Venus Express has been in orbit since April 11, 2006, and all but one of its instruments have been returning lots of data. There's been one fly in the ointment, however, and that's the failure of its Planetary Fourier Spectrometer (PFS). The instrument works all right, except that a tiny mirror that allows the instrument to point to different targets appears to be frozen, and it's frozen in the launch configuration -- so throughout Venus Express' lifetime, PFS has been resolutely staring at its calibration target and ignoring Venus. The PFS and engineering teams have been trying all sorts of creative gambits for trying to unstick that mirror, but nothing has yet worked.
As time goes on and the mirror is still stuck, they're getting less and less gentle in their attempts to revive the instrument. Over the last couple of weeks, they've tried the spacecraft equivalent of pounding PFS with a fist. According to Don Merritt, who works in Venus Express' Science Operations Center, they decided to try moving the mirror at the same time as the spacecraft got a jolt either from its reaction wheels or its thrusters. They wouldn't use up precious fuel just to try this out, so they waited for the right moment, when the spacecraft was going to be doing one of these jolting maneuvers anyway. Like most three-axis-stabilized spacecraft, Venus Express maintains its pointing using reaction wheels, several (usually three) weighted wheels that spin inside the spacecraft. If the spacecraft speeds up the wheel in one direction, it will rotate in the opposite direction; then it applies the brake to the wheel to halt the rotation. Because friction wastes some of the energy necessary to spin and brake the spacecraft, reaction wheels tend to spin faster and faster over time, so once in a while a spacecraft has to do a "momentum dump," or despin its wheels while compensating with the thrusters. So the Venus Express team commanded PFS' mirror to move during one of these momentum dumps, in the hopes that it would kick the mirror free. Sadly, it appears not to have worked.