Something's gone wrong aboard the planet-hunting Kepler spacecraft. On Friday evening, mission manager Charlie Sobeck announced that Kepler had entered "emergency mode." This is the lowest functioning operation mode and, critically, consumes the most fuel.

The last time NASA contacted Kepler, on April 4th, the spacecraft was in good health. On Thursday, however, Kepler was found to have been in emergency mode for about a day and a half. Even though it takes roughly 13 minutes for messages to travel the 120 million km from Earth to the spacecraft, it is a positive sign that NASA can still communicate with Kepler. This leaves open the possibility of some technical repair.

Since its launch in 2009, Kepler has been a major success for NASA, finding about 5,000 candidate planets, with 1,000 of those already confirmed by ground-based observations. Moreover, it has firmly established the commonality of planets, from Earth-sized worlds to gas giants, throughout the Milky Way Galaxy.

These are not the first technical difficulties for the spacecraft, and the NASA operations team has proven up to the task before. In May 2013, the second of four reaction wheels that steer Kepler failed. This should have been fatal for a spacecraft that requires precise pointing to observe the transits of exoplanets in front of their parent stars. But NASA scientists and engineers devised a strategy to use pressure emanating from the Sun to mimic the function of a reaction wheel to help control the spacecraft. It has since operated in this K2 mode for just over two years.