Comet-chasing probe cleared for launch

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

(CNN) -- NASA will attempt to launch a comet-chasing mission on Wednesday, after determining that a mysterious dust on the $160 million spacecraft did not pose a threat.

The unexpected discovery last week of contaminants on the probe's top solar array panel prompted NASA to scrub a scheduled Monday liftoff.

But over the weekend technicians cleaned off the fine particle layer and resumed flight preparations, which included fueling the Delta 2 rocket that will carry the Comet Nucleus Tour (Contour) robot ship.

The origin of the dust remains under investigation The new tentative liftoff time from Cape Canaveral, Florida, is 2:47 a.m. EDT. Forecasters expect a 70 percent chance of acceptable launch conditions. If necessary, the launch can be postponed until July 25 without affecting the mission, NASA said.

Contour, designed to take the most detailed pictures ever of comet nuclei, could pass within 62 miles (100 km) of two or more comets.

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

Mission scientists hope it sheds light on the behavior and evolution of the primordial ice boulders, among the oldest objects in the solar system.

After orbiting Earth for some weeks, Contour will fire its main engine and begin looping around the sun, which will give it gravity boosts as it approaches its intended destinations.

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). But besides proximity, the two comets have hardly anything 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 was first detected only 70 years ago and has proven much more volatile, splitting recently into several pieces.

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

Comet Borrelly

Besides serving as the building blocks of planets, comets might have seeded Earth with the complex organic chemicals from which life arose, scientists speculate.

Detailed studies of comets could do more than offer clues about the origins of the solar system and life. They could help humans learn how to divert a potential killer comet on a collision course with Earth, space scientists said.

Previous comet mission 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, scheduled to lift off in 2004, is slated to collide with a comet in 2005.

The closest comet encounter so far took place in 1992 when the European Space Agency's Giotto passed within about 120 miles (200 km) of a comet.

Rosetta, another ESA probe, could launch as early as January. It is designed to fly to a comet, go into orbit around its nucleus and drop a mini-lander on its surface.