CAPE CANAVERAL, Fla. - NASA launched its first moon shot in a decade, sending up a pair of unmanned science probes yesterday that will help determine where astronauts could land and set up camp in years to come.

The liftoff came one month and two days shy of the 40th anniversary of the first lunar footprints. The mission is a first step in NASA's effort to return humans to the moon by 2020.

An Atlas V rocket carrying the two spacecraft blasted off in late afternoon, ducking through clouds and providing an exhilarating start to the $583 million mission.

The probes should reach the moon in four to five days - or by early next week.

One, the Lunar Reconnaissance Orbiter, will enter an orbit around the moon for a high-precision mapping mission. The other, the Lunar Crater Observation and Sensing Satellite, will swing past the moon and go into an elongated orbit around Earth that will put it on course to crash into a crater at the moon's south pole in October.

The moon-impacting part of the mission is a quest to determine whether frozen water is buried in one of the permanently shadowed craters. Water would be a tremendous resource for pioneering astronauts.

The plume of ejected material from the impact - more than 350 tons of soil and rock - should rise as high as six miles and could be visible from the United States.

When it comes time to launch astronauts to the moon, NASA wants to avoid putting them down on an uneven surface, near boulders or in a crater.

"The Apollo program accepted risk and was able to have safe landings," said Richard Vondrak, project scientist for the orbiter. "But we want to return to the moon, make repeated landings in some areas, and be able to go there with a higher degree of safety."