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LCROSS Loses Much of Fuel Supply

By Doug Messier
Parabolic Arc
August 25, 2009
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NASA’s LCROSS lunar probe used up most of its fuel supply over the weekend when it was hit by a control problem with its Inertial Reference Unit, causing excess thruster firing. Initial reports were that about 70 percent of the fuel was used up.

LCROSS is composed of a Centaur upper stage and a small satellite. The upper stage will crash into the south pole of the moon while the satellite flies through the resulting plume to search for water and other elements. Having sufficient fuel is crucial in order to precisely guide the Centaur into a crater likely to contain water.

NASA officials believe this is still an achievable goal if nothing else goes wrong. Below are some up-to-date Tweets from NASA Ames Director Pete Worden from Tuesday afternoon PDT as well as the official NASA press release sent out on Monday evening.

@worden: The remaining fuel allows us to still hit any of the candidate targets if we don’t have another problem

@worden: Scary news — been working the LCROSS anomaly that caused us to use up almost all reserve fuel. Fabulous team though. We WILL succeed!

Spacecraft Anomaly
Mon, 24 Aug 2009 08:58:30 PM PDT

Upon starting an early morning communications pass on Aug. 22, 2009, the Lunar Crater Observation and Sensing Satellite (LCROSS) mission operations team discovered the spacecraft had experienced an anomaly.

According to spacecraft data, the LCROSS Inertial Reference Unit (IRU) experienced a fault. The IRU is a sensor used by the spacecraft’s attitude control system (ACS) to measure the orientation of the spacecraft. The anomaly caused the spacecraft ACS to switch to the Star Tracker Assembly for spacecraft rate information and caused the spacecraft’s thruster to fire excessively, consuming a substantial amount of fuel. Initial estimates indicate that the spacecraft still contains sufficient fuel to complete the full mission.

LCROSS mission operations declared a ‘spacecraft emergency’ and were allocated additional communications time on the Deep Space Network. The team conducted procedures to mitigate the problem and were able to restart the IRU and reduce fuel consumption to a nominal level. Automatic operations procedures also were implemented to minimize the possibility of another IRU anomaly from occurring while the spacecraft is out of contact with the ground. Since the re-start, IRU has not experienced any additional problems.

The team continues to actively assess and mitigate the situation and is in contact with the manufacturers of the IRU and star tracker to investigate the root cause of the problems. Mission managers remain optimistic the LCROSS mission can reach its successful conclusion with projected impact at the lunar south pole currently set for 4:30 a.m. PDT on Oct. 9, 2009.

LCROSS is a low-cost, highly risk-tolerant, fast-tracked mission of opportunity that was co-manifest with the Lunar Reconnaissance Orbiter. Both spacecraft launched from Cape Canaveral, Fla., on June 18, 2009. The main LCROSS mission objective is to confirm the presence of water ice in a permanently shadowed region near a lunar pole.