A large asteroid known for more than a century appears to actually be a comet in disguise, astronomers say.

Most asteroids are chunks of metallic rock that have virtually no atmospheres. Tens of thousands of asteroids circle the sun inside what's known as the main asteroid belt, a doughnut-like ring that lies between the orbits of Mars and Jupiter. (Explore an interactive solar system.)

By contrast, most comets are loose clumps of dirt and ice thought to originate in the Kuiper belt, far beyond the orbit of Neptune. When a comet's oval-shaped orbit brings it close to the sun, its ices vaporize and the comet develops its signature halo of gases and dust.

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 (113 kilometers) 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.

"It could be that all asteroids with a diameter of less than, say, 150 kilometers [93 miles] have ice inside," said Dale Cruikshank, a comet expert at NASA Ames Research Center in California who was not involved in the Catalina observations. (Also see "Water Discovered on an Asteroid—A First.")

Comets Revived by Cracks in Insulation

It's unclear why some comets would cease activity, but one idea is that they go into "stealth mode" after too many passes near the sun.

"If they come near the sun often enough, they can build up a layer of nonvolatile and [non-reflective] material that acts as an insulating crust. This appears to be what happened" with 596 Scheila, Catalina's Larson said.

Based on this idea, a comet that ceases spewing material will remain inactive unless something happens to reactivate it. The simplest trigger is if another object strikes the crusty comet and cracks its insulation, allowing ice and gas trapped inside to gush out.

This scenario is "an attractive idea, but it's not the only option," NASA Ames' Cruikshank said. "These objects can crack and break for other reasons as well."

Shifting material inside an MBC—caused, for example, by the gravitational tug of a passing body—might also loosen the surface layer.

Researchers at NASA Ames have recently submitted a proposal for a space mission to visit an MBC.

The mission "would go out and look at a couple of these things, because MBCs are potentially very important to the issue of where the water to make Earth's oceans came from," Cruikshank said.

Hybrid Comets Brought Water to Earth?

The origin of water on our planet is a mystery because—according to current models of planet formation—water could not have existed on early Earth.

"We think the Earth formed hot and dry ... and hot rock is not very good at trapping water," said comet expert David Jewitt at the University of California, Los Angeles, who also did not participate in the new 596 Scheila observations.

Some scientists have speculated that, as ancient Earth cooled down, water and other volatiles were added as a kind of "late veneer" by icy comets that collided with the planet. (See "Comet Swarm Delivered Earth's Oceans?")

A problem with this theory is that recent spacecraft rendezvous with comets have shown that comet water has a slightly different makeup than Earth water, containing a higher concentration of a heavier version of hydrogen called deuterium.

Some scientists point out, however, that so far all of the comets studied directly have come from far out in the solar system. MBCs have a different origin, so their water might be a chemical match for our planet, they say.

MBCs "are a source of water the detailed composition of which we do not know," Cruikshank said.

Also, because MBCs originate closer to Earth, they may have been more likely to strike the planet than conventional comets, UCLA's Jewitt added.