It was a remarkable feat of precision space navigation by engineers and scientists at the NASA Ames Research Center in Mountain View, and gave the scientists their first-ever chance to understand material that has lain hidden beneath the moon's surface for billions of years.
The spacecraft was 25,000 miles above the moon at about 7:30 p.m. Thursday when it aimed the Centaur rocket casing at its target deep inside a crater on the moon 60 miles wide named Cabeus. So precise was the mission control team's aim that the Centaur hit barely more than 1500 feet from the exact center of the 12-mile-wide assigned spot, and it dug an impact crater 60 feet wide and 13 feet deep.
But cameras aboard ther spacecraft did not see the brilliant plume of rising debris they had expoected from the impact, and Michael Bicay, science director at the Ames center, speculated that the Centaur might have hit solid bedrock rather than the mix of sand and smaller rocks the scientists had expected.
A few seconds after the mission's planned impact time of 4:30 a.m., the LCROSS mission control team announced that their spacecraft's instruments had detected the first thermal evidence that the crash had succeeded.
At that instant of the Centaur's impact, its "shepherding satellite" - as the spacecraft itself is affectionately called - was less than 375 miles above the moon's surface. Four minutes later the satellite itself plunged into the same crater, creating a smaller crater of its own.
Cabeus was chosen as the mission's target because it is more than two miles deep and its steep, high walls have kept its flat surface in total darkness since it was formed by the impact of some unknown meteorite or comet a billion years or more ago.
For 10 years now other observations by spacecraft have detected evidence of hydrogen and molecules called hydroxyls in many lunar craters, but inside Cabeus, permanently in shadow, that hydrogen could well be in the form of permanently frozen water ice.
And that's precisely what this mission is all about: to find out whether immense quantities of water do in fact lie beneath the moon's dry and lifeless surface. The moon has been bombarded ceaselessly by meteors and comets ever since it formed, and since comets typically contain large amounts of water, scientists reason that water indeed will be detected when other spacecraft and and an army of earthbound astronomical observatories analyze the rocks and dust that the LCROSS mission has kicked up from the dark lunar shadows into the sunlight above Cabeus crater.
"There's water down in that crater, and we're going to dig some of it up," said Anthony Colaprete, the mission's chief scientist, at a press conference Thursday.
Before the two impacts, Colaprete and Daniel Andrews, the project director, estimated that the Centaur's impact would lift 385,000 tons of lunar rocks and dust in a cloud of debris nearly six milles high, and the shepherding satellite's impact would send about half that mass up into the sunlight too.
LCROSS was launched from Cape Canaveral last June as a kind of hitchhiker aboard another spacecraft called the Lunar Reconnaissance Orbiter, and the LRO has been orbiting the moon ever since on an independent mission to map the moon's entire surface. Now it is gathering images of the impact for later analysis by scientists here on the ground.
Ames offficials had opened the guarded center for the night to the public, and hundreds of space enthusiasts had gathered in the open for the night to watch a program of three movies on a giant screen and await the impact.
When they saw an image of the crash transmitted from LCROSS far above and the debris plume was barely visible instead of spectacularly bright, the crowd in the cold open air - and some in tents - was disappointed.
"It was fun to hang out with my friend Dylan, but it's kind of like I got up at 3 in the morning for nothing," said Bobby Howie, 9, of San Mateo.