Kepler K2
NASA's Kepler spacecraft, launched in 2009 on a mission to search for exoplanets, is in a safe mode amid concerns the spacecraft may be finally running out of fuel. Credit: NASA

WASHINGTON — NASA’s Kepler spacecraft has paused science observations upon receiving indications that the spacecraft may be finally running out of fuel after more than nine years of operations.

In a status update distributed July 6, NASA said mission managers halted a current set of observations known as Campaign 18 and placed the spacecraft into a “no-fuel-use safe mode” July 2 after receiving indications of what the agency called an “anomalous” drop in fuel pressure in the spacecraft.

That safe mode, mission officials said, will preserve the 51 days of “flawless” observations collected during Campaign 18. The spacecraft will remain in that safe mode until Aug. 2, when it will resume operations for a previously scheduled downlink of data through the Deep Space Network.

Engineers will also evaluate the status of the spacecraft’s propulsion system to determine if observations will continue. For now, the mission is planning a Campaign 19 that is scheduled to begin Aug. 6, after the downlink of the Campaign 18 data is completed. “If the observed drop in fuel pressure is indicative of the spacecraft running out of fuel, then Campaign 19 may not be completed,” mission officials said in their status update.

NASA launched Kepler in March 2009 on a mission to search for planets around other stars by detecting minute, periodic dips in brightness of those stars as planets pass in front of, or transit, those stars. In Kepler’s primary mission, lasting until May 2013, the spacecraft looked at a single area of the sky.

The failure of two of the spacecraft’s four reaction control wheels, used to accurately point the spacecraft, forced NASA to end that primary mission. Engineers developed an alternative technique to point the spacecraft, using the two remaining wheels, solar pressure and the spacecraft’s thrusters. That approach allowed Kepler to look at different parts of the sky for about 80 days at a time.

Kepler has been operating under that extended mission, known as K2, since 2014. Although the spacecraft remains in good health, the limiting factor on its life has been its supply of hydrazine fuel. When Kepler exhausts its remaining fuel, it will no longer be able to accurately point, effectively ending the mission.

Accurately measuring the amount of fuel remaining is a challenge, and engineers have relied instead on data like the drop in fuel pressure as a sign that the spacecraft is running out of fuel. Engineers expected that Kepler would use up its remaining fuel some time this year but weren’t sure exactly when given the uncertainties in measuring the fuel on board.

Engineers planning for this wanted to be sure to maximize both the data collected and returned to Earth. “It’s like trying to decide when to gas up your car. Do you stop now? Or try to make it to the next station?” wrote Charlie Sobeck, a system engineer for Kepler, in a mission update in March. “In our case, there is no next station, so we want to stop collecting data while we’re still comfortable that we can aim the spacecraft to bring it back to Earth.”

In a statement, NASA said it will provide an update on the status of Kepler after the downlink of Campaign 18 observations in early August. The mission is, for now, planning to start Campaign 19 Aug. 6 while also continuing preparations for Campaign 20, which would begin in mid-October. The mission will continue even after the end of spacecraft operations in order to fund analysis of the data collected to date.

Kepler, though, has defied some earlier predictions about its impending demise. “I’m cautiously optimistic we’ll make it through Campaign 16,” said Jessie Dotson, K2 project scientist at NASA’s Ames Research Center, during a town hall meeting about the mission at an American Astronomical Society conference in January. Campaign 16 ended in late February. “Anything past that is gravy.”

Jeff Foust writes about space policy, commercial space, and related topics for SpaceNews. He earned a Ph.D. in planetary sciences from the Massachusetts Institute of Technology and a bachelor’s degree with honors in geophysics and planetary science...