An artist's depiction of the LCROSS moon-smashing mission as the Shepherding Spacecraft (left) pulls free of the Centaur upper stage impactor.
Officials are hurriedly looking for ways to save fuel on NASA's $79 million lunar impactor mission after a crisis Saturday caused the spacecraft to burn more than half of its remaining propellant.
The Lunar Crater Observation and Sensing Satellite used about 140 kilograms, or 309 pounds, of maneuvering fuel to maintain the probe's orientation in space Saturday, according to Dan Andrews, the mission's project manager at Ames Research Center.
LCROSS is tugging a 41-foot-long Centaur rocket stage on a circuitous route through space. Scientists are preparing for a fleeting series of observations as the spent booster is released for a suicidal plunge into the moon on Oct. 9.
The goal is to hit a shadowed crater near the lunar south pole to see if water ice deposits reside there.
The 6-foot-tall shepherding spacecraft's attitude control system was specifically designed to handle the unusual job of positioning the 47-foot-long stack as it flies toward the moon.
"It was a tough day, as you can imagine," Andrews said. "But what it's done is given us a razor's focus on how to manage the remainder of the propellant."
LCROSS is now perilously close to its built-in propellant margins, and Andrews said the team will probably have to cancel some activities that are not crucial to the mission.
"Our estimates now are if we pretty much baseline the mission, meaning just accomplish the things that we have to (do) to get the job done with full mission success, we're still in the black on propellant, but not by a lot," Andrews told Spaceflight Now late Tuesday.
LCROSS now has between 20 pounds and 40 pounds of extra propellant that could be used in unplanned activities, a relatively thin margin for satellite operations.
"We can finish this mission, but it makes our sensitivity to something happening quite high," Andrews said.
The anomaly occurred out of view of ground stations, so LCROSS engineers were not aware of the problem until the antenna acquired the spacecraft later Saturday morning.
"When we acquired the signal, we discovered that the thrusters had been very busy and that the propellant in the tank was reduced quite a bit," Andrews said.
After regaining contact with LCROSS, controllers traced the fault to the Inertial Reference Unit, a sensor that measures the craft's attitude. Software on the satellite detected the issue and automatically switched attitude determination to the star tracker system as designed.
"Those star tracker rates are inherently noisier, but they were noisier than we had anticipated," Andrews said.
Thrusters struggled to keep the six-sided spacecraft pointed in the right direction, but data from the star tracker caused the control system to use a substantial amount of fuel.
"Our system was working very hard to keep up with them, and in doing so was expending a lot of propellant. And because we were out of view, we couldn't do anything about it," Andrews said.
Engineers recovered the IRU and reduced propellant usage back to normal levels, according to Andrews.
"We stopped the bleeding pretty quickly," Andrews said.
When the anomaly occurred, LCROSS was turning the Centaur around to warm up the rocket to evacuate water ice carried into space. Officials want to get rid of the trapped water to ensure the accuracy of navigation and science results.
NASA is now working with the suppliers of the reference unit and star tracker assembly to find the root cause of the problem, the agency said in a statement.
In the meantime, controllers have changed the satellite's software logic to decrease its sensitivity to similar events.
"For example, if we find that the spacecraft in inclined to do a bunch of thruster firing activity, or if we notice one of the key avionics elements failed, we would rather go into a free drift than expend a bunch of propellant and seal our fate," Andrews said.
NASA recognizes LCROSS as a risk-tolerant mission, meaning the project has a "moderate chance" of not achieving success.
"We've made our spacecraft very simple," Andrews said. "What that did is it allowed us to fit into an incredibly tight cost and schedule box."
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