Something substantial has broken off an icy 50-kilometre object beyond the orbit of Saturn, leaving puzzled astronomers trying to figure out why.
Comets have been seen breaking up before, but only after heating when passing close to the Sun or a gravitational disturbance following a close encounter with a planet.
However, at 1.9 billion kilometres, this object is very far from the Sun. Another mysterious feature is that much more gas and dust is escaping from the breakaway fragment than from the parent body. The disintegration has created a dust cloud more than 100,000 km across and which is several times brighter than the original object was before the event.
The object, called 60558 Echeclus, was discovered in 2000 and is a “centaur” – part rocky asteroid and part icy comet. Its new activity, revealed in images taken on 2 April, makes it look “really strange”, says William Romanishen of the University of Oklahoma, US, one of the team that took the images. “The first thing that came to mind was a collision.”
Earlier observations showed Echeclus rotates about once every 26 hours, so a fragment would need a push to escape its gravity, says Paul Weissman of the Jet Propulsion Laboratory, who spotted the original cloud of gas and dust around Echeclus on 30 December 2005.
Such an impact on a comet-like body has not been observed before. But there are other possibilities, says Steve Tegler of Northern Arizona University, US, who works with Romanishen. He says it is most likely that the event was caused by explosive sublimation of volatile ices in Echeclus, resulting in material being blasted off.
Tegler says the evaporating ice is probably carbon monoxide, with vaporises at about that distance from the Sun, where the object’s temperature is about 80 degrees Kelvin, close to the sublimation temperature. No one has yet analysed the gas composition.
Another puzzle is the difference in activity between the main nucleus and the fragment. Freshly exposed ices normally sublimate, so “you’d expect equal activity from both pieces”, Wiessman says. But the nucleus does not look very active.
Echeclus was discovered by the Spacewatch telescope in 2000, and at first looked like an asteroid. Then Weissman found it was surrounded by a coma, so astronomers also classed it as a periodic comet, 174P. The photos from 2 April show the coma has now spread out.
Echeclus belongs to a group of more than 100 centaurs with orbits well outside the main asteroid belt. Although originally from the distant Kuiper belt, they now orbit the Sun between Jupiter and Neptune, but will be ejected from those unstable orbits within tens of millions of years. Cometary clouds have been reported around three other centaurs too.
Echeclus is currently moving towards the Sun on its 35-year orbit, and will pass closest to our star – about 880 million km – in April 2015. Other centaurs have become active as they moved inward, Tegler says. But none have shown such dramatic activity.