Kepler's planet-finding mission may be at an end
BY STEPHEN CLARK
Posted: 15 May 2013
NASA's planet-hunting Kepler telescope, stationed 40 million miles from Earth, has lost the ability to precisely point toward stars, and scientists fear the mission's quest to identify rocky planets around other stars is over.
"When they looked at the telemetry, they saw indications that reaction wheel no. 4, although it was being commanded to speed up, was actually not moving," said John Grunsfeld, head of NASA's science mission directorate. "This is something that we've been anticipating for a while, unfortunately."
Ground controllers communicate with Kepler twice a week, and engineers established radio contact with the observatory as scheduled Tuesday to find the spacecraft was in safe mode. Kepler placed itself in safe mode Sunday when its on-board computer detected the craft drifting from where it was supposed to be pointing, officials said.
NASA is not ready to give up on the mission, but Kepler will be unable to discern the scant signatures from small rocky planets like Earth unless engineers can restore the telescope's pointing ability.
"We're not ready to call the mission over," Grunsfeld said.
Engineers say the evidence from telemetry suggests an internal failure in the reaction wheel.
"The team, as a whole, is saddened by this," said Charlie Sobeck, Kepler's deputy project manager at NASA's Ames Research Center in Moffett Field, Calif. "It took a pause yesterday to consider what the consequences are, but nobody is looking to retire yet because there is a lot more work yet to do."
Sobeck said controllers could command the reaction wheel to reverse direction or apply more torque to the device to get it to budge.
"Like any stuck wheel you might be familiar with on the ground, we could try jiggling it, commanding it back and forth in both directions, or we could try forcing it through whatever the resistance is that's holding it up," Sobeck said.
Until NASA can restart the craft's two remaining healthy reaction wheels, Sobeck said Kepler is in a stable configuration using its rocket thrusters and natural pressure from the solar wind to maintain its orientation for battery charging and communications with Earth.
The $600 million mission, which launched in 2009, has acquired data leading to the discovery of 132 confirmed planets, plus more than 2,700 candidate worlds requiring further scrutiny.
It caught the imagination of scientists and the public alike, with its first discoveries coming two weeks after the release of James Cameron's "Avatar" sci-fi film set on a resplendent fictitious Earth-like world named Pandora.
"This field of exoplanets, which is still a relatively new field, has generated enormous excitement in the public, from kids fo elder statesmen," Grunsfeld said.
Reaction wheels have troubled Kepler since July 2012, when the first of the craft's four wheels failed after showing erratic friction levels for several months.
The wheels were manufactured by Goodrich Corp., which was acquired last year by United Technologies Corp. Goodrich reaction wheels on satellites owned by Globalstar, the U.S.-based mobile communications firm, suffered from a defect soon after launch in 2010 and 2011. The manufacturer of the satellites, Thales Alenia Space, designed a software patch to rectify the problem.
Ball Aerospace and Technologies Corp. was the Kepler spacecraft's prime contractor.
Kepler surpassed its mission design life of three-and-a-half years, but its mission was extended last year through 2016 to allow more time to validate the existence of the smaller, Earth-sized worlds in the mission's listing of candidate planets.
Engineers will first attempt to remedy the problem on the newly-failed wheel, but Sobeck said the control team could also try again to reactivate the reaction wheel that failed last year in an attempt to restore pointing to the Kepler spacecraft.
Kepler was launched into an Earth-trailing solar orbit and is now flying about 40 million miles from Earth, according to Sobeck.
"Unfortunately, Kepler's not in a place where I can go up and rescue it," said Grunsfeld, a former astronaut and Hubble telescope repairman.
The wheels spin in two directions between 1,000 and 4,000 rpm, generating momentum to maneuver the spacecraft and keep Kepler's 3.1-foot telescope steadily pointed toward a field of stars in the constellations Cygnus and Lyra.
"We need three wheels in service to give us the kind of precision that's necessary to find planets," said Bill Borucki, Kepler's principal investigator at Ames. "That's what makes this mission work - the fact that we can point with extreme precision. Without three wheels, it's unclear if we can do anything of that order."
Paul Hertz, director of NASA's astrophysics programs, said officials will study if Kepler could be repurposed for another mission requiring less stable pointing.
"We will do studies to understand the status of the vehicle and its scientific and technical capabilities, then decide if the science is valuable enough to continue operating it," Hertz said, adding the decision on Kepler's future would take several months.
Astronomers focused their research with Kepler on approximately 156,000 stars within Kepler's 10-degree field-of-view, using the observatory's 95-megapixel camera to stare at the sky and detect the dimming of stars caused by the passage of planets between stars and the Kepler spacecraft.
By observing a fraction of the sky for exoplanets, scientists can extrapolate their findings across the entire galaxy.
Scientists hoped to continue the telescope's survey and learn more about how common hospitable Earth-sized planets are in the Milky Way galaxy.
The likely answer, scientists say, is they are everywhere.
"We've found that Earth-sized plants are very common, they're often in the habitable zone, and they're around all types of stars," Borucki said.
The mission's time observing was not without glitches. Besides the reaction wheel trouble, raw images from Kepler's digital camera were polluted with bands of noise from the instrument's electronics.
Speaking at a global space conference in Italy in October, Borucki said his team devised complex computer algorithms to dispose of the meaningless data and clear up the imagery for analysis, describing how researchers must calibrate every pixel of each Kepler image before trusting the data.
The extra work added time and cost to Kepler data analysis, Borucki said.
Another complication was the twinkle of light coming from stars in Kepler's view, meaning scientists needed more time to confirm a candidate was an actual planet and not an artifact of stellar variability.
Kepler's top objective was to find potentially habitable Earth-sized planets around sun-like stars. Astronomers believe they have found hundreds of giant Jupiter-sized planets, some circling hellishly close to their parent stars.
Those are the easy ones.
Scientists are more interested in spotting planets in the habitable zone, a "Goldilocks" band where temperatures are just right to sustain liquid water and life.
Smaller planets in orbits farther out from the hosts take more time to confirm, with scientists needing several "transits" to confirm the dip in starlight is actually from a planet. For planets like Earth, such a discovery would take several years of observations to verify.
Borucki admitted Wednesday scientists may be unable to find as many "Earth analogs" as originally hoped, but he said he was optimistic such a world lies in Kepler's extensive data catalog.
"I would have been even happier if it continued another four years because we would have better data about more stars and smaller planets, and more in the habitable zone," Borucki said. "That would have been, in a sense, frosting on the cake, but we have an excellent cake."
Kepler's listing of 2,740 candidate planets only covers about half of the mission's data. Two years of Kepler observations still need to be studied, Borucki said.
Grunsfeld and Borucki said they expect most of the 2,740 candidates will be checked and verified as real planets.
"These planets range all the way in size from Earth's moon to many times bigger than Jupiter," Borucki said.
"We have excellent data for an additional two years," Borucki said. "We expect to find lots of planets. We expect to find them in the habitable zones, and we expect to find small ones. I think our most interesting and exciting discoveries are coming in the next two years, so the mission is not over."
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