An array of sensors stringing from the Earth to the moon will have four minutes to seek the tell-tale signature of lunar water when the $79 million LCROSS mission reaches its climactic conclusion early Friday.

The LCROSS shepherding satellite observes the impact of the Centaur. Credit: Northrop Grumman

A Centaur rocket stage left over from its June launch aboard an Atlas 5 rocket will play the role of an artificial meteor, striking the moon enough vicious force to dislodge more than 350 metric tons of soil from the lunar surface.

The striking velocity is expected to be 5,600 mph, about twice the speed of a rifle bullet, according to Dan Andrews, the LCROSS mission's project manager at the Ames Research Center in Moffett Field, Calif.

The impact at a site called Cabeus crater is scheduled for 1131:19 GMT (7:31:19 a.m. EDT).

The Centaur, a 41-foot-long thermos-shaped cylinder drained of rocket fuel, remained attached to the LCROSS shepherding satellite for more than 112 days. The spacecraft tugged the 5,000-pound stage three times in an unusual high-altitude polar orbit around Earth to line up for impact.

The two vehicles parted ways on schedule at 0150 GMT (9:50 p.m. EDT) as springs pushed the spacecraft apart at a rate of about 2.3 feet per second.

The shepherding satellite next fired its thrusters in a braking maneuver around 0230 GMT (10:30 p.m. EDT).

"That's where we slow down the shepherding spacecraft and make sure we have that four-minute separation before the two impacts," Andrews said.

Controllers overnight will upload updated command sequences for LCROSS to automatically perform during and after the first Centaur impact.

Those commands govern hundreds of spacecraft maneuvers and other activities to keep the probe's nine instruments pointed at the debris cloud, collect data and send the information back to Earth live.

"We are bent pipe, meaning we don't have any store-and-forward (capability). There's no extended mission for LCROSS, unfortunately," said Tony Colaprete, the LCROSS principal investigator.

LCROSS will meet the same fate as the Centaur just four minutes later, meaning the spacecraft's instruments will be sending all their data back to Earth as it is gathered.

The suite of sensors include a photometer to measure the brightness of the Centaur's impact flash.

Three water-sniffing spectrometers, the workhorses of the mission, will determine the chemical composition of the lunar dust excavated by the impact.

Five cameras will also be taking live slow-rate video as the LCROSS mothership approaches the moon. They will first image the Centaur as it strikes the surface, then observe the ejecta plume and the estimated 66-foot-wide crater left by the rocket's impact.

"We have to live within a very constrained bandwidth, so that limits how much data we can actually get down. It also means we have to use a pretty aggressive compression on the analog cameras in particular to get them in at the rate we want for science," Colaprete said.

The cameras will downlink video at a rate of one or two frames per second, officials said.

"Right at impact, when there's a flash, we turn off our visible camera and we go solely to our near-infrared camera, which is better for detecting the flash because the near-infrared portion of the flash lasts longer," Colaprete said.

The visible, thermal and near-infrared cameras will all be used to monitor the curtain of lunar dust thrown into space by the Centaur.

In the final minute of its mission, the LCROSS thermal cameras will turn toward the Centaur's impact crater for last-chance observations of the fresh mark left behind by the rocket stage.

LCROSS itself will hit the moon at 1135:38 GMT (7:35:38 a.m. EDT), based on the latest navigation data.

Although LCROSS will have the most spectacular view of the fiery lunar smash, a network of telescopes across North America will be pointing at the moon for complementary observations.

At least four major telescopes at the summit of Hawaii's Mauna Kea, plus observatories in California, Arizona and New Mexico, will focus their instruments at the moon's south pole.

"We have a whole cadre of professional astronomers and groups that are observing," said Jennifer Heldmann, the coordinator of the LCROSS observation campaign.

The Hubble Space Telescope's STIS instrument will also observe vapors kicked up by LCROSS to aid the search for water. Several more satellites in Earth orbit will also attempt to image the moon at the time of impact.

NASA's Lunar Reconnaissance Orbiter, launched on the same rocket as LCROSS, will fly over Cabeus shortly after the impacts to monitor changes from its perch 31 miles above the moon.

The target for the impact is Cabeus, a depression adjacent to the lunar south pole stretching 60 miles across and some two-and-a-half miles deep.

The objective is to upheave enough material for instruments to detect the water ice scientists believe resides inside the crater.

Researchers proposed the LCROSS mission after previous spacecraft observed elevated levels of hydrogen inside permanently shadowed craters at the moon's poles. Scientists think water is the leading candidate to explain the hydrogen concentrations.

"We're going someplace where we are motivated to go by neutron observations of this hydrogen concentration," said Colaprete said.

A fleet of other missions, including India's Chandrayaan lunar orbiter and a pair of NASA planetary probes, discovered signs of hydrogen compounds extending further away from the poles.

NASA announced that news last month as the most convincing evidence yet that water and another hydrogen compound, called hydroxyl, exist in dormant states in equatorial regions on the moon.

"Then you have something in these craters...that is 10, 20 or 30 times more concentrated and maybe 1,000 times more extensive in terms of distribution and depth than what they saw," Colaprete said.

Whatever is inside the crater, Colaprete said they do not expect to have any definitive science results immediately.

"It should be clear that we don't anticipate saying anything about the presence of absence of water immediately," Colaprete said.

It could be a few weeks before scientists are prepared to announce what was detected during the impact.