It's been dubbed Earth's attic, a keystone for understanding the early history of the inner planets, and even a potential safe-deposit box for evidence of life early in the solar system's history.
By whatever label, the moon's star appears to be rising.
Even before President Bush unveiled his space policy earlier this month, which set a return to the moon as a top priority for US space exploration, scientists have had their sights set on Earth's rocky companion.
A European Space Agency orbiter, SMART-1, was launched last September and now is hurtling around the moon in an ever-tightening series of ellipses. By December, the craft is expected to begin its mapping mission. Next August, Japan is set to launch the first of two missions to the moon, with a second set for 2005.
In the US, teams of researchers are laying plans for a sample-return mission from the moon's dark side later this decade - a mission that scientists placed high on a list of objectives for solar-system research this decade. In addition, China and India have given the moon a lofty place in their nascent space-exploration programs.
Now, with impetus from the White House, the pace of lunar exploration could accelerate - first with robots, then with humans again for the first time in more than 30 years.
For some researchers, just getting to know the moon better will be worth the effort. For others, such work is vital as a steppingstone to using the moon as a base for research, manufacturing, and as a gateway for space exploration. That would include manned missions to Mars, should they become a reality.
Either way, "it's totally fallacious to say, 'We've been there, done that,' " says Alan Binder, lead investigator of NASA's successful, 19-month Lunar Prospector mission, which was launched in 1998 and mapped chemical elements on the moon's surface.
Researchers have spent more than 200 years and used everything from hammers and chisels to lasers and seismographs to study their home planet's structure and history, he adds, "and we still don't know everything. We have decades, if not centuries, of exploration to undertake on the moon before we understand it scientifically."
Still, based on results from the Apollo program and from the US Clementine and Lunar Prospector missions in the '90s, "the level at which we know the moon is tremendous compared with other planetary bodies," says James Head III, professor of geological sciences at Brown University. He has called the moon a "keystone" to understanding the early evolution of Mercury, Venus, Earth, and Mars.
Indeed, building on the foundation of earlier moon missions, "we're now asking more sophisticated questions," says Paul Spudis, a researcher at the Johns Hopkins University's Applied Physics Laboratory in Laurel, Md.
One of the enduring scientific legacies of Apollo, for instance, was an appreciation for the way in which collisions with other objects shape planetary bodies. Now, says Dr. Spudis, questions have shifted from whether cratering is important to how the cratering rate has changed with time and what that tells us about the Earth-moon system and the evolution of Earth's environment.
Moreover, the moon's pocked surface, Spudis says, has become the geological "clock" against which the time scales of planets are measured. Researchers have dated moon rocks from regions the Apollo astronauts visited, counted craters in those regions, then used the results to estimate the ages of cratered features on other planets.
"Now, suppose we're wrong, perhaps because we didn't go to the right places or collect the right samples" from the moon? he asks. "We still need to know: What is the real history and time scale of the moon, and when did large-scale cratering begin and end?"
Some answers may come with a sample-return mission NASA is planning for the South Pole-Aitken Basin, an ancient crater on the far side of the moon that measures roughly 1,550 miles across and six to eight miles deep.
The crater is so deep that researchers expect rock samples to yield a treasure trove of information about the structure of the moon's crust, the composition of its upper mantle, and the way the moon's interior arranged itself under the influence of gravity as it evolved. Such insights, scientists say, could tease out the story of the geological evolution of other rocky planets as well.
As if to tantalize researchers further, some scientists have posited that because the lunar surface is essentially pristine compared with Earth's, and because meteorites from Mars have been found on Earth, it may be possible to find relics from other inner planets strewn across the lunar surface.
Some 4 billion years ago, astronomers say, the inner planets endured what has become known as the Late Heavy Bombardment, which lasted roughly 200 million years. During this period, Earth, our moon, and other inner planets would have exchanged material as they took direct hits from asteroids or encountered collision debris from objects striking their inner-solar system counterparts.
On average, these planetary shards could amount to more than 22 tons of terrestrial material spread over each 38-square mile patch of lunar surface, estimates John Armstrong, a research professor at Weber State University in Ogden, Utah. More distant planets would have progressively less representation.
Other researchers calculate that the amount of material, if it exists at all, would be much less. Still, Dr. Armstrong suggests that these cosmic shards on the moonscape - particularly those from Earth - may yield unique clues to Earth's first billion years and perhaps hold geochemical evidence of early life.
Yet even as planetary scientists look forward to new opportunities to study
humankind's surrogate for other planets, others are looking long-term at commercial opportunities - ranging from supplying companies with data about the moon to building manufacturing
plants and power plants on the lunar surface.
Dr. Binder, who heads the nonprofit Lunar Research Institute and the for-profit Lunar Exploration, Inc., sees scientific gains and a great deal of money to be made from future lunar activities. In the short term, robotic exploration makes sense, he says. And the use of the moon as a place to learn to "live off the land" for future planetary exploration is also a logical approach.
Scientists and engineers have been devising ways to beam energy from solar-cell installations on the moon via microwaves to Earth. Former Apollo 17 astronaut Harrison Schmitt has been working with fusion-energy researchers at the University of Wisconsin who are devising reactors that use a heavy isotope of helium as fuel. The isotope is abundant on the moon.
There is also indirect evidence of possible water in the moon's polar craters. The Clementine and Lunar Prospector spacecraft detected large amounts of hydrogen there. Binder and others hold that if water is present, it's likely to be bound to dust grains as small particles, rather than blocks of ice. Yet even if water is not present, he says, the hydrogen can be "mined" and combined with oxygen - abundant in lunar minerals and soil - to make water.
The two mapping missions in the 1990s revealed deposits of other materials - such as silica, iron, titanium, magnesium, and aluminum. These could be used for a range of products manufactured on location.
"We have the raw materials for a civilization" to build the structures it needs on the moon, Binder says.
Beyond Earth's orbit
Here are some key flybys and landings in 40 years of space exploration:
Mercury: Mariner 10 flew past the planet in 1974-75 and shot photos of 45 percent of its surface.
Venus: Russian landings and Pioneer flybys in the mid to late '70s examined its sulfuric-acid rain and carbon-dioxide atmosphere. The Magellan mission in 1989-90 produced detailed maps of Venus.
The moon: Six Apollo landings from 1969 to 1972 represent the first - and only - times man has walked a surface beyond Earth.
Mars: Three decades before today's rovers, a Viking landing in 1976 helped scientists better understand the planet with photos of possible ancient rivers and lake beds.
Jupiter, Saturn, Uranus, Neptune: Launched in 1977, Voyager 1 and 2 flew by all four planets, as well as their rings, moons, and magnetic environments.
SOURCES: NASA, The Planetary Society