A large asteroid known for more than a century appears to actually be a comet in disguise, astronomers say. On December 11 astronomer Steven Larson of the Catalina Sky Survey in Arizona spotted what appeared to be a faint comet not currently in any comet databases. Larson later realized the cometlike body is traveling along the same circular, stable orbit as an asteroid named 596 Scheila. Discovered in 1906, the space rock is more than 70 miles wide.
The scientist thinks the body belongs to a mysterious group of solar system hybrids called main belt comets, or MBCs, which have orbits like those of asteroids yet display comet-like activity. "Most MBCs are small things, but this is the first time that a large asteroid has been observed to show cometary activity," Larson said. Astronomers have positively identified only five other MBCs to date, but experts think there could be millions more such hybrids cruising through our solar system in inactive states.