PHOENIX — Spacecraft controllers are working to restore control of NASA’s Kepler astronomy spacecraft after it entered an “emergency mode,” disrupting science observations, the mission’s manager said April 8.

In a statement, Charlie Sobeck, Kepler mission manager, said that spacecraft operators found the spacecraft was in emergency mode during a scheduled communications session with the spacecraft April 7. Sobeck described that emergency mode as the spacecraft’s lowest operational mode and is “fuel intensive.”

Kepler appeared to enter emergency mode about 36 hours before the communications session, but Sobeck didn’t state what might have caused the incident. The spacecraft was operating normally during its prior communications session April 4.

Work to recover the spacecraft is hindered by its distance from the Earth of nearly 120 million kilometers. Sobeck said the mission has declared a spacecraft emergency to get priority access to NASA’s Deep Space Network to aid in efforts to recover the spacecraft.

Controllers found the spacecraft was in emergency mode during a communications session originally intended to reorient the spacecraft for its next observational campaign. Kepler is currently in an extended mission known as K2 where it observes regions of the sky for about three months at a time to search for extrasolar planets and study other astronomical phenomena.

The K2 mission emerged from an earlier problem with the spacecraft. The failure of two of the four reaction wheels on Kepler, used to provide stable pointing, forced NASA to end Kepler’s primary mission of observing the same region of the sky in 2013, four years after launch. Engineers developed an alternative pointing mode using the spacecraft’s remaining reaction wheels and solar pressure to allow it to observe regions of the sky for months at a time while minimizing the use of propellant.

Until the recent emergency mode, Kepler had experienced few problems during its extended mission. “The spacecraft has operated beautifully, with scarcely a whiff of trouble,” Sobeck wrote in a March 11 mission update. Spacecraft engineers, he said then, had adjusted spacecraft parameters to take “a bit more risk” to improve the efficiency of science observations. “In response, the spacecraft appears to have rewarded our trust by operating more smoothly than it has at any other time in its history.”

That good performance had been a boon for scientists, who have been using Kepler for studies ranging from continued searches for exoplanets to studies of supernova explosions. At the time Kepler went into emergency mode, controllers were preparing to begin its ninth observing campaign of the K2 mission.

Even if spacecraft controllers recover Kepler, the fuel used during emergency mode could shorten the mission’s lifetime. Fuel is one of the key limiting factors in the life of the spacecraft, along with communications as the spacecraft drifts away from the Earth in its orbit around the sun. Kepler had been using less fuel than originally projected, making further extensions of its mission feasible.

Prior to the current anomaly, project officials were optimistic Kepler could operate well into 2018. “The spacecraft health is very good,” said John Troeltzsch, Kepler program manager at Ball Aerospace, the mission’s prime contractor, at an American Astronomical Society meeting in Florida in January. “We’re in good shape to get out to Campaign 17,” an observing run planned for mid-2018.