NASA put its famous planet-hunting telescope to sleep because it’s almost out of fuel

The Kepler Space Telescope is almost out of fuel, which means its life is coming to an end, NASA announced today. The space agency says it put the planet-hunting spacecraft into a “hibernation” safe mode this past Monday, and that a plan to reactivate Kepler next month could burn out whatever fuel remains.

NASA launched the Kepler Space Telescope in 2009 in an effort to learn more about the number and frequency of planets in our galaxy. To the delight of many, scientists using Kepler have found an abundance of exoplanets, or planets outside our solar system. The spacecraft, which is some 94 million miles away from Earth, has scanned just a small section of our galactic neighborhood, but its efforts have led scientists to discover 2,650 confirmed planets so far.

They’ve come in all shapes and sizes, too. The planets that researchers have turned up range from big and weird — like a Jupiter-sized planet orbiting a binary star system — to ones that are closer in size and orbit to Earth. Each discovery has taught us more about how planets form, how many kinds of planets there are, and even how our planet came to be. And many rewards still lie in Kepler’s bounty. There are thousands more unconfirmed discoveries, and researchers continue to find new ways to rummage through Kepler’s data trove.

NASA says it plans to turn Kepler back on in early August, when it will order the spacecraft to point its antenna at Earth to download the data from its most recent survey of the sky. It’s not clear if there’s enough fuel to do that transfer. If there is, though, once the transfer’s complete, NASA plans to start what will be the 19th discrete “observation campaign” of Kepler’s secondary “K2” mission, which was started in 2014. The maneuvers required to point the antenna toward Earth are the most fuel-intensive ones that Kepler performs, and at any point, the spacecraft’s tank could finally run dry.

NASA knew that Kepler would one day run out of fuel, and when it started the K2 mission the agency originally forecast being able to squeeze just 10 observation campaigns out of what was left in the tank. But putting the spacecraft in safe mode is a sign that Kepler is truly running on fumes. So now, “returning the data back to Earth is the highest priority for the remaining fuel,” NASA says.

Kepler has entered hibernation before, and the space telescope has run into a fair share of trouble in its nine year run. When it slipped into a similar mode in 2016, it caused NASA to declare a temporary “spacecraft emergency” while the team worked to bring the telescope back online.

But Kepler’s biggest trouble came in 2012, when two of the telescope’s four gyroscopic “reaction wheels” stopped working. The momentum generated by these wheels was used to make fine adjustments to the telescope’s targeting. Losing one was fine, but losing two was a potential death sentence. Kepler had finished its initial mission by this time, and it appeared that NASA might wind down the telescope’s operation — until a clever solution emerged inside the agency to use the pressure that the Sun’s rays exerted on the spacecraft’s solar panels as a stand-in for one of the wheels. That fix gave Kepler its second life with the K2 mission, which it’s still performing today.

NASA has already launched a successor to Kepler, so even when it does die, the hunt for exoplanets will continue. The Transiting Exoplanet Survey Satellite, or TESS, hitched a ride off the planet on a SpaceX Falcon 9 earlier this year, and has already snapped its first image of the galactic sky. TESS has a field of view that’s 400 times bigger than Kepler’s, which will let it study hundreds of thousands more stars than its predecessor. TESS will also search for planets around stars that are tens to hundreds of light years away from us, as opposed to Kepler, which studied stars that are thousands of light years from our solar system.