From: baalke@zagami.jpl.nasa.gov   
      
   Royal Astronomical Society Press Notice   
      
   Issued by Jacqueline Mitton, RAS Press Officer   
   jmitton@dial.pipex.com   
   tel: +44 (0)1223-564914   
      
   CONTACTS:   
      
   Jonathan Horner   
   Theoretical Physics, University of Oxford   
   Phone: (+44) (0)1865 273977 (not 18 - 31 August)   
   Mobile phone: (18 - 31 August) 07900 988145   
   E-mail: j.horner@physics.ox.ac.uk   
      
   Professor Mark Bailey   
   Armagh Observatory   
   Phone: (+44) (0)28 3752 2928   
   E-mail meb@arm.ac.uk   
      
   Date: 5 August 2003   
      
   PN03-31:   
      
   MAKING SENSE OF CENTAURS AND THEIR KIN   
      
   It's time for astronomers to abandon the traditional ways they've categorised   
   comets and distant asteroids and for them to look at all the small bodies   
   populating the outer solar system in a fresh light, according to four   
   researchers writing in the 21 August issue of the Monthly Notices of the Royal   
   Astronomical Society.   
      
   A new classification devised by Jonathan Horner of Oxford University, with   
   collaborators Dr Wyn Evans (of Oxford and Cambridge Universities), and   
   Professor   
   Mark Bailey and Dr David Asher (both of Armagh Observatory) promises a   
   semblance   
   of order to replace much of the confusion about the variety of objects coming   
   under the titles comets, Centaurs, trans-Neptunian objects, Kuiper Belt   
   Objects etc.   
      
   These bodies give important clues as to how our planetary system formed but   
   many   
   of them have changed orbits significantly over the lifetime of the solar system   
   due to the gravitational influence of the four giant planets -- Jupiter,   
   Saturn,   
   Uranus and Neptune. As is the case with plants and animals, a good   
   classification scheme is the first step in understanding how groups with   
   different characteristics relate to each other, and to tracing their   
   evolutionary paths back to their origins.   
      
   "Minor bodies between Saturn and Neptune are often described simply as   
   'Centaurs' and those beyond Neptune simply as 'Kuiper Belt Objects' -- but this   
   is not very enlightening as their histories and fates may be very different,   
   regardless of where they happen to orbit now", says Jonathan Horner. In the   
   same   
   way, the traditional arbitrary labelling of comets as 'long-period' or   
   'short-period' has not been very consistent or helpful.   
      
   The group known as Centaurs are a particular puzzle because they are like   
   hybrids with some comet-like and some asteroid-like characteristics. Many of   
   them are over 100 km across, much larger than the nucleus of a typical comet,   
   yet several have been seen surrounded by a cloud of gas and dust like the coma   
   of a comet. The first Centaur, called Chiron, was discovered in 1977. Since   
   then   
   more than 100 roughly similar objects have been found.   
      
   Building on previous research by a number of different astronomers, Horner and   
   colleagues base their scheme on the fact that the long-term changes to the   
   orbit   
   of a body in the outer solar system are controlled by one or two of the four   
   giant planets. The important factors are the object's distance from the Sun at   
   its closest and farthest points (perihelion and aphelion). These main classes   
   are then subdivided into four types (I, II, III and IV) to take account of the   
   angle the body's orbit makes with the main plane of the solar system. For   
   example, Chiron is an SU IV object, with its perihelion in Saturn's zone of   
   control and its aphelion in Uranus's. By contrast, the Centaur Pholus is SN   
   III.   
   (N stands for Neptune.)   
      
   Professor Mark Bailey comments, "One of the first things our new scheme has   
   done   
   is to help us understand how the very diverse set of objects we are calling   
   Centaurs can be divided into groups with orbits that have broadly similar   
   characteristics. For instance, we can see at a glance that the orbits of Chiron   
   and Pholus evolve in different ways and over different periods of time. Now we   
   can more effectively trace back the history and predict the likely future for   
   the different groups of Centaurs. We can examine the chances that one could   
   cross into the inner part of the solar system and, if so, on what time-scale.   
   The enormous amount of dust that would come off such an object as it approached   
   the Sun would be an environmental hazard for Earth."   
      
   --- SoupGate-Win32 v1.05   
    * Origin: you cannot sedate... all the things you hate (1:229/2)