Launched in 2009, Kepler was intended as a “planet hunter” telescope. It finds planets by observing stars for long periods of time. To make long observations, Kepler needs to be able to point in the same direction very precisely, and it must be able to adjust its direction if it starts to drift. So how do you keep a telescope oriented?
On Earth, a telescope can be mounted to the ground, and any change in direction can be made by orienting the telescope relative to its mount on the ground. But for a space telescope there is nothing to mount it to. This means there are only two ways to adjust the orientation of a space telescope: thrusters and gyroscopes.
Thrusters are basically small rockets. They release a small amount of propellent, and the telescope moves a bit in the opposite direction. It’s Newton’s third law of motion in action. With multiple thrusters you can adjust the orientation of the telescope. But there are two disadvantages to this method. The first is that every time you make a thrust you lose a bit of fuel. The second is that it is difficult to make thrusts with the precision necessary for Kepler.
Gyroscopes use a different approach. A gyroscope is basically a spinning wheel. When the wheel is spinning, it resists changing direction (a property known as conservation of angular momentum). You can see this effect in the video. With three gyroscopes you can orient a telescope in any direction, and you can keep it in a specific direction very precisely.
The Kepler spacecraft has 4 gyroscopes (called reaction wheels), but about two years ago one of them started acting wonky and was shut down. This wasn’t a huge deal, since the telescope can get along just fine with 3 gyroscopes. But last year a second reaction wheel malfunctioned, and that put the mission at risk.
So what could be done? They basically tried two options. The first was to start up the first reaction wheel in the hope that it would function well enough to be used. The second was to use a combination of thrusters and the two remaining gyroscopes. But neither of these solutions were successful.
So it was feared that Kepler’s planet hunting days were over. There are other projects the telescope could be used on, but its orientation simply wouldn’t be precise enough to detect planets. This would have been really disappointing, but it wouldn’t mean a failure of the mission. Kepler completed its mission in 2012, and then entered an extended mission phase because it was still functioning well. It has gathered tons of data that has yet to be fully analyzed, so there is plenty to keep astronomers busy.
But it turned out the failure of the reaction wheels wasn’t the death of Kepler. Using some clever tricks, a new project known as K2 was devised. But that’s a story for another time.