[Baa-ebulletin 00545] Large main-belt asteroid (596) Scheila exhibits 'cometary' appearance

[BAA electronic bulletins service]

The first well-observed case has been reported of a large main-belt asteroid 
apparently exhibiting 'comet-like' behaviour.

Steve Larson of the Lunar and Planetary Laboratory, University of Arizona, 
has reported that images of the minor planet (596) taken on December 
11.44-11.47 UT with the 0.68-m f/1.8 Schmidt telescope at Catalina show the 
object to be in apparent outburst with a comet-like appearance, exhibiting a 
total V magnitude of about 13.4 and an envelope that extends 2' north and 5' 
west of the central condensation (CBET No.2583 issued December 12).  Note 
that the predicted magnitude of this asteroid at the time of Steve's 
observation is about 60% fainter than the observed value.

BAA member Peter Birtwhistle obtained images from Great Shefford Observatory 
between December 12.178-12.204 showing the presence of a large arc to the 
north and smaller arc to the south in a field 3'x3' in size - See:
http://www.birtwhistle.org/Gallery(596)Scheila.htm

(596) Scheila is a large asteroid (diameter ~113 km) situated in the outer 
regions of the Main Belt, having a dark surface (albedo = 0.038). A rotation 
period of 15.8 hours has been reported and from the amplitude of its 
lightcurve(0.09 mag) it must be relatively spherical in shape.  In other 
words it is a very ordinary, rather typical asteroid with no special 
features.  Likewise its orbit is definitely asteroidal in nature and not 
cometary.

It has been speculated that we have just witnessed the aftermath of a 
high-speed collision between (596) and a small, non-descript object, perhaps 
no more than a metre or two in size.  The two conjoined arcs visible in 
Peter's stacked image are reminiscent of the arc-shaped material of 'Comet 
P/2010 A2' imaged by the Hubble Space Telescope in January of this year - 
See:
http://hubblesite.org/newscenter/archive/releases/2010/07/image/a/
For this latter case, it has now been shown that this phenomenon was not 
cometary but was indeed the result of a collision between two small bodies 
in the asteroid belt.

Further observations of the evolution of the 'debris', dust, etc. around 
(596) Scheila should make it possible to discriminate between the collisonal 
hypothesis and a cometary origin.  My money is on the former.  Indeed, 
high-resolution spectroscopy of the emitted cloud of material could provide 
a unique opportunity to probe the composition of an asteroid by analysing 
the nature of the expelled material.  Let's hope that an 8-metre or 10-metre 
class telescope will be put to work doing just this in the very near future. 
Checks should also be made on images of (596) taken in the past to see 
whether an associated coma can be found at some other epoch.  If such a coma 
were to have occurred in the past then this would demonstrate recurrent 
activity characteristic of a true comet.

Richard Miles
Director, Asteroids and remote Planets Section
BAA