WASHINGTON, (AFP) - A NASA spacecraft is just hours from its long-anticipated encounter with a comet, and skywatchers hope the flyby will shed light on how the comet's surface has changed since it skimmed by the Sun in 2005.

The US space agency's Stardust-NExT mission spacecraft "is within a quarter-million miles (402,336 kilometers) of its quarry, comet Tempel 1, which it will fly by tonight," NASA said in a statement.

"The spacecraft is cutting the distance with the comet at a rate of about 10.9 kilometers per second (6.77 miles per second or 24,000 mph)."

The pair are expected to get closest to each other, at a distance of 200 kilometers (124 miles), at 0440 GMT on Wednesday.

The Stardust-NExT spacecraft will be rapidly snapping pictures of the Tempel 1 comet as they pass.

Space experts are curious to see how a trip around the Sun has affected the surface of the Tempel 1, which is about six kilometers (3.7 miles) wide and travels on an orbit that brings it as close to the Sun as Mars and as far away as Jupiter.

Tempel 1 was last glimpsed in 2005 by NASA's Deep Impact mission as the comet was shooting toward the Sun on its five-year orbit between Mars and Jupiter.

Deep Impact pummeled the comet with a special impactor spacecraft and the material that came out was a surprise to scientists: a cloud of fine powdery material emerged, not the water, ice and dirt that was expected.

Deep Impact also found evidence of ice on the surface of the comet, not just inside it.