Space is a tough environment to work in. Sometimes, successes like the Mars Exploration Rovers or the Cassini orbiter make it seem otherwise, but every space mission is really a series of minor miracles. Unfortunately, it looks as if the Kepler spacecraft, one of the most exciting missions in recent memory, may have run out of its good luck. Yesterday, mission controllers announced that Kepler had lost a second of its four reaction wheels to mechanical failure. Without at least three wheels, science operations cannot continue.
(Full disclosure: the Kepler spacecraft is operated by my employer, the Laboratory for Atmospheric and Space Physics. It's a sad day around here...)
Reaction wheels are kind of like gyroscopes. They spin quickly, providing a source of angular momentum to the spacecraft. Varying the speed of a wheel causes the telescope to rotate in the direction that wheel is spinning. In this way, very small changes can be made to stabilize the craft, much finer than via rocket control. Reaction wheels also operate on electricity, which conserves precious maneuvering fuel. Since we live in a three-dimensional world, we need three reaction wheels in order to fully manipulate the spacecraft.
Because they are some of the only moving components on many spacecraft, reactions wheels are a common point of failure. For this reason, Kepler carried four, instead of the required three. One failed about nine months ago, leaving Kepler with no remaining backups. Now, with only two wheels left, mission controllers must also use maneuvering thrusters to orient the telescope. Unfortunately, this usually isn't accurate enough for scientific observations.
All spacecraft are designed to operate for a so-called primary mission. Kepler's primary mission lasted three and half years, and it completed that last year. In one sense, any life the spacecraft has after its primary mission is 'borrowed time.' Sometimes, like with Cassini, you get lucky and can extend the mission two (or more!) times. Sometimes not. The Kepler mission had been extended through 2016, but that was conditional on having an operating spacecraft.
Even if Kepler has made its final observation, its impact on astronomy has been enormous. In just four years, it has identified more than 2700 possible planets. I think we'll always look bak at the history of extra-solar planets as being pre-Kepler and post-Kepler. The vast quantity of data collected by the telescope has also benefitted other areas of astronomy. Asteroseismology, for example, has made huge strides in the last few years. The study of star-quakes, asteroseismology has benefitted enormously from the precise, repetitive observations made by Kepler.
Just like every other field in science, the study of exoplanets will go on after Kepler. Perhaps most exciting is Gaia, slated to launch later this year. Instead of monitoring the brightness of many stars, like Kepler did, Gaia will monitor their positions. Small wiggles in the position of a star could suggest an orbiting planet, just as a small dip in brightness has already done.
We don't need Gaia, however, to tell us what Kepler already has: that we have far more galactic neighbors than we ever thought possible.