NASA craft to skim over comets

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

(CNN) -- An upcoming NASA launch will send a spacecraft on its way to what scientists hope will be the closest encounter yet with the nucleus of a comet.

In fact, if the $160 million probe performs as billed, it will fly within 62 miles (100 kilometers) of two or three of the primordial ice boulders.

The Comet Nucleus Tour (Contour), slated to kick off July 1 from the Cape Canaveral Air Force Station in Florida, could shed light on the behavior and evolution of comets, the smallest and perhaps oldest bodies in our space neighborhood.

"We believe they hold the most primitive materials in the solar system and that they played a role in shaping some of the planets, but we really have more ideas about comets than facts," Contour scientist Joseph Veverka told reporters on Wednesday.

After orbiting Earth for six weeks, Contour will fire its main engine and begin looping around the sun, which will give it gravity boosts as it closes in on its quarry.

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 kilometers). But the two comets are hardly similar in other ways.

Encke, one of the most observed comets, 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.

"The key to the Contour mission is to visit a diverse range of comets, from an evolved comet such as Encke, to a younger comet like [Schwassmann-Wachmann 3], or even a new comet never seen in this part of the solar system," mission researcher Mary Chiu said.

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

NASA has other missions in progress or in the works dedicated to comet exploration. 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.