The lack of dust, however, does not mean that a search for evidence of water on the moon has failed, experts said. Telescopes, using ultraviolet filters, will continue to look for clues that water vapor was splashed into the lunar sky by the impact.
NASA engineers sent final instructions to Lunar Prospector just before it sped out of sight behind the moon. Rockets were set to fire automatically to crash the craft on the lunar surface.
David Folta, the guidance officer at the Goddard Space Flight Center in Greenbelt, Maryland, said that since Prospector's radio signals were not picked up again, the spacecraft must have hit the moon. The precise impact point, however, wasn't clear.
"Everything went normally," said Folta, "so we have every reason to believe it made it to the impact site."
Had the automatic rocket firing behind the moon not gone as planned, the spacecraft would have reappeared, still in orbit and sending radio signals. But Folta said, "We never heard from it again."
David Goldstein, a University of Texas scientist observing from the MacDonald Observatory in west Texas, said no dust cloud or other visible indication of impact was detected by telescopes focused on the lunar south pole.
"We didn't see any evidence of the impact," said Goldstein. "We didn't see any dust. It is a bit of a disappointment."
Goldstein said the MacDonald telescopes continue to take data, however, recording measurements in invisible ultraviolet light. The data will be analyzed in a search for the chemical signature of water, a process that may take weeks.
The 354-pound spacecraft was to complete its highly successful voyage of discovery by smashing near the lunar south pole, into a shadowed crater that scientists believe may contain frozen water.
Experts calculated that the spacecraft hurtling at 3,800 mph would explode against the moon with enough force to send a plume of dust and vapor spiraling into the lunar sky. Telescopes watching from Earth and orbit were to search that plume for evidence of water.
It was hoped there might also be a visible plume or dust cloud from the impact, but there were no immediate reports of such sightings from about a half dozen observatory telescopes focused on the moon.
Lunar Prospector was launched Jan. 6, 1998, and spent 18 months circling the moon. It completed more than 6,800 orbits, dipping to within 6.2 miles, as its five instruments analyzed the moon's chemistry, gravity and magnetic fields. Findings from one of the instruments led to the plan to crash the spacecraft into the surface.
In readings over the lunar poles, the instrument detected the chemical signature of hydroge, a finding that to scientists suggests the presence of water. By some estimates, there could be as much as 200 million metric tons of water mixed in the top 18 inches of lunar soil near the poles.
Only a fraction of that - about 100 pounds - may be vaporized by the spacecraft crash, experts believe. But that would be enough to prove the presence of water.
Folta said guiding the spacecraft to a precise impact in the crater was "really a tricky maneuver," but it everything went on schedule.
The crash site will become the final resting place for Eugene Shoemaker, a U.S. Geological Survey astronomer who was one of the world's leading authorities on lunar impacts. After he died two years ago in a car crash, a lipstick-size metal container filled with his ashes was glued to one of Prospector's braces before the craft was launched.
Lunar Prospector's data gathering has resulted in a series of discoveries and new scientific tools, including:
- Tentative evidence that water ice exists in shadowed craters near the moon's south and north poles.
- The first precise gravity map of the entire lunar surface.
- Confirming the presence of local magnetic fields that create the two smallest magnetospheres in the Solar System.
- The first global maps of the moon's elemental composition.