Another giant leap

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.

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