In a brute-force search for ice on the moon, an empty 5,000-pound rocket stage traveling twice as fast as a rifle bullet crashed into a permanently shadowed crater near the moon's south pole Friday, presumably blasting out tons of debris for examination by an instrumented probe that carried out its own kamikaze plunge four minutes later.
While the initial impact at 4:31 a.m. PDT did not prove especially dramatic--it was not even visible in real-time video from the Lunar Crater Observation and Sensing Satellite (LCROSS)--scientists said a camera sensitive to temperature variations clearly recorded the flash of the Centaur rocket's catastrophic crash.
More important, spectroscopic data indicated the presence of material of some sort above or near the impact point in a murky crater known as Cabeus, and instruments aboard NASA's Lunar Reconnaissance Orbiter observed the Centaur crater and confirmed a plume of debris. But it was not immediately clear how extensive the plume was or how much material was blasted out.
Principal investigator Anthony Colaprete said it would take several days to analyze the data from the $79 million LCROSS experiment and reach a consensus on whether or not water ice was, or was not, detected.
"Life is full of surprises, we want to be careful and not make a false negative or a false positive claim," he told reporters after the impact. "I'm excited we saw variations in the spectra because that means we saw something, and it was not just blackness. The information's there, we just need to get to it."
Asked if he had seen anything in the initial data to indicate the presence of ice, Colaprete said he had not yet had time to look for the telltale signals.
"We're going to take our time and build up a case for water in the ejecta, if it's there, or a case against it if it's not there," he said. "And then understand if we're seeing variations, what do these variations mean? We've got to understand that before we say anything."
Interestingly, a closeup of the thermal flash of the Centaur impact showed an elongated smear of light and not a concentrated flare as one might expect from a near straight-in impact. Colaprete said his team would look into what that might mean. Topographic data collected by other satellites indicated a relatively flat floor where the impact occurred.
LCROSS was launched June 18 as a companion payload to NASA's $504 million Lunar Reconnaissance Orbiter spacecraft. Working in a 31-mile-high orbit, LRO is designed to create a high-resolution map of the moon's surface to help identify sites for future manned missions.
It also will measure the solar and cosmic radiation that future lunar explorers will face, and map out the surface topology, mineralogy, and chemical composition of Earth's nearest neighbor. One year will be spent scouting future landing sites, followed by three years of purely scientific observations.
While LRO was launched directly to the moon by a powerful Atlas 5 rocket, LCROSS and the booster's empty Centaur upper stage were sent into a looping four-month orbit back around the Earth.
The spacecraft was designed to aim itself and the attached Centaur stage back at the moon, targeting a permanently shadowed crater near the south pole. Mission managers initially selected a crater known as Cabeus A, but after additional analysis of topographic data, the target was switched to nearby Cabeus, a crater measuring some 62 miles across and about 2.5 miles deep.
LCROSS successfully separated from the Centaur stage at 9:50 p.m. Thursday and then rotated 180 degrees to aim its instruments forward. A small rocket firing slowed LCROSS to ensure the proper four-minute separation from the Centaur.
Analysis of telemetry indicated the trajectory was right on the money--the Centaur is believed to have hit the surface within about 210 feet of the planned target--and LCROSS presumably flew through an ejecta cloud of some sort.
"Everything really worked out well," Colaprete said earlier. "The spacecraft flew perfectly, the instruments performed, honestly, better than expected in some cases. We got interesting results. But again, these are just initial results...I can certainly report there was an impact, we saw the impact, we saw the crater and we got good measurements, spectroscopic measurements, which is what we needed of the impact event.
"So we have the data we need to actually address the questions we set out to address."
The search for water ice on the moon is one of the holy grails of modern lunar exploration. Data from other spacecraft, including the Lunar Reconnaissance Orbiter, show the presence of hydrogen, possibly from water ice, in the top three feet or so of lunar soil. Scientists initially believed ice from comets could be expected primarily in permanently shadowed craters near the moon's poles, but more recent data indicated the presence of trace amounts over broad regions.
"It could be water, it could be methane, it could be hydrocarbons or organics," Colaprete said during a pre-impact briefing. "From a scientific standpoint, this is incredibly important. Whatever the moon has collected over the last 3.5 billion years in terms of water, organics, materials from comets, asteroids, the sun, could be trapped in these pockets on the moon. It's a time capsule, it's a window into the past of the entire inner solar system, of Earth."
Finding ice on the moon could be critical to future exploration or even colonization. With unlimited solar power, ice can be converted into water, oxygen, and hydrogen rocket fuel. Finding ice on the moon also would raise the possibility of similar deposits in similar environments across the solar system.
"Water in terms of exploration is very important," Colaprete said. "Even if we don't go back to the moon, it is a principle resource throughout the solar system. On Mars and beyond. The old Mars mantra was 'follow the water.' And really, that extends in my mind through the entire solar system and the entire universe. And so really, LRO and LCROSS are the first directed, focused steps in that direction on the moon."