[meteorite-list] Hybrid Comet-Asteroid in Mysterious Break-Up (60558 Echeclus)

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Wed Apr 12 12:30:30 2006
Message-ID: <200604112349.k3BNniC08751_at_zagami.jpl.nasa.gov>

http://www.newscientistspace.com/article/dn8976-hybrid-cometasteroid-in-mysterious-breakup.html

Hybrid comet-asteroid in mysterious break-up
Jeff Hecht
April 11, 2006

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.
Received on Tue 11 Apr 2006 07:49:44 PM PDT