Back to the moon -- but why?

From CNN Interactive writer Andy Walton

After a blockbuster 1997, NASA is launching 1998 with a return to its old stomping grounds. The Lunar Prospector probe, slated for a January liftoff, will map the surface of the moon -- much of which is still mysterious.

But why send another ship to the moon? Haven't we been there, done that and bought the commemorative key chain?

In a word, no.

Of all that's "out there," the moon stands alone -- it's right next door in planetary terms, the dominant sight in the night sky, the object of an international obsession decades ago.

So close and so visible, the moon mocks us and piques the curiosity of scientists and laymen almost to the point of insanity. The very word "lunatic" dates from a time when it was thought that the full moon inspired madness.

Yet little is known about our gray neighbor. All of the manned and unmanned missions of the 1950s, 1960s and 1970s focused on a fairly narrow band of the moon. Only 25 percent of the lunar surface has been mapped in detail.

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One important question sprung to light in 1994, when the Clementine probe found signs of water far below the lunar surface. While the moon has no water of its own, the impact of long-ago comets -- which are, after all, balls of ice -- may have left ice crystals below.

The Apollo missions did not stray far from the lunar equator, landing on dusty "seas" where exposure to the sun evaporated any water long ago. At the shady poles, where Prospector will focus its search, scientists believe ice crystals lurk beneath the surface.

"It now appears from data beamed back from Lunar Prospector, that indeed, there is frozen water on the moon. "

Why does water matter?

Simply put, it's the single most essential substance for humans and their machines.

"Water is probably one of the most valuable strategic materials we can find in the solar system," muses Dr. Paul Spudis of the Lunar and Planetary Institute.

Water, of course, is a necessity for human life. But it can also be split into hydrogen and oxygen -- the two components of rocket fuel.

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All of which is fine, of course. But why on Earth would anyone want to build a rocket on the moon? It's a weighty question that demands a lightweight answer.

The moon's gravity is a fraction of Earth's, and the escape velocity -- the speed that a spaceship has to reach to escape its gravitational pull -- is a mere 2.38 kilometers (1.4 miles) per second, compared to Earth's 11.2 km/second (about 7 miles/second).

That's why experts treat a base on the moon as essential to manned missions to the planets. Launching from the moon leaves more fuel for the rest of the trip, and extends the range of the mission.

Exploration of the moon is not a done deal, any more than the American West was settled at the end of the Lewis and Clark expedition. The missions until now have produced a vast wealth of information, but crucial questions remain unanswered -- questions Lunar Prospector will begin to answer.

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