NASA’s Kepler space telescope has once again entered safe mode, most likely due to an attitude error. The spacecraft, during a scheduled semi-weekly contact, was found oriented with the solar panels facing the sun, slowly spinning in the Sun’s direction. The spin made it difficult to communicate with the space telescope.
“This is something that we’ve been expecting for a while, unfortunately,” said NASA Science Chief and former astronaut, John Grunsfeld. “I wouldn’t call Kepler down and out just yet.”
NASA attempted to return to reaction wheel control as the spacecraft rotated into communication. However, although initially all three working reaction wheels responded and stopped the rotation, reaction wheel number four remained at full torque while the spinning rate was set to zero. NASA identified this behaviour as a structural failure within the reaction wheel. At that point, Kepler was then transitioned back to Thruster Controlled Safe Mode. Reaction wheels are attitude control devices that allow spacecraft to maintain a position in space. Kepler’s reaction wheels are used to maintain a very accurate position while pointing at stars, while looking for planets outside the solar system. The space observatory needs three functioning reaction wheels to operate. Four reaction wheels were originally mounted on the spacecraft, with the fourth acting as a redundancy in case of the failure of one wheel. However, wheel number two had already failed in July 2012 and now wheel number four appears to have failed as well, after experiencing problems in the past few months.
Kepler is about to enter into its Point Rest State which is a loosely pointed, thruster controlled state that minimizes fuels usage while providing a continuous X-band communication downlink. In that state, Kepler could potentially last a few more years according to NASA. The agency will also attempt in the next weeks to recover wheel functionality and investigate possible hybrid mode, using both wheels and thrusters for the attitude, but it is unlikely that the spacecraft will return to the previous high pointing accuracy.
Kepler entered an extended mission phase in November 2012, after outliving its originally planned 4 year mission. The good news is that even if data collection stops, the information that has been gained from the mission so far will keep scientists busy for years to come.
Below, lecture on the Kepler mission at Villanova University: