Probe starts quest for comet encounters

Drawing of Contour approaching the rocky, icy core of a comet.

(CNN) -- An unmanned NASA spacecraft was launched Wednesday morning from Cape Canaveral, Florida, beginning a deep space expedition that promises to return the most detailed pictures ever of comets.

The Comet Nucleus Tour (Contour) probe could pass within 62 miles (100 km) of three of the primordial ice boulders during its flight, intended to last more than four years.

Astronomers hope the $160 million mission sheds light on the behavior and evolution of comets, among the oldest objects in the solar system.

Contour, after orbiting Earth for about six weeks, will fire its main engine and begin looping around the sun, giving it gravity boosts as it approaches its intended destinations.

COMET FACTS Interactive comet tour    THE MISSION Contour, a closer look

The first stop is comet Encke in November 2003. The second is comet Schwassmann-Wachmann 3 in June 2006.

Both will be relatively close to Earth during the encounters -- within 31 million miles (50 million km). Besides proximity, the two comets have little else in common.

Encke, one of the most observed comets for centuries, has passed near the sun thousands of times and releases little gas and dust.

In contrast, Schwassmann-Wachmann 3, first detected only 70 years ago, has proven much more volatile and recently split into several pieces.

Equipped with high-resolution cameras and instruments to study comet dust, gas and nuclei, Contour might visit a third comet if the conditions are right.

Comet Borrelly

In addition to serving as the building blocks of planets, comets might have served as the building blocks of life, seeding Earth with complex organic chemicals from which life arose, according to space scientists.

Previous comet missions include NASA's Deep Space 1, which last year snapped the best images yet of a comet nucleus when it passed within 1,200 (2,000 km) of comet Borrelly.

Other NASA comet expeditions are in progress or in the works. Stardust, launched in 1999, should fly near a comet in 2005 and return to Earth with dust samples in 2006. Deep Impact, is slated to lift off in 2004 and collide with a comet in 2005.

Rosetta, a European Space Agency mission that could launch in January, is expected to orbit around a comet and drop a mini-lander on its surface.