Solar pressure may help Kepler return to planet-hunting duties
By David Szondy
December 1, 2013
Last August, it looked as if the NASA's Kepler space telescope was as good as scrap due to the failure of its attitude control system. Now the space agency proposes what it calls the K2 mission concept, which will give the unmanned probe "second light" by using the Sun to regain attitude control and allow Kepler to resume its search for extrasolar planets.
Launched in 2009, Kepler's primary mission was to seek out extrasolar planets by measuring dips in starlight caused by orbiting planets passing in front of their parent stars. This proved very successful with over 3,000 candidates identified so far, but it’s a technique that requires extreme precision and for the spacecraft to remain absolutely stable during observations.
Last May, Kepler’s planet-hunting days appeared to come to an end as the second of its four attitude-controlling reaction wheels failed. Designed for a degree of redundancy, Kepler could have continued on three wheels, but the loss of two made it impossible to keep the craft stable. Despite several efforts to regain attitude control, NASA abandoned repairs in August.
Ironically, it was the primary cause of this instability that may bring Kepler back into service. It’s the pressure of sunlight that pushes Kepler out of proper alignment and for which the reaction wheels were supposed to compensate. Engineers from NASA and Ball Aerospace have been studying the problem and have concluded that it should be possible to use the Sun as a "third wheel."
If Kepler were aligned so that its orientation is nearly parallel to its orbit around the Sun, instead of the sunlight pushing Kepler out of alignment, the force of the sunlight would be evenly distributed across the spacecraft's surface and balance against the force of the remaining two reaction wheels and stabilize it. Kepler wouldn't have many choices of which way to point, but where it was pointing could be planet-hunting grounds.
NASA is currently testing the feasibility of the K2 mission concept. Last October, Kepler was aimed at the constellation of Sagittarius and caught a full-frame view for a 30-minute exposure. The resulting image quality came within five percent of primary mission quality. The space agency is now conducting tests to determine if the technique can allow Kepler to maintain the required pointing control for days or weeks.
Whether or not K2 goes ahead depends on the results of NASA's 2014 Senior Review and the acceptance of a budget for the mission.