Revealed: How NASA saved the Kepler space telescope from suicide

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Suddenly, a signal

The first lot of data picked up by the Deep Space Network from the probe suggested the thrusters, the main communications hardware, and the telescope’s two remaining reaction wheels were dead. The telemetry was sent by Kepler's emergency radio gear.

That sounds bad but, in fact, it was good news. It was highly unlikely that all of those systems had been damaged or shut down – only a catastrophic accident could have caused that, and that would have knocked out the communications systems completely. The more likely situation was that the data coming from the probe's individual subsystems was faulty.

The team established that emergency mode had kicked in about 30 hours before that fateful early morning call on April 8, before the telescope was due to reorientate itself. That told them the uploaded maneuvering commands hadn’t borked the system.

The next step was to get the telescope into safe mode. This brings the primary and secondary main computers back online but keeps the instruments shut down to minimize power use. Then they stopped the spin and got a constant feed of data to and from the spacecraft, which speeded things up dramatically.

After rebooting the main computers the readings from the thrusters, wheels and instruments showed all systems were normal. This confirmed the theory of a monitoring software fault, and it appears the telescope fooled itself into emergency mode.

"We don’t yet know what spawned the problem, and we may never know, but the first effects that we’ve found were a sudden series of alarms that caused the onboard fault protection to react," Sobeck said.

"The alarms themselves seem to be erroneous. As a result, the spacecraft’s response didn’t address the real situation, only the situation that was reported. In such conditions the resulting actions can, and this case were, detrimental rather than helpful."

The incident has now been classed as a "transitory event" – meaning it’s an unknown problem that sorted itself out. It's the opposite to a "hard failure," meaning some component had blown out and can’t be recovered. But while Kepler is back online, its future is limited.

Squeezing out the last few years

Sobeck said initial fuel readings showed that the telescope’s emergency maneuvers had expended a lot of propellant. "We lost more fuel than I had hoped, but less than I had feared," he said.

Fuel pressure in the tanks was uneven, he said, so it would take months of measurements before the final fuel load can be accurately gauged. The telescope is unlikely to finish its K2 star scan before the last of its fuel is exhausted and it becomes impossible to use the instrument.

Kepler has had a damned good run since it was launched into space in 2009. It was designed to last three and a half years but held up reasonably well and has been thrifty with its thruster fuel. In 2012 one of the four reactions wheels used to keep it stable failed, and a second failed less than a year later. Even then NASA engineers found a way to keep it online.

After months of calculations, the boffins worked out that the two remaining reaction wheels could be balanced against the force of charged particles streaming from the Sun and hitting the spacecraft’s solar panels. It’s that kind of hackery that made the K2 mission possible.

The key to success in situations like this is to keep a clear head and be methodical, Sobeck said. Gather a team who knows what needs to be done and the order in which to do it.

"I was impressed with the commitment, which everyone on the team demonstrated, and the cool, thoughtful approach that was taken," he said. "I was impressed with everyone’s ability to help when they could, and to stay out of the way when they couldn’t."

Kepler will continue to run for as long as it can, scouting for the images of planets passing in front of stars. It's sobering to remember that until the 1980s we'd never even seen a planet outside of the Solar System. But Kepler has discovered hundreds, if not thousands of planets, and maybe someday we'll find one that's just like home. ®


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