Hybrid comet-asteroid in mysterious break-up

http://www.newscientistspace.com/art...s-breakup.html

a.. 09:30 11 April 2006
b.. NewScientist.com news service
c.. Jeff Hecht
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.

Explosive sublimation
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.

Unstable orbits
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.