Mechanical trouble imperils Kepler

Devin Powell

Just over four years after it was launched into orbit, NASA's Kepler space telescope has broken down. On 12 May, after tilting in an unexpected direction, it entered a protective safe mode and stopped collecting data. Efforts to get the spacecraft going again failed when a wheel critical for pointing the telescope refused to spin.

NASA isn't ready to give up on the mission, which launched in 2009 and was extended last year to 2016. Running on thrusters, Kepler has the fuel to stay in orbit for months or perhaps years as engineers try to fix the problem from 40 million miles away. But with two of its four reaction wheels now out of commission — the first stopped working last July — the spacecraft's search for planets around other stars is clearly in trouble.

"I wouldn't call Kepler down and out just yet," said John Grunsfeld, associate administrator of NASA's Science Mission Directorate in Washington, DC, during a teleconference today. Nonetheless, he said, "we do need three wheels to do the exoplanet science."

The agency is still trying to determine whether it can collect useful data of any sort with Kepler if project engineers cannot restore a third reaction wheel to working order.

Kepler looks for stars that dim periodically as some of their light is blocked by a passing planet. To date the telescope has found 132 confirmed planets, with 2,730 candidates still awaiting verification. Thanks to Kepler, astronomers now know that planets are common. Dozens of potential planets about the size of Earth have turned up, as well as many bigger planets orbiting in the 'habitable zone'  around their stars where water could exist as a liquid.

But the mission's primary goal — calculating exactly what fraction of planets are Earth-sized — remains unfulfilled. Neither has the telescope yet found a planet as small as ours orbiting in a habitable zone around a Sun-like star.

Such a planet could still turn up in the coming years as scientists continue to sift through Kepler's data, much of which hasn't yet been analysed.

"I'm very optimistic that the data we have will allow us to accomplish that," says William Borucki of NASA, the principal investigator for the Kepler mission. "I think the most interesting, exciting discoveries are coming in the next two years." /Nature