Asteroids, chunks of rock and metal orbiting the Sun, were once considered “vermin of the skies” by astronomers. This disparaging term originated when a long-exposure image of a deep sky object on a glass plate was ruined by the trail of a distant asteroid moving across the sky. Yet today these Solar System left overs are proving to be increasingly interesting in their own right. In March 2014 we found out how one asteroid surprised astronomers by its possession of a hitherto unsuspected feature.
Asteroid 10199 Chariklo is a distant member of the Sun’s family, its orbit carries it from beyond Saturn to almost as far as Uranus (13.1 to 18.7 AU). It is named after a nymph from Greek mythology. One of the Centaurs, a diverse group of asteroids with some comet-like properties found between Jupiter and Neptune but mostly beyond Saturn’s orbit, Chariklo is a sizeable body estimated to some 250 km (155 miles) across making it comparable to the Saturnian moon Enceladus. It is thus the largest known Centaur but until recently it was otherwise unremarkable.
What changed our view of Chariklo is the discovery of a ring system around it, the first such feature found around a Solar System body smaller than a gas giant planet. In June 2013 astronomers across South America were observing Chariklo passing in front of a star, an event called an occultation. The observers recorded two dips in the star’s apparent brightness just before and after the occultation. The only way to explain this was by assuming there was a set of rings around the asteroid. This exactly parallels how the rings of Uranus were discovered in 1977.
It seems the ring system consists of a set of two rings of fine debris, possibly rich in water ice, about 20 km across. The two rings are only seven and three kilometres wide, separated by a clear gap of nine kilometres. There is a real possibility that there is a small moon among the asteroid’s rings. Almost certainly the rings were born when a collision with a smaller asteroid blasted millions of tonnes of debris off Chariklo’s surface. Presumably these will be found to be common features around asteroids including those in the main belt and near-Earth objects. Space explorers beware!
A ring system detected around the Centaur (10199) Chariklo (Braga-Rigas et al.)
(Article by Colin Johnston, Science Education Director)