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, which helped astronomers discover thousands of exoplanets since its launch nearly a decade ago, has ended operations after running out of fuel, the agency announced Oct. 30.

In a briefing with reporters, agency officials said that Kepler ended operations after exhausting the last of its hydrazine fuel used for attitude control. The spacecraft had been in what NASA called a “no-fuel-use” safe mode since it was contacted by controllers Oct. 19.

“NASA’s original planet hunter, the Kepler Space Telescope, has run out of fuel,” said Paul Hertz, director of NASA’s astrophysics division. “This is not unexpected, and this marks the end of spacecraft operations for Kepler and the end of the collection of science data.”

Project officials had expected Kepler to run out of hydrazine some time this summer. In July, and again in September, the spacecraft entered safe mode after showing signs of declining fuel levels. Both times, Kepler was able to transmit scientific data back to Earth and begin a new observing campaign.

It was at the end of that second effort to return data and begin a new observation campaign that controllers noticed a sharp drop in fuel pressure. “We saw it drop from 90 psi [pounds per square inch] all the way down to 25 psi” over a few hours, said Charlie Sobeck, project system engineer for Kepler at NASA’s Ames Research Center. “That’s what told us very, very clearly that we were now out of fuel and operating on fumes.”

All that is left for the Kepler spacecraft is a brief decommissioning effort, Sobeck said, that includes turning off fault protection on the spacecraft and shutting down its radio transmitter so that it does not inadvertently start transmitting in the future, causing interference. The sequence of commands for doing so has been transmitted to the spacecraft, awaiting a final command from the ground to run them.

Sobeck said controllers will wait until they have “good visibility” on the Deep Space Network so they can observe that the commands run as planned. “I suspect we will be doing that next week or maybe the week after,” he said.

The end of spacecraft operations means the end of new Kepler data, but project scientists said that the observations Kepler collected since its March 2009 launch will continue to be analyzed by astronomers for years to come, yielding new discoveries.

What Kepler found was that planets around other stars are commonplace. As of Oct. 29, Kepler had detected 2,681 exoplanets, with an additional 2,899 exoplanet candidates awaiting confirmation, said Jessie Dotson, Kepler project scientist at NASA Ames.

“It has revolutionized our understanding of our place in the cosmos,” Hertz said. “Before we launched Kepler, we didn’t know if planets were common or rare in our galaxy. But now we know, because of the Kepler Space Telescope and its science mission, that planets are more common than stars in our galaxy.”

The heir to Kepler, the Transiting Exoplanet Survey Satellite (TESS), launched in April and started science observations this summer. Like Kepler, TESS looks for very small, periodic dips in brightness of stars caused when planets pass in front of, or transit, those stars. However, while Kepler spent its prime mission looking at a single, very small region of the sky, TESS is performing an all-sky survey focused on the nearest and brightest stars.

“We’re hoping to find our near-neighbor planetary systems,” said Padi Boyd, TESS project scientist at NASA’s Goddard Space Flight Center. “We’re confident that TESS is going to find thousands more planets, just like Kepler did.”

The driving force behind Kepler was Bill Borucki, the now-retired principal investigator for the mission at NASA Ames. Borucki sought to develop what ultimately became Kepler for decades, eventually convincing NASA that a space telescope that could perform precise photometry of thousands of stars, needed to detect the brightness variations caused by transits, was feasible.

“He put together a like-minded team of scientists and engineers,” Hertz said of Borucki. “They worked through the technical problems. They convinced their peers and they convinced NASA that this was a mission that had to be done. And, boy, are we glad that he did that.”

The spacecraft, whose total mission cost was about $700 million, worked well right up to the end, other than the failure of two reaction control wheels in 2013 that ended the spacecraft’s primary mission and led to the development of an alternative mission, called K2, that allowed Kepler to continue observations of regions of the sky for weeks at a time up until it ran out of hydrazine. “We collected every bit of possible science data and returned it all to the ground safely,” Sobeck said.

“I think we were all extremely impressed with what it was doing for us,” Borucki said of Kepler. He noted the spacecraft had occasional glitches, but nothing that could not be corrected. “I never felt it was that much of a pesky object. It was really something I could be fond of.”

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...