Harrison Schmitt was mimicking a really bad skier coming down a rough Alpine slope. "Can't keep my edges," he joked, making two-footed hops one way and then another. "Shhhoomp. Shhhoomp. Little hard to get good hip rotation."

The geologist/astronaut was near the end of the final work shift on the moon and indulging in a brief burst of childlike, low-gravity play as he descended a hill. His buddy, Apollo 17 commander Eugene Cernan, followed suit with a gleeful whoop.

It was Dec. 13, 1972, and the astronauts had been laboring in feverish haste for three days in a sun-flooded valley on the edge of the Sea of Serenity. Huge boulders had rolled most of the way down the slopes of the surrounding mountains, delivering an ancient cornucopia of otherwise inaccessible highland rock samples.

Gunning their rover over the lunar dunes, the astronauts had driven as far as five miles from their spaceship -- the longest trips of any moon expedition -- and sometimes lost sight of it entirely. They had struggled with the effects of moon dust as it infiltrated and fouled the vehicle and prospecting tools.

Ham-handed and awkward in spacesuits, they had scooped and hammered like maniacs, maneuvered in and out of the buggy, clambered up and down steep inclines.

The harvest that day would constitute the last chunks of any alien celestial body to be returned to Earth in this century.

Lunar exploration had begun in 1959, when unmanned probes from the Soviets' Luna spacecraft first flew by and then struck the moon's surface. Around the same time, NASA's Pioneer 4 passed within 37,000 miles of the moon. Subsequent U.S. robotic missions, such as Ranger, Surveyor and Lunar Orbiter, photographed the lunar surface in preparation for the safe landing of astronauts.

Eleven manned Apollo missions flew between 1968 and 1972. Nine of them traveled all the way to the moon and six dispatched two-man landing parties to explore, conduct experiments and gather samples. They brought back almost 850 pounds of moon rock.

With the success of Apollo 11, the political impetus behind the program faded, as did public interest. But NASA continued as planned. Apollo 12 was spectacularly hit by lightning during launch, and yet the mission went on to a pinpoint lunar landing and an astronaut excursion to retrieve a camera from the defunct Surveyor 3 robot craft.

Apollo 13 gained the limelight briefly when an oxygen tank exploded en route and a tragedy was averted by human heroics (later recounted in a hit movie). Apollo 14 was the first mission successfully dedicated entirely to scientific exploration. Only on the last three flights were the crews equipped with improved life-support packs and the battery-powered rover, which enabled them to stay out longer and extend their range.

The Apollo program greatly advanced knowledge of the moon's origins, composition and surface, notes Paul Spudis of the Lunar and Planetary Institute in Houston. "But the more important story, to me, is the unexpected scientific legacy of Apollo, and that is the importance of `impact' as a geological process."

Spudis said the insight has "revolutionized earth and planetary sciences," revealing that hypervelocity collisions of asteroids, comets and other space rubble are fundamental processes in the solar system that built the planets and guided the course of biological evolution on Earth through mass extinctions.

Samples grabbed up by Schmitt and Cernan in the Valley of Taurus-Littrow, for example, told the story of a titanic asteroid impact some 3.8 billion years ago that had unleashed the power of billions of H-bombs, throwing up lunar crust and pulverizing or melting it for hundreds of miles around. Once scientists had used the Apollo trove to learn the geological signature of meteorite impacts on the moon, Spudis said, they were then able to recognize signs of similar events on Earth -- notably the Yucatan imprint of the huge bolide that wiped out the dinosaurs 65 million years ago.

But planetary geologists say all the Apollo missions combined only scratched the lunar surface. No crew included a professional scientist until Schmitt. And after Cernan and Schmitt departed, more than two decades passed before another craft was sent to study the moon -- and then it was from orbit. In 1994, a tiny experimental Defense Department spacecraft called Clementine made worldwide headlines when it discovered possible indirect evidence for water ice in a permanently shadowed miles-deep crater at the lunar south pole.

And in January 1998, NASA launched the Lunar Prospector into a 62-mile-high polar orbit around the moon on a mission to map the surface, measure its composition, search for uranium, iron and other elements, and probe the gravity and magnetic fields. Almost immediately, Prospector also found signs of water ice buried in the dark of the moon's poles.

As the Prospector mission ends on July 31, its handlers plan to crash the craft into the suspected site and send up a cloud of water vapor that might be detectable from Earth and space-based observatories. This could provide definitive proof that the ice is really there.

The immediate future of moon exploration is uncertain. Ambitious plans for further human forays, colonization, lunar astronomical observatories and spaceports for solar system exploration are all on indefinite hold. There are no U.S. robotic missions on the books, though some are in development, such as one at Carnegie Mellon University that would land a robot geologist to search for water ice at the moon's south pole. The European and Japanese space agencies are also designing lunar robotic missions.

If it turns out there is lunar ice in sufficient quantities to become a valuable natural resource for future human colonists, general interest in the moon might be revived, scientists say. Since any ice found on the arid moon most likely was delivered by comets, it could also provide researchers a rare record of comet activity in the inner solar system over the eons.

Other attractions for future explorers, robotic or human, include clues accumulated in the unprotected lunar surface dust during billions of years of bombardment by comets, asteroids and solar particles -- clues not only to the moon's history, but to that of the sun and the entire solar system.

CAPTION: Above, rover craft used by Apollo 17 crew, the last humans on the moon. Right, Lunar Prospector is positioned atop launch vehicle.

CAPTION: Clementine craft is depicted in artist's rendering on its planned trajectory to the moon.