Since the last American astronaut left footprints on the moon 25 years ago, human exploration of Earth's familiar companion has taken a back seat to space shuttles, a space station, and high-profile unmanned missions to places like Jupiter and Mars.
But with the confirmation last week that large amounts of ice or frost rest in the dark recesses of craters at the moon's poles, the moon's star may once again be on the rise.
Initial readings from the tiny Lunar Prospector spacecraft orbiting Earth's companion point to from 10 million to 300 million metric tons of water on the moon - enough to "enable a modest amount of colonization for centuries," says William Feldman, a researcher with the Los Alamos National Laboratory in New Mexico and a co-investigator on the Lunar Prospector team.
Such a colony, or cluster of colonies, could well turn the moon into a lunar Legoland, where Mars-bound spacecraft or large structures for orbit could be built and launched for a fraction of the cost of building and launching them from Earth.
"The implications are tremendous," says Alan Binder, the lead scientist on the Lunar Prospector project.
Launched by the National Aeronautics and Space Administration on Jan. 7, the $63 million Lunar Prospector is the first of several dedicated moon missions planned for the coming years. Next year, Japan is scheduled to launch its Lunar-A spacecraft, which consists of an orbiter that will drop probes deep into the lunar surface, although not at the poles. Four years later, Japan hopes to launch Selene, which will put an unmanned lander on the moon to study its composition. Meanwhile, the European Space Agency is studying a proposal to put an orbiter around the moon in 2000, to be followed by a lander in 2001. The lander's target is the rim of a crater at the moon's south pole.
Follow-up missions from the US may come first from the private sector. Dr. Binder says he wants to conduct 10 more missions - including sample return missions - with private financing through his Lunar Research Institute, based in Gilroy, Calif. He hopes to launch his first mission within the next two years. A start-up company on Long Island, Applied Space Resources Inc., is planning a sample return mission for the year 2000.
Such missions, especially at the moon's poles, are needed to confirm Lunar Prospector's findings, mission scientists say, because while their craft's evidence is convincing, it is still indirect. As the orbiter circles over the moon's poles, it uses neutrons as a probe for hydrogen, one of the two elements that make up water. Measuring the energy levels of neutrons at the moon's surface with a device known as a neutron spectrometer, the researchers found the hydrogen signature they sought. Its strength left water as the most likely explanation for what they saw, because it is the most stable form of hydrogen on the moon.
Lunar Prospector's results confirm findings in 1994 by the Clementine spacecraft that the moon held water. Those results suggested water amounts of 100 million to 1 billion metric tons.
The difference between the Clementine and Lunar Prospector estimates lies in the techniques they use to take their measurements, according to Stewart Nozette, a researcher at the Lawrence Livermore National Laboratory in Livermore, Calif., and the architect of the Clementine project. The neutron spectrometer can "see" only into the top 2 meters of soil. The radio signals of Clementine penetrated deeper, and required a formation several meters thick for the signal to "bounce" back to Earth. Binder and Dr. Feldman acknowledge that the initial results from Lunar Prospector could be off by a factor of 10 in either direction - a situation that they expect will improve as the orbiter continues to gather data during its year-long mission, which it will augment with an extended mission within 10 kilometers of the surface.
BUT some scientists caution that economics remains the key factor in determining whether any water found is worth exploiting.
"This finding by Lunar Prospector is primarily of scientific interest at this time," notes Wesley Huntress, NASA's associate administrator for space science. "A cost-effective method to mine the water crystals from within this large volume of soil would have to be developed if it were to become a real resource for drinking water or as the basic components of rocket fuel to support future human explorers."
It's a point Binder acknowledges as well. Extracting the water from the soil is easy enough, he says, but building "mining" machinery to work in temperatures of minus 250 degrees F. or lower could be daunting.
Still, says Scott Hubbard, Lunar Prospector mission manager, "the implications of these findings could be significant, and possibly profound."