Artist's conception of NASA's Kepler planet hunter. The cross-shaped images indicate the spacecraft's observing field.

Artist's conception of NASA's Kepler planet hunter. The cross-shaped images indicate the spacecraft's observing field.


NASA's planet hunter safe again, for now

NASA has regained control of its exoplanet discovery satellite Kepler following a fraught few days during which the spacecraft had put itself into a protective “emergency mode.” What went wrong is not yet clear, but on Sunday morning controllers had the spacecraft in a stable state with its communications antenna pointing toward Earth. Data from the spacecraft are being downloaded and analyzed to find out the cause of the problem.

Kepler had finished its last observing campaign on 23 March and was in a “rest” state waiting for the next one, which was due to begin last week. The emergency mode began some 14 hours before the observations were due to begin. This is the first time Kepler has had to resort to emergency mode in its 7 years in space. Investigations into the event will continue throughout this week, NASA says. 

NASA launched Kepler in 2009 to search out roughly Earth-sized planets around sunlike stars. It does this by staring at a few select parts of the sky and monitoring the brightness of 150,000 target stars over long periods. If any of those stars dimmed slightly for a while and then brightened again that could be a sign that an orbiting planet has passed in front of it. This “transit method” proved hugely successful: In more than 4 years of operation it detected 4696 candidate exoplanets, of which 1041 have been confirmed by other detection methods or statistical techniques.

Already beyond its designed mission, NASA had planned to operate Kepler for another few years, but that was put in doubt in 2012 when one of its four reaction wheels—devices necessary for pointing the spacecraft accurately—failed. Kepler could keep working with three reaction wheels but when a second failed in May 2013 the mission seemed all but lost. However, mission engineers worked out a way to steer the spacecraft—not as well as before, but good enough—using a combination of its thrusters, the surviving reaction wheels, and the pressure of sunlight on its solar panels. Dubbed K2, the new mission was opened to suggestions from astronomers and since June 2014 has been counting certain exoplanet-star combinations, studying stellar structure and activity, looking for the progenitors of supernovae, and discovering binary stars, among other things.

Mission controllers discovered that Kepler was in trouble again last week. During a recent check in with the spacecraft on 7 April they found that it had put itself into emergency mode, its lowest-activity operational state. It had gone into emergency mode 36 hours earlier, just before Kepler was about to begin a campaign to detect exoplanets by gravitational microlensing. Prior to that, the spacecraft had been in good health. To complicate matters, Kepler is 120 million kilometers from Earth, so messages take 13 minutes to travel to the spacecraft and back. “It was the quick response and determination of the engineers throughout the weekend that led to the recovery,” Kepler mission manager Charlie Sobeck said in a statement.

Earth-based observatories that are collaborating with Kepler in the microlensing campaign will continue observing this week while the spacecraft has its health check. The required observing window onto the center of the Milky Way remains open until 1 July.

It is “a huge relief that the K2 mission will continue,” says astronomer Sara Seager of the Massachusetts Institute of Technology in Cambridge. “Already K2 has yielded 2 dozen exoplanets with more than 250 awaiting confirmation.”