Scientists are hoping that the crafts impact, scheduled for July 31, will bring up vapors which prove ice deposits exist on the moon. Such a find would suggest that humans could eventually colonize the moon with less difficulty than previously expected.

Scientists have strong but indirect evidence of lunar ice deposits. In March 1998, the Prospector detected large quantities of hydrogen at the moons poles, suggesting the presence of ice. It is unknown, however, if the detected hydrogen is actually part of water molecules, or hydrogen in another form. An impact-induced vapor cloud would prove the deposits.

NASA describes the project as "low probability, high pay-off," and estimates the chances of success at 10 percent. While scientists have picked a target a crater on which the sun never shines under which they say ice deposits are more likely to exist, there is no guarantee that the site is the best. Even if the craft releases vapor, the cloud could be so thin that space- and land-based telescopes focused on the crash may not be able to detect it.

But should the mission succeed, proof of water deposits would mean the moon might someday support human colonies. Water molecules could also be broken down into a form that would allow space shuttles to be refueled on the moon. In September, scientists estimated that the moon may hold up to six billion metric tons of ice.

Lunar Prospector 's $63 million mission will end on impact. NASA says the crafts finale is worth it, despite the slim chances of success. "The costs involved in this end-of-mission scenario are trivial," NASA wrote in a press release, "and the possibility of a positive detection of ice is sufficiently interesting that the approach will be tried."