After decades of flirting with the moon, scientists hope to give it a great big smack this week. In the predawn hours of July 31, they intend to slam the Lunar Prospector craft into the lunar south pole with an impact force equal to that of a two-ton car hurtling in at more than 1,100 miles per hour.

Their purpose is to kick up enough water vapor to answer conclusively the question of whether there is frozen water on the moon. If it is present in great enough quantities, as strongly suggested by recent evidence, lunar H2O (hydrogen and oxygen) could be of immeasurable scientific and possibly economic value.

NASA's $63 million Prospector mission to map the moon's surface composition, launched in January 1998, was scheduled to end in a kamikaze plunge about now in any case. So the team decided to take this long-shot chance to make it a useful one. But they give themselves only about a 10 percent chance of success: that is, a clear detection of either water vapor or its byproduct, the hydroxyl (OH) ion.

At best, said Prospector lead scientist Alan Binder of the Lunar Research Institute, the crash might loft as much as 40 pounds of water vapor--just enough to be detectable by waiting telescopes on the ground and in space. Until now, the evidence, although convincing to Binder and others, has been indirect and open to interpretation.

The plan is for Prospector to dive in at a shallow, near-horizontal angle, barely clearing the rim of the 26-mile-diameter target crater, and plow into the base of the far wall. But engineer David Goldstein, who led the University of Texas group that proposed the plan, offers a daunting list of things that can go wrong: Prospector's solar-powered batteries could be depleted during the prolonged darkness of the partial lunar eclipse on Wednesday. Its rockets might not fire normally as it exhausts its last remnants of fuel. It could hit the crater rim. It could skip across the target area like a stone on a pond. Or it could hurtle past a huge pile of snow and dig up nothing but dust--"and we'd never know."

On Thursday, the team is to begin to reposition the 354-pound craft, currently orbiting just 20 miles high, for the long-shot endgame. They will increase its spin rate so that centrifugal force will drive the last drop of fuel to the thrusters, and raise the altitude as much as the fuel supply permits to steepen the impact angle and maximize the depth of the impact. They are to send the final commands just before Prospector disappears behind the moon shortly before impact. The crucial final engine firing will take place during this communication blackout. When Prospector reemerges above the lunar south pole, assuming no problems, it will be skimming over the surface in the direction of Earth at a little more than a mile per second (about 3,800 mph), and perhaps 10 seconds from impact.

At this point, a battery of telescopes, including the Hubble and the Submillimeter Wave Astronomy Satellite in Earth orbit, along with the earthbound Keck Telescope in Hawaii and the McDonald Observatory in Texas, will be trained on lunar ground zero. The scientists theorize that about 2 percent of the dusty surface layer in the target area is water ice in the form of crystals, like a fine frost, or possibly chunks like ice cubes.

The plume of dust thrown up by Prospector should fall back to the surface almost immediately, but the vapor cloud could last for hours, according to University of Texas astronomer Edwin Barker. The water molecules should expand into the vacuum and then fall, bouncing around over thousands of square miles of hot, sunlit lunar surface like cold droplets thrown into a hot skillet. Meanwhile, the molecules will be broken down by the sun's rays, forming a "a thin haze, like a skullcap" of OH molecules around the lunar southern hemisphere. The array of telescopes on and around Earth will try to record all this in visible, infrared and ultraviolet wavelengths.

In order to detect the water, scientists noted wryly, they have to send their craft "where the sun don't shine." Although lunar samples returned by the Apollo astronauts suggested that the moon's interior is dry, its surface is bombarded with water-rich comets. Scientists had suspected that some of the deposited water could have migrated to permanently dark areas at the lunar poles.

In 1994, a $75 million experimental Pentagon spacecraft called Clementine, the first U.S. mission to the moon since the Apollo program, beamed radio waves briefly into dark spots at the lunar south pole. The radar echo was altered in ways consistent with ice, rather than the rock powder characteristic of most lunar terrain. Although the interpretation of the Clementine data has been controversial, it proved at the least that there were lunar sites where ice might lurk. Clementine found the South Pole-Aitken Basin, an impact crater more than 2 billion years old, to be more than 1,500 miles in diameter and up to 8 miles deep, making it the largest and deepest crater known in the solar system. Its floor is peppered with many smaller depressions in perpetual shade.

Inside these "cold traps," the temperatures never rise above about 280 degrees below zero Fahrenheit, and water ice at the bottom might exist for billions of years. If so, according to Paul Spudis, of the Lunar and Planetary Institute in Houston, it could contain a remarkable record of the rhythms and origins of the comet flux in the inner solar system over the eons.

While many scientists argued that the Clementine data could be a statistical fluke, Lunar Prospector took just weeks to find a similar result using a different technique, with more sensitive instruments, and flying four times closer to the surface. Prospector's neutron spectrometer measured hydrogen, mapping its abundance by location. In this way, the team could infer the presence of water ice in the polar regions.

Some scientists argue that water on the moon would be worth many times its weight in gold, making the establishment of a permanent lunar colony much cheaper. Spudis has called the desolate polar traps potentially "the most valuable piece of real estate in the solar system."

However, other scientists have cautioned that ice at the poles could be too remote for use in the more temperate equatorial zones, where lunar bases would be located. The logistical barriers, they argue, could be insurmountable.

But before speculating too much about the price of a lunar snow cone, scientists first want to find out for sure what is there. If Prospector's finale is fruitful, Binder said, it will eliminate the need for another $60 million mission to go and find out.