On the evening of 21 February 1906, A. Kopff discovered asteroid 596 Scheila.

  It was found to have a fairly typical Main Body orbit, meaning that it lives comfortably in the middle of the Asteroid Belt between Mars and Jupiter.  Beyond that, it never attracted much notice, being just one amongst billions of asteroids. 

This all changed over the weekend, when Steve Larson of the University of Arizona’s Lunar and Planetary Laboratory looked at an image of 596 Scheila dated 11 December 2010 and noticed that it was surrounded by a fuzzy coma, blowing back into a tail.  A quick check of archived images (from 18 October to 3 December) showed that, up until recently, the asteroid had looked like all other asteroids at this distance:  A featureless dot, indistinguishable from a star.  So what’s changed?  What’s up with 596 Scheila?

One possibility is that it collided with a small meteoroid, throwing up a cloud of dust and debris.  The solar wind would blow on the dust cloud, gradually sculpting it into a shape similar to what we see now.  But the other possibility, both more likely and more exiting, is that this 596 Scheila is not an asteroid at all but a comet lurking in an unusually round orbit!  What’s happening is that, as the comet reaches perihelion (the point in its orbit when it is closest to the Sun), it gets just warm enough for water ice to start melting.  In the vacuum of space, it boils off immediately and refreezes into a cloud of tiny ice particles.

Why do we think the comet scenario is more likely?  Well there’s a precedent – it’s not the first such comet to be found.  Another such comet, 133P/Elst-Pizarro, has been observed showing exactly the same behaviour several times, always at perihelion.  In fact, thanks to discoveries like these, we now have an entirely new class of solar system objects: Main Belt Comets.

Mike Simonsen has more detail in his article for Universe Today, posted here: http://www.universetoday.com/81576/asteroid-sheila-sprouts-a-tail-and-coma/