The Kepler spacecraft is once again in good health, following a glitch that sent the planet-hunting observatory in emergency mode.

Mission engineers at NASA managed to repair the vehicle, using a step-by-step process, while it was 75 million miles from Earth.

Kepler is currently in the second stage of its mission, exploring nearby stars for signs of alien worlds. When mission planners attempted to contact the craft on April 7, they found the vehicle had placed itself into a shutdown mode, usually reserved for emergencies. In this state, the solar panels on the vehicle turn to face the sun, as the observatory slowly rotates in space.

Following a series of commands, engineers were able to place Kepler into safe mode on April 15.

Controllers switched Kepler to a point rest state, during which time the vehicle uses little fuel, and priority is given to communication with controllers. On April 12, NASA officials were able to download enough information about the spacecraft to diagnose the problem.

After that, engineers were able to devise a series of commands designed to switch on non-critical segments of the spacecraft.

The rescue plan was first tested on a Kepler simulator. When the plan worked there, the instructions were sent to the Kepler spacecraft.

"The recovery started slowly and carefully, as we initially merely tried to understand the situation and recover the systems least likely to have been the cause," said Charlie Sobeck of the Ames Research Center, managed by NASA. "Over the last day and a half, we've begun to turn the corner, by powering on more suspect components. With just one more to go, I expect that we will soon be on the home stretch and picking up speed towards returning to normal science operations."

Engineers are still diagnosing the data returned by Kepler, in order to deduce the events that could have led to the observatory entering emergency mode. However, fault indicators within the observatory are designed to be sensitive, and it is possible systems were overwhelmed by several alarms going off at one time.

"The thing to keep in mind is K2 had been remarkably trouble free and I think we'll be trouble free going forward. I don't believe this signals the end of the mission at all," Sobeck said.

Kepler was launched in March 2009, in order to hunt for planets around other stars. Since that time, it has found more than 1,000 confirmed exoplanets, as well as 3,500 suspected planetary candidates.

This is not the first time mission engineers have been able to bring Kepler back from a crisis. In 2013, the second of four reaction wheels gave out, eliminating navigational ability. Engineers found a way to use pressure from sunlight to guide the observatory. This revitalized mission became known as K2, and now includes observations of supernovae and other celestial events.

While Kepler is in safe mode, the vehicle allows one more error to take place. However, if another alarm had signaled while Kepler were in emergency mode, NASA would have lost the $600 million observatory.

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