Once in a while an asteroid comes along that changes things.
On January 6, 2010 the Lincoln Near Earth Asteroid Research team discovered a main-belt asteroid that was experiencing an outburst making it look like a comet. Comets aren't normally found in the asteroid belt and early indications suggested that this was an asteroid that had suffered a collision, which produced it's comet-like outburst and tail. A photo released yesterday (below) taken by the Hubble Space Telescope reveals that the object's tail does not resemble that of a normal comet and that the bright star-like nucleus is strangely offset from the tail itself. All of this tends to support the idea that this display is the result of the collision of two asteroids - an event never before witnessed.
Seeing an asteroid have a comet-like outburst reminds me of the case of a comet that apparently converted into an asteroid. Back in November 1949 Albert Wilson and Robert G. Harrington were using the 48-inch Schmidt (now called the Samuel Oschin Telescope) taking photos for first Palomar Sky Survey when they discovered a comet. Comet 107P/Wilson-Harrington was photographed by the duo over three nights but there was great uncertainty in the orbit and it was eventually lost.
Thirty years later, in November 1979, Eleanor Helin was observing at Palomar when she discovered a new asteroid temporarily dubbed 1979 VA. You can see it as the streak in the center of the image below.
In the early 1990s it was determined that Comet Wilson-Harrington and asteroid 1979 VA were the same object. The asteroid is now known as 4015 Wilson-Harrington and is thought to be a comet that lost all of its icy volatiles, or in essence a "burned out" comet. For more on the discovery of Wilson-Harrington have a look at this page over at Cometography.
Why do astronomers think that Wilson-Harrington is a dead comet and not an asteroid that had a collision back in 1949? It's orbit.
Notice that Wilson-Harrington's orbit is elliptical like a typical comet. At its farthest point from the Sun it is out in the asteroid belt, but at its closest point it comes in a little closer to the Sun than Earth is. Just like a typical comet, this one had its 1949 outburst when it was close to the Sun and its heating.
By the way, as of a few days ago the count of the total number of asteroids discovered at Palomar Observatory stood at 23,366.