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Kepler reaction wheel still problematic after mitigation
BY STEPHEN CLARK
SPACEFLIGHT NOW

Posted: April 11, 2013


NASA's Kepler telescope, peering at the sky from a vantage point more than 40 million miles from Earth, is showing troubling signs of wear and tear, and engineers are studying how to keep the planet-hunting observatory running with jittery vision.


Artist's concept of the Kepler spacecraft. Credit: NASA
 
One of the mission's four reaction wheels stopped working last year, and officials are worried another wheel could fail at any time. Three wheels are needed to keep the observatory pointed toward a field of stars looking for signs of planets.

The Kepler mission, which launched in 2009 on a quest to find planets around other stars, has been a remarkable success, according to mission officials. Scientists using data from the telescope's 95-megapixel camera have confirmed the discovery of 115 exoplanets, and the mission's data archive has evidence for more than 2,700 planet candidates.

What the data tell astronomers about the planet population in the Milky Way galaxy is astounding. One study by scientists at the University of Auckland released in April, combining Kepler statistics with a technique called gravitational microlensing, estimates there may be up to 100 billion Earth-like, habitable planets in the Milky Way.

Kepler's $600 million mission is to find Earth-sized planets in the right orbits around stars, where temperature are not too hot or too cold to support life. Astronomers look for dips in starlight caused by planets passing between Kepler and a parent star, and they need multiple transits to verify the signature is a real planet and not a false positive.

But noisy data from Kepler's camera and flickering light from Kepler's target stars have hampered astronomers as they comb through data looking for planets, forcing NASA to extend the mission from 2013 through 2016 to allow more time to finish the mission.

Launched with four spinning momentum wheels to keep the telescope steadily pointed toward a field of more than 150,000 stars, Kepler has now lost one of the pointing wheels and is at risk of losing another, officials said.

Before a reaction wheel failed in July 2012, the device showed erratic friction over several months. A second wheel has showed intermittent friction for several years without, but it has continued working.

The wheels normally spin between 1,000 and 4,000 rpm in both directions, according to Charlie Sobeck, Kepler's deputy project manager at NASA's Ames Research Center in Moffett Field, Calif.

The momentum generated by the spinning wheels controls the attitude of the spacecraft, which must precisely point toward stars to detect signatures of extrasolar planets.

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.

Three of the wheels are needed for Kepler's 3.1-foot telescope to have enough sensitivity to detect the minuscule signatures of Earth-sized planets.

In an exercise of caution, mission managers switched off Kepler's reaction wheels for 10 days in January, hoping the break would redistribute lubricant inside the wheel assemblies, reducing friction and allowing the units to cool down.

But friction in wheel no. 4, which has showed friction for much of Kepler's mission, actually increased in the month following the "wheel rest" period.

"The elevated friction did not come down after resting," Sobeck said in an interview with Spaceflight Now. "I don't think we had strong hopes that it would, but it was something we had yet to try. During February, in fact, it seemed to elevate somewhat higher and then come back down to its normal elevated level. When we looked at the detailed friction levels, we saw some very brief friction spikes - what we call torque spikes - in the friction. I don't know that we've ever seen that before."

The spikes last several minutes, and engineers have ruled out a connection to solar flares, which can temporarily saturate the telescope's guidance system and cause the momentum wheels to behave erratically.

"It could be a reaction to something internal to the wheel going on, something sticking or starting to wear out some of the internal components," Sobeck said.

Sobeck said the high torque was "not terribly alarming" because the troublesome wheel has operated with high friction for several years, but the intensity and frequency of the behavior has caught the attention of engineers, who recommended a series of mitigation measures last year to extend the lives of the wheels.

The wheel rest period was the last mitigation effort available for Kepler.

"We're still not sure what this means, but we don't have a lot of options left," Sobeck said, adding there are no plans for another wheel rest period.

If another wheel stops spinning, Kepler will be unable to finish its mission as planned. The telescope's will lose sensitivity because the spacecraft will be unable to keep as steady as designed.

"I think it would be difficult, but so far we don't see anything that says we couldn't do some sort of science on two wheels," Sobeck said. I don't think we could do the kind of science Kepler is currently doing."

Officials have not finished an analysis of the worst-case scenario for Kepler, but the mission's other two reaction wheels show no signs of wear, leaving managers hopeful the mission could continue collecting some type of valuable observations even with another failure.

"We would lose precision in the photometry that we do. We probably could not do a point-and-stare type of mission which is what Kepler was designed to do. We would have to go into some sort of scanning mode where you back and forth among the fields. Without a shutter, that makes the processing challenging. It can be done, but it's something that would be difficult to do."

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