Free Astronomy Magazine

SMALL BODIES cultation timings it is possible to determine the diameter and approximate shape of the asteroid (i.e., that projected during the few minutes duration of the event). It is conse- quently important to arrange observing points also along the borders of the theo- retical outer boundary of the cast shadow, since the missed oc- cultation on the part of some observers is equally important as its recording by others. Observations of this type of phe- nomena are com- monplace activities for astronomers as, in fact, they are quite recurrent during the year, with some of them also well within the reach of small telescopes. But those involving objects of particular interest are rare, and one of these was precisely that where Chariklo play- ed the leading part. Like all Centaurs, also Chariklo has an unsta- ble long-period orbit, shaped in such a way as to make it move between the giant planets of the solar system, between Sat- urn and Uranus in this case, with a “gravity predilection” for the latter to which is al- most bound by a 4:3 orbital resonance (a Chariklo year lasts 63.17 terrestrial years while that of Uranus 84.32 years, hence 4 years of the first last nearly 3 years of the second). When discovered in 1997 by James Scotti (Spacewatch program), Chariklo demoted from its status of largest centaur the already mentioned and better-known Chiron: 250 kilometres across the first and 230 kilome- tres the second. Besides the need to verify the dimensions of the asteroid, initially esti- mated from the thermal properties of al- T he Chariklo system “seen” from a few thou- sand kilometres away. The aster- oid has a very low albedo, about half that of the rings. In the video to the right a simulation of the asteroid’s approach (depic- ted in its most likely shape). [ESO/L. Calçada M. Korn-messer N. Risinger]