Watery prospects: Shoot the moon.

Some spacecraft end their missions by sailing quietly into oblivion, but the impending demise of Lunar Prospector could make quite a splash.

Observations by the craft, which has orbited the moon since January 1998 and is running low on fuel, have indicated that several craters at the lunar north and south poles contain frozen water. Prospector's final moments--set for a few minutes before 6 a.m. EDT on July 31--may provide the best proof yet that water really exists on the moon. If astronauts can easily extract it from the lunar soil, water will be a valuable resource and could spur efforts to colonize the moon.

If all goes according to plan, the tiny craft will crash into Mawson crater, a 60-kilometer-wide dent at the moon's south pole. Because the crater's rim is high enough to prevent sunlight from ever illuminating the bottom, Mawson is an ideal place to harbor frozen water. An abundance of hydrogen atoms in Mawson, previously revealed by Prospector, also indicates the presence of water (SN: 10/10/98, p. 239).

Hurtling into the crater at 1.3 km per second, Prospector may shoot up a plume of water vapor--if Mawson contains as much ice as astronomers hope. Previous estimates suggest that the concentration of frozen water in Mawson could be as much as 2 percent and that the impact could heat as much as 18 kilograms of ice to a temperature of 400 kelvins.

The densest part of the water plume could remain aloft for 16 minutes, calculates David B. Goldstein of the University of Texas at Austin. Goldstein and his colleagues, who proposed sending Prospector to a watery grave, describe their analysis in the June 15 GEOPHYSICAL RESEARCH LETTERS.

Of all the observatories that will cast their eyes on the moon this July 31, the Submillimeter Wave Astronomy Satellite has the best chance of detecting a water plume, says Goldstein. That's because its detectors are tuned to a wavelength at which water absorbs light. The Earth-orbiting craft, however, won't have a clear view of the moon until a half hour after the crash.

Even if the Submillimeter satellite misses the plume, that won't put a damper on the observations. Sunlight striking the rising column of water vapor will separate it into hydroxyl (OH) molecules and hydrogen, and these constituents should linger in the tenuous lunar atmosphere for several hours. Instruments joining the lunar-gazing party include the Hubble Space Telescope and the McDonald Observatory, near Fort Davis, Texas.

"A positive spectral detection of water vapor or its photo-disassociated byproduct, OH, would provide definite proof of the presence of water ice," says Goldstein.

Both Goldstein and Alan B. Binder of the Lunar Research Institute in Gilroy, Calif., the chief scientist for Lunar Prospector, note that a failure to detect water would not rule out water's existence on the moon. For instance, Prospector could miss its intended target. Plowing into the lunar surface at a glancing angle of 6.5 [degrees], the craft might hit the crater's rim rather than its floor.

In addition, Mawson may not contain sufficient water to generate a detectable signal, or the models researchers have developed could be wrong. Binder and his colleagues put the overall probability of detecting a water signal at 10 percent.

Nonetheless, Prospector's controlled crash on July 31 "is a lot better than just running out of gas," comments James W. Head of Brown University in Providence, R.I.