Since I promised everyone a more regular input to this blog, I guess I should try to live up to that promise. I’m afraid I took some time off over the holidays after such a busy year.
The Kepler spacecraft uses four distinct attitudes for its observations. We’ll monitor the same part of the sky all the time, but as the spacecraft orbits the sun, the geometry of it all requires that we periodically roll the vehicle a quarter of a revolution (90 degrees) to keep sunlight falling on our solar panels, and the radiator that keeps the detectors cool pointed to deep space. The focal plane has been built with four-fold symmetry and the mission has been designed such that we roll the spacecraft exactly 90 degrees, about every 91 days. That’s four times an “orbital year”, and we conveniently term them our Spring, Summer, Fall and Winter attitudes. The spacecraft orbits the Sun in 372 earth days. We launched into the Spring attitude, and since then we’ve been rolling the vehicle per plan. Just before the holidays, we rolled to the Winter attitude, so now the vehicle has “seen it all”! We’ve now successfully operated in all four of the attitudes that we will use of the entire duration of the mission over and over again.
Each roll has been a bit of an adventure, as we are rolling to an attitude we’ve never been at before. While taking science data, we guide the spacecraft with fine guidance sensors located in the four corners of the focal plane, so we use the same guide stars, season after season. The star trackers mounted on the outside of the spacecraft that provide our coarse pointing during non-science activities (like downloading data to the ground) are pointed more or less off to the side and see different stars in different seasons. This means we do have to be a bit careful with each roll. Last month, one of the new star tracker guide stars proved to be tracking poorly and we had to switch stars to get stable enough to transfer over to the fine guidance sensors.
And although we’ve already been at the Spring attitude, we have changed the way we track stars with the fine guidance sensors and we’ll have to be a bit careful to get it right. But at least the star trackers will recognize their view. A year after launch, come March, the trackers will once more see the same stars they saw when we first started taking science data.
Charlie Sobeck, Kepler Chief Engineer