Kepler mission may be over

The telescope that has discovered thousands of exotic, quirky worlds — and a few tantalizingly Earthlike ones — orbiting distant stars is no longer capable of finding planets, at least temporarily and probably for good. Officials with NASA’s $600-million Kepler space telescope announced May 15 that an essential piece of hardware on the spacecraft has failed.

Since May 2009, Kepler has been staring at 170,000 stars and looking for tiny shadows cast by planets crossing in front of them. To enable Kepler to make such precise measurements, engineers installed four pointing devices, called reaction wheels, that turn the telescope and keep it dialed in on its stellar targets. One of the wheels stopped working last July, but the telescope requires only three.

On May 14 Kepler scientists learned that the spacecraft had entered safe mode, which occurs when something is awry. When they tried to restore the telescope to normal operations, another reaction wheel failed to activate. That wheel had been behaving erratically for months, so its failure was not a total surprise. “This is something we’ve been anticipating for a while,” said John Grunsfeld, NASA’s associate administrator of the science mission directorate.

For now, Kepler will enter a fuel-saving mode that keeps the spacecraft in constant communication with Earth. Charles Sobeck, Kepler’s deputy project manager, said the mission team will take the next few weeks to put together a plan of action for trying to restore one of the failed reaction wheels. “We’re not ready to call the mission over,” Grunsfeld said. “I wouldn’t call Kepler down and out just yet.”

Unlike NASA’s Hubble Space Telescope, which was repaired in space in 1993, Kepler is too far away from Earth for astronauts to visit and fix it. If engineers cannot restore the wheels from the ground, then Kepler officials will determine what kind of science can be done with the telescope’s limited capabilities.

The announcement comes not even a month after Kepler astronomers announced the discovery of the most Earthlike worlds found to date: a pair of rocky planets orbiting the star Kepler-62 that have a good chance for harboring liquid water (SN 5/18/13, p. 5).

While the telescope’s search for planets may be over, researchers’ analysis of the data it collected is not. Kepler data has yielded more than 2,700 likely planets and 132 confirmed ones, with more yet to come. In all, Kepler collected almost exactly four years’ worth of data, and the last year or so of that has barely been analyzed. That means there is still a chance that astronomers will find the signal of an Earth-sized planet orbiting at a life-friendly distance from a sunlike star. “I’m optimistic the data we have will allow us to accomplish that,” said William Borucki, Kepler’s principal investigator.

Still, the news comes as a major disappointment to astronomers. The primary goal of the Kepler mission was to determine how common planets similar to Earth are in the galaxy. Even if Kepler’s existing data yields a handful of Earth analogs, that big question may not be answerable.

Further complicating matters is that the brightness of stars naturally fluctuates over time, making the process of picking out planets extra difficult. Last year NASA extended the Kepler mission to 2016 so that researchers could overcome that natural variability and pick out more planets.

Still, Kepler’s original mission was scheduled for four years, and that’s exactly how long it did its job. Early last year, when Kepler was still at full health, Borucki commented on the importance of the mission: “There is no mission that’s comparable,” he said. “Kepler is the greatest mission NASA has ever flown.”