The Kepler Space Telescope's Planet-Hunting Days Are Over, But It's Been A Great Run

NASA has finally called it. Months after the Kepler Space Telescope lost a second reaction wheel—and after months of trying to find a way to bring it back online—the space agency has declared Kepler’s planet hunting days to be over. Now, the mission’s science team is pursuing plan b and considering what else the spacecraft can do in its current condition.

When Kepler launched in 2009, it was a marvel among planet hunters. Its 4.5 foot mirror  funnels light from distant stars into a 95-megapixel camera that can see changes in that light’s brightness as subtle as 10 parts per million. It’s these little dips in light that signal a planet might be orbiting a distant star.

Kepler can see that distant light in such impressive detail because of its ability to stay steady in space, and can do this thanks to reaction wheels. Reaction wheels are motors that spin between 1,000 and 4,000 times per minute, and when they spin in one direction the spacecraft spins the opposite way. By carefully adjusting the precise orientation and speed of these motors on the telescope’s three axes of motion, it can focus on a distant well enough to see a tiny planet passing in front of a star. Kepler’s precision is akin to someone pointing to a soccer ball in New York City’s Central Park while standing in San Francisco.

But it turns out the reaction wheels Kepler used were problematic from the start. The mission science team knew that similar wheels built by the same manufacturer had experienced crippling problems: on NASA’s Far Ultraviolet Spectroscopic Explorer in 2001, on Japan’s Hayabusa mission in 2004 and 2005, on NASA’s Thermosphere, Ionosphere, Mesosphere Energetics and Dynamics (TIMED) satellite in 2007, and the agency’s Dawn mission in 2010 and 2012.

By the time reaction wheel problems started piling up, it was too late to make major changes to Kepler’s system. Instead, minor fixes were made, like replacing ball bearings. But Kepler was going to launch with its reaction wheels. The silver lining, mission planners saw, was that the primary mission was only three and a half years. The wheels were expected to last that long at least.

Kepler launched with four reaction wheels, three primary and one backup. It lost the first wheel in July of 2012, but the spare was brought online and the mission continued. The telescope completed its primary mission in November and started its four year extended mission. Then, a second wheel failed this past May and dealt the death blow. Kepler can’t focus anymore, meaning its days of finding those dips in distant starlight are over.

But Kepler’s done a lot in its time. When the mission launched, we knew about just a handful of exoplanets. The first pair was found in 1992 by astronomers Aleksander Wolszczan and Dale Frail were. Studying the pulsar (the pulsating remains of an exploded star) PSR1257 +12 with the Arecibo radiotelescope in Puerto Rico, they found an unpredicted pattern of pulses. It looked like two planets were orbiting the stellar remains, and it turned out that was the case. By 2009, astronomers had found and confirmed the existence of 374 exoplanets.

Kepler increased that number by an order of magnitude. So far, the mission has added 132 confirmed exoplanets and nearly 3,000 candidate bodies. And there are more to find in the terabytes of data the telescope has sent back to Earth that astronomers have yet to comb through. And some mission scientists are confident that this data, which was gathered as the team learned more about the mission, will hold the most interesting planetary discoveries yet.

Whatever astronomers do find in the still unexplored data, it won’t take away from Kepler’s main discovery: Earth-like planets, which we once thought were exceedingly rare bodies, are actually fairly common. Kepler has found some of the most Earth-like exoplanets we know about, and we've found several super Earth orbiting in their stars habitable zones, taking the idea of extraterrestrial life from science fiction to possible future science fact. It's also found extreme planets that orbit so close to their stars they risk boiling away.

As for the telescope itself, scientists are working out what they can do with it in its current condition.First, they will identify possible science opportunities for a two-wheel Kepler mission. Then, and depending on what these studies reveal, NASA will consider the scientific priorities of a two-wheel Kepler mission taking into account NASA’s Transiting Exoplanet Survey Satellite launching and the James Webb Space Telescope, which are set to launch on their own exoplanet hunting missions in 2017 and 2018 respectively.

In the mean time, preparations remain underway for the second Kepler Science Conference, which will take place from Nov. 4 to 8 at NASA's Ames Research Center. This will be an opportunity to share not only the investigations of the Kepler project team, but also those of the wider science community using publicly accessible data from Kepler.