What’s the difference between a comet and an asteroid? The distinction is hazy at best. Thanks to the recent discovery of ice on an asteroid in the main belt, the lines have been further blurred.
Comets are typically thought to be small masses of rock and ice. They originate in the outer Solar System and are characterized by vapor tails that stream out as the comet ventures close to the Sun. Most asteroids, on the other hand, are relatively large and waterless rocks that typically orbit in the inner Solar System. They have no observable tails. Astronomers admit that these are loose definitions, and in recent years scientists have uncovered exceptions to both.
In 2006, astronomers observed tails of vapor and dust coming from a few small objects in the main asteroid belt. Never before had they seen this type of comet-like behavior from objects that were thought to have formed so near to the Sun. In April 2010, two independent research teams reported in the scientific journal Nature that the asteroid 24 Themis is completely covered by a thin layer of ice.