WASHINGTON — NASA’s Kepler spacecraft, which went into a safe mode in July amid concerns the exoplanet observatory was running out of propellant, has again gone into safe mode as astronomers fear its mission may be nearing an end.

In a brief statement Aug. 24, NASA said the spacecraft went into a fuel-conserving “sleep mode” after transmitting the data it collected from its previous observing campaign earlier in the month. The observing campaign was interrupted by the safe mode triggered by the low-fuel warning July 2.

“It is unclear how much fuel is still on board,” the agency said in its statement. “NASA is looking into the health of the spacecraft and determining a full range of options and next steps.”

Officials were optimistic that Kepler would be able to start its next round of observations, known as Campaign 19, in early August despite the low fuel warnings. “Campaign 19 will begin as planned on Aug. 6,” said Gary Blackwood, manager of NASA’s exoplanet exploration program, during a July 29 meeting of the agency’s Exoplanet Exploration Program Analysis Group.

However, NASA did not start Campaign 19 on Aug. 6. Instead, in an Aug. 9 statement the mission said the data from the previous session, Campaign 18, had been successfully transmitted back to Earth. “We are monitoring the spacecraft very closely and will provide more information when its status has been fully assessed,” the project stated in a website post.

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, enabling the discovery of ultimately thousands of exoplanets.

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 in a series of observing campaigns that continued until July.

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 remaining hydrazine on the spacecraft is challenging, so project officials have been anticipating the mission’s end for several months, not sure exactly when the spacecraft would run out of fuel.

Because Kepler, in a heliocentric orbit trailing the Earth, doesn’t pose a reentry and collision hazard, the spacecraft can continue to operate until it completely exhausts its fuel. “We can leave it in whatever state we want so we hadn’t given it a lot of thought,” Charlie Sobeck, system engineer for Kepler, said in a NASA podcast earlier this year about planning for Kepler’s end of mission. “Now that we are here most of our thought is going in to, ‘How do we get the most final science off the spacecraft and down into the hands of scientists before it goes in to that final resting spot?’”