A year after being sidelined by a positioning system failure, NASA’s planet-hunting Kepler space telescope is back at work on a new and expanded mission called K2.

The telescope was launched in 2009 to find Earth-sized planets suitably positioned from their parent stars to support liquid water on their surfaces, a condition believed to be necessary for life. Analysis for a true Earth analog -- one that circles a sun-like star -- is still under way, but scientists already have added 974 confirmations and 3,846 candidates to the list of nearly 1,800 planets discovered beyond the solar system.

The telescope works by capturing slight changes in the amount of light coming from about 150,000 target stars, some of which were caused by orbiting planets passing by, or transiting, relative to Kepler’s line of sight.

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After four years of observations, the telescope lost the second of four gyroscope-like wheels, which are used to maintain the observatory’s laser-like focus on its target stars. At least three wheels are needed to steady the spacecraft -- or so engineers thought.

Telescope manufacturer Ball Aerospace came up with a plan to use two wheels and a combination of pressure from sunlight and tiny thruster burns to maintain orientation.

“After the loss of the second reaction wheel there were many doubters that we could do anything to repurpose the spacecraft,” Kepler project scientist Steve Howell, said at the American Astronomical Society meeting in Boston this week.

Any lingering doubts ended last week when Kepler completed a 5.5-week engineering test to check out its new pointing system.

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“We drift about one pixel about every six hours and then we thruster fire and return to the same position. It’s a very, very good pointing,” astronomer Thomas Barclay, with NASA’s Ames Research Center in Moffett Field, Calif., said at the American Astronomical Society meeting in Boston this week.

In its first nine days of science observations, the K2 campaign already has turned up three more candidate planets, all around the size of Jupiter, circling relatively bright stars, Barclay said.

To balance the telescope with pressure from sunlight, Kepler must be nearly parallel to its orbital path around the sun, which is slightly offset from the orbital plane of Earth. The so-called ecliptic plane -- the band of sky containing the constellations of the zodiac -– is Kepler’s new hunting ground.

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Rather than continuously stare at a single patch of the sky for years, K2 science is limited to 80-day increments. That means it will no longer be able to search for Earth-sized worlds circling sun-like stars (which would transit only once a year), but it does open the search for planets around different kinds of stars.

“K2 can do some really high impact research ... stuff Kepler couldn’t,” Barclay said.

Scientists are particularly interested in finding planets that bridge the gap between rocky worlds, like Earth and the inner planets of the solar system, and the gas giants, like Neptune, Jupiter and the outer planets. They also hope to find young stars with planets still incubating in protoplanetary disks and detect transits of planets still in the formation stages.

“It’s a very exciting field for us,” Howell said.

NASA has approved a two-year K2 mission. The telescope is believed to have enough fuel for another year of operations beyond that.