The wheels have literally come off the Kepler space telescope, NASA’s primary instrument for detecting planets beyond our solar system. Last week's malfunction leaves just two of the four stability-controlling wheels operational, jeopardizing the sun-orbiting spacecraft's continued mission. Since its launch in 2009, the telescope has discovered more than 2700 candidate exoplanets orbiting distant stars, including several Earth-sized planets that are within their star’s habitable zone. (See: Space Telescope Finds 3 Super Earths) Two wheels that helps guide the Kepler space telescope have stopped functioning and the craft has been placed in safe mode. Courtesy of NASA. Kepler’s photodetector array can register more than 100,000 stars at a time, said Scott Hubbard, a consulting professor of aeronautics and astronautics at Stanford University’s School of Engineering, who helped guide the Kepler mission when he served as director of NASA Ames Research Center. In order to detect exoplanets, the telescope must remain perfectly still so that the stars do not wander across the optics. To help hold its gaze, the telescope has a series of four gyroscope-like reaction wheels. Three of the four wheels must be functioning for the telescope to remain stable. One of the devices failed about a year ago and was shut off, and now a second wheel has ceased functioning, pausing the spacecraft’s operations. The root cause of the error is not yet known, but the spacecraft is now stable and in the safe mode. “Because this is a new operating mode of the spacecraft, the team will closely monitor the spacecraft, but no other immediate actions are planned,” the Kepler team said in a mission manager update. “We will take the next several days and weeks to assess our options and develop new command products.” Hubbard says there are two possibilities for salvaging the spacecraft. “One is that they could try turning back on the reaction wheel that they shut off a year ago,” he said. “The other scheme, and this has never been tried, involves using thrusters and the solar pressure exerted on the solar panels to try and act as a third reaction wheel and provide additional pointing stability.” “With the failure of a second reaction wheel, it’s unlikely that the spacecraft will be able to return to the high pointing accuracy that enables its high-precision photometry,” the Kepler team said. No decision has been made to end data collection. Kepler completed its primary three-and-a-half-year mission, which, based on its success, was extended four more years in November 2012. “It completed its primary observation phase and had entered its extended science phase,” Hubbard said. “We’re already in the gravy train period — there’s still a year and a half’s worth of data in the pipeline that scientists will analyze to identify other candidate planets, and there will continue to be Kepler science discoveries for quite some time.” The telescope has paved the way for additional missions, he said, including TESS (Transiting Exoplanet Survey Satellite) and TPF (Terrestrial Planet Finder), which will continue to search for Earthlike planets in the near future. For more information, visit: kepler.nasa.gov