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Kepler illustration
NASA is now considering alternative uses of the Kepler spacecraft after efforts failed to fix two reaction wheels needed to perform its primary exoplanet search mission. (credit: NASA)

Kepler seeks a new mission


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NASA has, over the years, developed a public reputation for pulling off tasks that seemed initially to be difficult or improbable. From performing critical repairs on the Hubble Space Telescope to landing a one-ton spacecraft on the surface of Mars, NASA has successfully completed enough projects with high levels of difficultly to lead some in the public to think that the space agency can complete every such challenging mission. But, sometimes, the improbable is just that, even for NASA.

“I think all of us who were investigating the wheels at that point really had very little hope that we would be able to recover the wheels,” Sobeck said.

That’s the case with recent efforts to try and revive one of two failed reaction control wheels on the Kepler spacecraft. The failure of the second wheel in May put the planet-hunting spacecraft out of action, since Kepler carries four wheels and it requires three working ones for the precision pointing needed to carry out its mission (see “Kepler’s uncertain future”, The Space Review, May 20, 2013). Project officials held out hope—faint hope, to be certain—that it might be possible to restore one of the failed wheels, allowing normal operations to resume.

Those efforts to fix one of the wheels started in July. An update from Kepler mission manager Roger Hunter on July 24 offered some cause for optimism. While one of the failed wheels, designed wheel #4, could only spin in one direction, the other, wheel #2, could spin in both. “Although both wheels have shown motion, the friction levels will be critical in future considerations,” Hunter wrote.

In an August 2 update, Hunter reported that wheel #4 was spinning in both directions now. The issue, though, remained the amount of friction on both damaged wheels. “A constant friction level may be correctable in the spacecraft’s attitude control system, whereas a variable friction level will likely render the wheels unusable,” he wrote. The project, he added, was planning a test to measure how well the spacecraft operated on wheel #2 along with the two good reaction wheels.

Deputy project manager Charles Sobeck reported on that test at a media teleconference on August 15. Controllers put the spacecraft into what he called a “benign standby mode” to see how it operated using those three wheels. For the first couple of hours, things went well, he said. Controllers then pointed the spacecraft towards Earth to download data collected over the last few months. “During that time we did see an increase in friction in wheel #2,” he said. After about four hours, he said, “the friction went beyond what we could accommodate, and the error in pointing increased beyond the degree limit we had set on it,” temporarily putting the spacecraft into safe mode, he said.

“The results of that showed what we expected to see,” he concluded, “which is that the wheels are sufficiently damaged that they cannot sustain spacecraft pointing control for any extended period of time.” In other words, there’s no hope of getting Kepler back to its mission of looking for Earth-like extrasolar planets.

Sobeck, though, was not surprised by the failed bid to reactivate one of the failed wheels. “I think all of us who were investigating the wheels at that point really had very little hope that we would be able to recover the wheels,” he said, adding there was no track record at NASA of recovering a failed reaction wheel in that way. “We were doing what we thought was due diligence. We could have been surprised. We didn’t want to leave these wheels untested. But we had very little hope that was actually going to be recoverable.”

With only two wheels, Kepler is not able to do the fine pointing required to carry out its original mission: to look at a single star field, monitoring more than 100,000 stars for minute, periodic dimming caused when an orbiting exoplanet passes in front of it. Those observations would allow astronomers to calculate what they call “eta-Earth”: the frequency of Earth-sized planets orbiting stars in their habitable zones.

“It is not the end of the Kepler mission,” Borucki said. “The mission is done when you finish your analysis and have a result. We have all this data we have not yet analyzed and we expect many, many more discoveries.”

With additional observations to look for exoplanets in that manner no longer possible, but with Kepler otherwise in good condition, NASA is looking for potential alternative missions for the spacecraft. On August 2, the project issued to the scientific community a call for white papers, asking for their ideas on other uses for the spacecraft. White papers offering suggestions for alternative missions are due to the project on September 3.

Kepler officials already have some ideas of what they could do with Kepler given the less precise pointing it is now capable of. “The missions that have been suggested to us are of quite a large variety,” said Kepler science principal investigator William Borucki, ranging from searches for asteroids in our solar system to distant supernovae. It’s even possible Kepler could look for exoplanets, using a different technique: “microlensing,” where one star serves as a gravitational lens to allow detections of gas giant planets orbiting a more distant star.

As the Kepler science team examines the proposed missions submitted in the call for white papers, engineers will, in parallel, what kind of two-wheel operations are possible and the capabilities of Kepler in that mode. Those studies are due to NASA Headquarters this fall, said Paul Hertz, director of the astrophysics division of NASA’s Science Mission Directorate.

NASA, Hertz said, will then consider whether it makes sense to continue to operate Kepler in some alternate mission, or if the money spent on it—about $13 million in the current fiscal year—would be better spent on other programs. “In this era of constrained resources, continued operations of any our missions that have already completed their prime phases” must be carefully considered, he said. That decision would likely come in a senior review planned for next spring.

While Kepler’s observations of exoplanets may be at an end, its overall mission goes on, as scientists continue to analyze the data collected by the spacecraft since its launch in 2009. Borucki became a bit irate when one reporter asked about the end of the mission. “It is not the end of the Kepler mission. That is a terrible misstatement,” he said. “The mission is done when you finish your analysis and have a result. We have all this data we have not yet analyzed and we expect many, many more discoveries.”

“When we started the mission, when we conceived the mission, it was sort of like I was standing out in this desert,” Borucki said. “Now, I sort of feel like I’m standing at the bottom of the ocean, and I’m covered with an oceanful of data on all these possible planets.”

That data analysis, Borucki said, is likely to continue for about three years, as scientists run the backlog of data through the “pipeline” of computer systems that search for the periodic dimmings of stars that are the telltale signature of transiting exoplanets. The project team will also work to refine that data pipeline to pull out the signal of smaller planets from the observations, while dealing with noise in the form of stellar variability that turned out to be higher than expected before Kepler launched, in order to come up with their best estimate of eta-Earth.

More Kepler observations, Borucki said, would have helped in improving the statistics of planets in larger orbits and around stars hotter than the Sun, and to deal with the stellar variability noise problem. “We’re going to have to dig down hard to find these small planets,” he said, but noted Kepler has already detected some small exoplanets. “I’m confident we’re going to find what we expected, that we’re going to have sufficient statistics, but we’re going to have to work hard for the next couple if years.”

While Borucki and the Kepler science team might not have collected as much data from Kepler as they might have hoped, he said he felt “immense satisfaction” with what the mission has accomplished so far. “When we started the mission, when we conceived the mission, it was sort of like I was standing out in this desert,” he said. “Now, I sort of feel like I’m standing at the bottom of the ocean, and I’m covered with an oceanful of data on all these possible planets.”


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