Solar pressure may help Kepler return to planet-hunting duties

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.

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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.

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That looks possible. If Kepler was in low Earth orbit then this wouldn’t be possible as the main forces would be due to air drag from the upper atmosphere. But Kepler is the far side of the Moon where the force due to sunlight (reflection plus absorption and re-radiation as heat) is the main disturbing force. This is good news, other spacecraft have successfully operated on a reduced number of gyroscopes, and having a different patch of sky to look at will lead to many more candidate planets.All this presupposes that the next gyroscope will hold on for long enough for a sufficient number of observations. Also, the original star field in Sagittarius was chosen because pointing towards the galaxy centre gave the greatest number of stars to watch and the re-orientation would reduce the total number of star candidates. This may not be a bad thing, because one of the problems with Kepler in Sagittarius was a very large number of background stars that created enough stray light to make binary stars look like planets, and with light leakage from one star to another I spotted cases where the same “planet” appeared in the light curves of three or more separate stars.