Losing Direction

18 May 2013

The Kepler spacecraft is in trouble. 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 above. 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 a year 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 this week a second reaction wheel malfunctioned, and that puts the mission at risk.

So what can be done? Assuming the second wheel can’t be revived (and that looks to be the case), then there are basically two options. The first is to start up the first reaction wheel and hope it functions well enough to be used. The second is to use a combination of thrusters and the two remaining gyroscopes. This might buy the mission a bit more time.

Failing those two solutions, Kepler’s planet hunting days are 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 be 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 if Kepler fails that does mean the goal of finding habitable Earth-like worlds is over for now. We would likely have to wait for the Transiting Exoplanet Survey Satellite (TESS), which is scheduled to launch in 2017.