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Kepler illustration
Unless one of its two malfunctioning reaction wheels can be restored, Kepler is unlikely to make more observations of transiting exoplanets, although discoveries based on existing data may continue for years. (credit: NASA)

Kepler’s uncertain future

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For those in the exoplanetary science community who have closely followed the health of NASA’s Kepler spacecraft in recent months, the announcement around midday on Wednesday couldn’t have seemed like good news. “NASA will host a news teleconference at 4 p.m. EDT, today, May 15, to discuss the status of the agency's Kepler Space Telescope,” it stated. Telecons to discuss spacecraft status, called on just a few hours notice, are rarely bearers of good news. And there were already reports earlier in the day that Kepler had gone into a protective safe mode.

For months, project officials and the scientists who use Kepler had been closely watching wheel #4, whose friction levels started to rise early in the year.

The teleconference, featuring officials from NASA Headquarters in Washington and the Kepler project at NASA’s Ames Research Center, confirmed those fears. Kepler had indeed gone into a thruster-controlled safe mode during the most recent communications pass with Earth the previous day. “Our normal response to that condition,” said Charles Sobeck, deputy project manager for Kepler, “is to command it back to wheels.” That’s a reference to the reaction wheels normally used by Kepler to keep the spacecraft accurately pointed.

However, one of Kepler’s wheels, designated reaction wheel #4, failed to spin up even after commanding it to spin at full speed. “This is indicative of an internal failure within the wheel,” he said.

Kepler launched with four reaction wheels, and needs three for normal operations. One of the wheels, #2, failed last July. With wheel #4 also not working, Kepler’s exoplanetary observations, which have discovered more than 2,700 planet candidates and 132 confirmed exoplanets, cannot continue.

The news, while unfortunate, was also not that surprising. For months, project officials and the scientists who use Kepler had been closely watching wheel #4, whose friction levels started to rise early in the year. In January, engineers put the spacecraft into a “wheel rest” mode for ten days, interrupting science observations in the hope that the downtime would allow the wheel’s lubricant to redistribute and reduce friction levels. However, elevated friction levels remained when Kepler resumed science observations in late January.

Project officials held out hope that wheel #4 would not meet the same fate as wheel #2, noting that the changes in friction in wheel #4 were different from those in wheel #2 in the months leading up to its failure last year. In an interview last month, Sobeck said that they were taking additional steps to reduce the wear and tear on the wheel, including running the wheel at higher speeds and warmer temperature to increase the effectiveness of the lubricant (see “Revisiting exoplanets and dark matter”, The Space Review, April 8, 2013).

Those efforts, though, didn’t seem to change the behavior of the wheel. “Kepler’s reaction wheel #4 continues to exhibit signs of elevated friction levels and occasional torque spikes that appear to indicate a deterioration of the wheel bearing,” wrote Kepler mission manager Roger Hunter in an April 29 update. “While the wheel may still continue to operate for some time yet, the engineering team has now turned its attention to the development of contingency actions should the wheel fail sooner, rather than later.”

“We need to know more about the performance of the spacecraft before we can assess what kind of science we’ll be able to do,” Sobeck said.

With news that two of Kepler’s reaction wheels had failed, some observers were ready to write off the mission, but project officials weren’t ready yet to pen the spacecraft’s obituary. “We have given some thoughts of the kinds of things we could do” to try and recover one of the failed reaction wheels, Sobeck said. Wheel #2 was, unlike wheel #4, still moving somewhat when it failed last year; it was turned off then. Sobeck added it might be possible to get wheel #4 moving in the reverse direction. “We could try jiggling it, we could try commanding it back and forth in both directions, we could try forcing it through whatever the resistance is that’s holding it up.”

The process to develop a plan to try and recover one of those wheels will take weeks, Sobeck added. In the meantime, Kepler will remain in what he calls a “point rest state” to minimize the use of thrusters. In this mode, the spacecraft has a small “nodding” motion, pushed in one direction by thrusters and the opposite by solar pressure. The spacecraft can stay in such a state for years before running out of propellant, versus months for the more traditional thruster-controlled safe mode.

In the event neither of the spacecraft’s two failed wheels can be restored, NASA is looking at alternative uses of the spacecraft that don’t require the same degree of pointing accuracy needed to observe transits of stars by Earth-sized orbiting exoplanets. That could include searches for asteroids in our solar system or extragalactic objects, or possibly transits by larger, Jupiter-sized exoplanets. “We need to know more about the performance of the spacecraft before we can assess what kind of science we’ll be able to do,” Sobeck said.

That effort, said Paul Hertz, director of NASA’s astrophysics division, will likely take months to complete. “There’s a technical study the project needs to do to identify what the options are, and then we would have to do a scientific study to determine what the benefits of those options might be,” he said.

One challenge will be to identify alternative missions that can not only use Kepler, but are scientifically compelling enough to keep operating the spacecraft versus spending that money—about $20 million per year—on other missions. NASA holds biannual “senior reviews” of ongoing missions to ensure that the benefits of these missions warrant their costs; the most recent one was last year. “We would be making a decision next year as part of our normal process” about keeping Kepler in operation, Hertz said.

“I think the most interesting, exciting discoveries are coming in the next two years,” said Borucki. “The mission is not over.”

Even if Kepler never makes another exoplanet transit observation, though, the discoveries by the mission will continue for years. “We certainly have two years’ of data that have not been fully searched,” said William Borucki, the principal investigator for the Kepler mission and the man who fought for years within NASA to get Kepler built and flown. That work includes identifying “planet candidates” in the data and doing follow-up observations to confirm them. “We have at least two years of work ahead of us with the data we already have.”

Borucki bristled when one reporter on the telecon asked him to confirm that Kepler’s planet-hunting mission was over. “That’s not true at all. We have excellent data for the next two years that we expect to search and we expect to find lots of planets,” he said. “I think the most interesting, exciting discoveries are coming in the next two years. The mission is not over.”

“The mission has been phenomenally successful,” he continued, “and I really wouldn’t write it off at this point.”

Borucki said later, though, that he could see a time when Kepler’s mission would end. “I think we’ll declare the mission over when there is no possibility of getting critically important science, and also, when we look at the mission and what it might do in the future, and we compare that with other missions,” he said. “If it turns out other missions are going to give us more valuable data, I think Paul [Hertz] will probably ask us to shut this mission down.”

“It’s been a very long journey,” said Borucki of Kepler. “Right now, I am really delighted, frankly, with all that it’s accomplished. It was designed to operate for four years; it operated for four years. It gave us data like no other mission concept did. I’m just elated with how much we’ve accomplished. I’m not feeling sorry at all, I’m just feeling delighted and happy with what we’ve done.”