A glitch caused NASA’s water-hunting LCROSS mission to lose more than half its fuel, but the spacecraft is still on track to collide with the moon (Illustration: NASA)

NASA’s moon-colliding probe LCROSS lost more than half its propellant late last week after a glitch caused it to repeatedly fire its thrusters to try to orient itself. But the spacecraft is still on track to complete its mission to slam into the moon’s south pole in October.

The Lunar CRater Observation and Sensing Satellite (LCROSS) took off on 18 June and has been orbiting the Earth at about the moon’s distance in preparation for a lunar collision on 9 October. NASA hopes the impact will excavate material from one of the moon’s permanently shadowed craters, which could be rich in water that could supply future lunar outposts.

Mission managers made contact with the spacecraft on Saturday to conduct a ‘cold-side bake’. This manoeuvre flips the spacecraft to allow the sun to heat up and vaporise any residual water on the 2400-kilogram upper stage of LCROSS’s launch rocket, which will be sent crashing into the moon four minutes before the LCROSS spacecraft itself.

When the team made contact with the spacecraft, they discovered it had lost 140 kg of hydrazine propellant in the 20 previous hours, when the spacecraft was not in contact with Earth. The spacecraft’s tank originally held 306 kg of fuel; 50 kg remain.


“It was pretty apparent that the thrusters had been firing for quite a bit,” says LCROSS project manager Daniel Andrews of NASA’s Ames Research Center in Moffett Field, California.

Noisy data

The fuel was lost after a fault in one of the spacecraft’s data lines caused a change in the probe’s control system. It switched from the craft’s Inertial Reference Unit (IRU), which uses spinning gyroscopes used to gauge the spacecraft’s orientation, to a less precise star tracker, which orients the probe by observing the position of stars.

The spacecraft made an effort keep up with these noisier star tracker data, causing it to rapidly burn through fuel to keep up with the heavily fluctuating numbers.

The switch was triggered by a change in an indicator used to monitor the health of the IRU. The fault, which lasted just one second, may have been a passing glitch. But mission operators have not been able to eliminate the possibility that there is a problem with the IRU.”We’re not 100 per cent convinced that it isn’t a sign the IRU is about to fail,” Andrews says.

If the unit does fail, LCROSS can fall back on the star tracker for orientation information. The team is working on re-tuning the spacecraft so it does not respond as attentively to information from the unit.

The spacecraft has also been reprogrammed to switch from the IRU to the star tracker only if problematic signals last for at least five seconds, a change that should cut down on LCROSS’s sensitivity to fleeting glitches.

Extra fuel

Despite the heavy fuel loss, LCROSS is still on track to conduct its water ice search. “It turns out we didn’t need anywhere near that 300 [kilograms] to complete the mission. We just filled it up in case we had a bad day,” Andrews told New Scientist.

LCROSS still has more fuel than the mission requires – about 10 to 20 kg extra, Andrews says. The exact cushion depends on how many other activities, such as turns to look at Earth, LCROSS will conduct before the collision.

Because the spacecraft is already on a ballistic trajectory to the moon, it should still be able to intersect the moon without using up any additional propellant. The remaining fuel is needed to trim the spacecraft’s trajectory so that it hits the surface at the right place.

The team is expected to announce LCROSS’s target crater about a month before impact.