In 1972 the last Apollo astronauts came home from the moon, and that was that. The budget was tight, and they'd only found rocks. So for a quarter of a century at NASA, the moon was a dead issue.
No longer. In 1994 a military space probe called Clementine, sent to map the moon as a way of testing sensors for possible Defense Department use, found evidence of ice in the shadowed corners of craters near the moon's south pole. In 1998 a NASA probe called Lunar Prospector was sent to confirm Clementine's findings, and as it orbited the moon it found evidence of large amounts of ice in the lunar soil.
Neither ship completely settled the issue; Clementine relied on radar data, and Lunar Prospector did indirect chemical measurements. But ice on the moon? Lunar Prospector's readings, in particular, suggested there may be hundreds of billions of gallons of it. Engineers on Earth almost salivate at the thought.
"If we find water there, it will change the course of exploration," said Rusty Hunt of NASA's Ames Research Center in Mountain View, Calif. "If there's water near the south pole, we'd go there. The people who settled the old West were able to live off the land, so to speak, and we'd do the same."
Hunt is a flight director for a mission called LCROSS (Lunar CRater Observation and Sensing Satellite), which was launched in June, and is scheduled to crash into the floor of a crater near the lunar south pole on the morning of Oct. 9.
It will be a violent end, but, the engineers and scientists say they hope, a spectacular one. LCROSS is flying formation with the Centaur rocket that sent it into space, coasting a few hundred miles behind. On crash day, the empty booster will plow into the lunar soil at 5,600 miles per hour, digging a crater about the size of a swimming pool, and, they hope, kicking up 350 tons of material.
Instruments on board LCROSS will watch it all, taking precise measurements of the impact before the satellite crashes too, four minutes later. Earthly telescopes, plus the Hubble telescope in earth orbit, will watch from afar.
The theory being tested is that comets, which were common in the early eons of the solar system, pummeled the planets and their moons, leaving vast quantities of frozen water. Some scientists think they may account for a third of the water in the Earth's oceans and atmosphere today.
On the airless moon, most of the ice would have quickly vaporized (or, to be more precise, sublimated), escaping into space. But near the lunar south pole, there are craters whose floors are never heated by the light of the sun, perpetually low on the horizon. Perhaps ice remains there, easily reached with minimal digging. That is what LCROSS is after.