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Kepler's exoplanet survey jeopardized by two issues
BY STEPHEN CLARK
SPACEFLIGHT NOW

Posted: October 16, 2012


NASA's Kepler space telescope, a sleuth with electronic eyes, needs an extra four years to meet its goal of finding an Earth-sized planet in habitable zones around other stars, but a critical hardware failure aboard the probe this summer has managers worried the mission could end at any time.


Artist's concept of the Kepler space telescope. Credit: NASA
 
Launched in March 2009, the $600 million observatory is stationed in an Earth-trailing solar orbit and aims its 3.1-foot telescope toward constellations Cygnus and Lyra, observing a 10-degree-wide field containing 4.5 million detectable stars. Kepler is focusing on approximately 156,000 stars for the purposes of its research.

Kepler monitors the stars for dips in brightness, an indication a planet could be passing in front of it.

Astronomers using data from Kepler have confirmed 77 new planets beyond the solar system. Including data to be released by the end of October, there are nearly 3,000 candidates for exoplanets waiting to be verified by follow-up observations, according to William Borucki, the Kepler mission's principal investigator at NASA's Ames Research Center in Moffett Field, Calif.

But two issues with Kepler have the attention of scientists and engineers.

On July 14, one of the spacecraft's four reaction wheels stopped due to increasing friction. The spinning masses control Kepler's orientation in space and keep the telescope locked on to target stars.

"We have to guide very accurately, and we had four reaction wheels to do this guidance," Borucki said. "One of those was a spare, and we now have lost one of those four wheels ... The guiding is still great, but they've all had over a billion revolutions. If we lose another one, this mission terminates. We cannot track very well with two. We cannot track well enough to find planets."

Borucki said engineers will ensure Kepler's three active reaction wheels stay warm and alternate their rotation between clockwise and counter-clockwise directions. Officials believe the measures will help the wheels remain healthy.

"We're trying to understand how to protect those last three wheels," Borucki said. "People have studied these reaction wheels over the years and never came up with a good answer."

Kepler was initially intended to last three-and-a-half years, but NASA is keeping the telescope operational through 2016, in part due to difficulties processing and analyzing the wealth of data streaming down from the spacecraft.

Although Kepler's 95-megapixel digital camera is plenty sensitive, bands of noise appear in raw imagery streaming down from the telescope. Scientists have devised complex computer algorithms to remove the meaningless data, Borucki said.

"Even the tiniest noise sources are important to us, and we do have noise sources - noise in the electronics, in particular," Borucki said. "We call it rolling band, guider noise, there's oscillating amplifiers, etc. Every time we get a frame, we have to calibrate every single pixel in that frame."


Diagram of the transit method of detecting exoplanets, in which a telescope records a dip in a star's brightness caused by a planet. Credit: NASA
 
According to Borucki, the noise problem adds to the time and cost of analyzing Kepler data.

And despite the best estimates of scientists before Kepler's launch, most of the sun-like stars in Kepler's field-of-view show more variability than projected.

Coupled with the noisy data derived from technical causes, the variability issue means astronomers need more data to confirm a dip in a star's luminosity is from a transiting planet.

"Because of the high variability of stars like the sun, we're not going to get the final answers in four years," Borucki said. "We're going to get them in eight years."

Kepler scientists want a planet to pass in front of its parent star several times to weed out false positives from a transiting binary star or blips in the star's brightness. For a planet like Earth, it would normally take at least four years announce a confirmed discovery.

With many stars, Borucki said, scientists will need to observe double expected the number of transits before certifying a candidate as an exoplanet.

The Kepler team succeeded earlier this year in obtaining approval from NASA Headquarters to extend the mission from 2013 through 2016.

Most of the new planets announced early in Kepler's mission orbit hellishly close to their parent star, creating an environment inhospitable for life.

But Kepler is designed to find Earth-sized planets in a star's habitable zone, the cosmic sweet spot where scientists believe conditions could support life. Planets closer to their parent star are too hot, causing water to evaporate and starving life of vital nutrients. Colder planets lie further away from the star.

NASA announced last year Kepler's first new planet in a star's habitable zone, and the number of discoveries is expected to rise in the next few years if the spacecraft keeps functioning.

"We want to know if Earths are common or rare," Borucki said.

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