As well as being soundless, some craters near the moon's poles are in permanent shadow and are so cold that ice could remain there for eons. When the LCROSS rocket's upper stage violently collides with the surface of a shadowed lunar crater, the massive impact will jolt up a huge cloud, or 'plume,' of lunar material - soil and maybe even water ice. Finding water ice is the main purpose of LCROSS. However, if LCROSS does not detect ice, that would not rule out ice at the lunar poles, according to scientists.
If there is enough of it, water ice would be as valuable as gold to astronauts on the moon because launching anything into space from Earth's surface costs as much as $10,000 per pound (0.45 kilogram.) Astronauts could drink life-sustaining moon water or make it into rocket fuel. If there is adequate water near one of the lunar poles that astronauts could use, that water could save NASA huge sums of money.
"Our objective is to detect and measure water in the lunar soil," said Tony Colaprete, the LCROSS principal investigator and a planetary scientist at NASA Ames Research Center, Moffett Field, Calif. "It's just like prospectors used to do when they were looking for precious metals. They would drill a hole in the side of a riverbank, stick a piece of dynamite in there and then blast a chunk of earth off. They would then sift through the debris - using a variety of methods - to detect ores," said Colaprete. "They'd wash the debris into the river and use slurry - a mix of water and debris - to separate gold from the rest of the dirt."
"We're doing the same thing. We're blasting a hole in the moon about half the size of an Olympic-size swimming pool. Instead of slurry and a tin pan, we're using a suite of instruments both at the moon and on Earth (to detect water and other materials)," he said. "We expect to excavate at least 220 tons (200 metric tons) of moon dirt," Colaprete noted. The impact will be visible to a number of lunar-orbiting satellites and possibly also to Earth-based telescopes.
Image left: The south pole of the Moon. Click on the image to download high resolution photo.
What triggered NASA's interest in locating possible water near the moon's poles? Both the Clementine (in 1994) and the Lunar Prospector (in 1998) spacecraft found indirect -- but not definitive proof -- that water ice may exist in the dark shadows of craters in the lunar south pole area - gloomy, extremely cold places that never see the light of day. Lunar Prospector found evidence of hydrogen, which along with oxygen, comprises water. Soon, LCROSS mission scientists hope to find solid proof of water.
LCROSS will be a 'secondary payload' when it is launched for its journey to the moon in October 2008. That is, LCROSS will be a hitchhiker. It will ride the same rocket as the Lunar Reconnaissance Orbiter (LRO), another NASA mission to the moon. The rocket, an Atlas V with a Centaur upper stage, will launch from Cape Canaveral Air Force Station, Florida.
The LCROSS spacecraft will arrive in the lunar vicinity independent of the LRO satellite. Instead of arriving at the moon in a few days like LRO, LCROSS will orbit Earth twice for about 80 days, and then will strike one of the lunar poles in January 2009.
The reason that the LCROSS spacecraft will take so long to arrive at the moon is that the spacecraft will use 'lunar gravity assist' to change the second stage Centaur rocket's trajectory so that the space vehicle will strike its target near one of the moon's poles. During a gravity assist maneuver, a spacecraft approaches a planet or a moon, and the spacecraft's orbit is affected, causing the probe to shift direction.
Because of lunar gravity assist, LCROSS will approach the moon's poles with more velocity -- 1.56 miles per second (2.5 kilometers per second) -- and will strike the lunar surface more squarely -- at 70 degrees to the moon's horizon -- a steeper impact angle that will produce a bigger plume. LCROSS also will take longer to reach the moon, and NASA will have more time to track the spacecraft, control it and precisely aim it at a crater.
On the way to the moon, the LCROSS spacecraft's two main parts, the Shepherding Spacecraft and the Centaur second stage, will remain coupled.
As the spacecraft nears one of the moon's poles, the upper Centaur stage will separate, and then will impact a crater in that region. A plume from the upper stage crash will develop as the Shepherding Spacecraft heads in toward the moon.
The Shepherding Spacecraft will fly through the impact plume, sending back real-time images and spectra of the plume material taken by infrared cameras and spectrometers. After sending back these data, the Shepherding Satellite will become a 1,543-pound (700-kilogram) 'impactor' as well. The second impact will provide another opportunity for lunar-orbiting satellites and Earth-based observatories to study the nature of the lunar soil in the second, smaller plume.
In 1988 during the Lunar Prospector lunar orbital mission, scientists estimated that as much as 6 billion metric tons of water ice could be under about 18 inches of lunar soil in the craters. However, Lunar Prospector did not provide proof positive of ice. Scientists now are determining how best to detect water - if any -- in the mammoth plumes of moon dust that will result from the two LCROSS impacts. Because the impacts will be so complicated, scientists need to understand them before they happen. Then, researchers can properly plan the science observations scientists would like to make.
time exposed image of impact "An impact is a very complex event," Colaprete observed. "There are a number of processes that occur one after the other, and some simultaneously, each of which contains clues about the moon's materials and the impact."