Since its launch in 2009, the Kepler spacecraft has found thousands of planets outside our solar system. One of NASA's least known but most remarkable—and important—missions, it has quietly changed our understanding of our place in the cosmos, providing hard information about the chances of life in the far reaches of space.
But its planet-hunting days may be done.
"On Friday October 19, during a regularly scheduled spacecraft using NASA’s Deep Space Network, the team learned that the spacecraft had transitioned to its no-fuel-use sleep mode," the Kepler team said in an . "The Kepler team is currently assessing the cause and evaluating possible next steps."
NASA knew the spacecraft was low on fuel, making it only a matter of time before the supply was exhausted. The space agency had already put the craft into no-fuel-use sleep mode once this month. But there was another mode change, which wasn't initiated by NASA—meaning the craft may have done it automatically. (An October 12 report also indicated that Kepler was having trouble stabilizing, though NASA was able to get it stable enough to download data from its last observing campaign.)
The Kepler Space Telescope began as a four-year planned mission to monitor the same patch of sky near the Cygnus constellation for transiting planets. "Transiting" refers to a planet crossing between its star and Earth, causing dips in light, which are detected by photometers. The changes in light are minute, but they're enough to determine the approximate size of a planet.
In 2013, Kepler broke a third reaction wheel, a part that it uses to keep the instruments stable, seemingly imperiling the mission. Using pressure from sunlight along with strategic fuel use, engineers were able to prolong the lifetime of Kepler. They started looking for larger planets and called this mission K2. The telescope has found more than 300 new planets during its second run.
The original Kepler mission found 2,327 planets, while K2 found 325. An additional 2,244 Kepler and 493 K2 planet candidates await confirmation that they are indeed new worlds, a process that typically involves using ground-based observatories to look for the candidates in question.
Even if Kepler wakes up this time, it appears that the end is near. But not before it told us about more than 5,000 planets and potential planets that we never knew existed.
That, space fans, is a legacy.