects
Minor Planet Groups/Families
NEO Impact Hazard

Last Revised: 2003 August 13th
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