WASHINGTON — NASA’s Kepler spacecraft has once again entered a safe mode as the aging spacecraft appears to be running out of fuel.

In a brief statement Oct. 23, NASA said that, during a routine communications session on Oct. 19, controllers found that the spacecraft had entered a “no-fuel-use sleep mode” that disrupted operations. “The Kepler team is currently assessing the cause and evaluating possible next steps,” the agency stated.

NASA didn’t disclose what those possible next steps are or a timeline for making a decision. “NASA is still analyzing the data to determine the next steps, and will provide an update when we can,” project spokesperson Alison Hawkes said Oct. 25.

Two prior observing campaigns by Kepler, in July and September, were also interrupted by problems that appear to be due to the spacecraft running out of hydrazine fuel for its thrusters. The spacecraft uses thrusters to properly point the spacecraft, and when its hydrazine is exhausted the mission will end.

In both previous cases, NASA was able to downlink the data that Kepler collected during those interrupted observing campaigns and then start a new observing campaign. The latest observational effort, called Campaign 20, started Oct. 14.

“We can see that the pressure in the fuel tanks on the thrusters is dropping,” said Paul Hertz, director of NASA’s astrophysics division, during an Oct. 22 meeting of the Astrophysics Advisory Committee. “It hasn’t run out of fuel yet, but we’re not going to be surprised when it runs out of fuel.”

He said that when controllers did see signs of problems, they would try to download the data they collected, as they did the previous two times. However, transmitting data, which requires reorienting the spacecraft so that its high-gain antenna is pointed to Earth, is a more fuel-intensive mode of operations than routine observations.

“We’re not going to be surprised, whether it’s a day from now or a month from now or two months from now, when we find out Kepler has run out of fuel,” he said.