R.I.P. Kepler, the planet-hunting telescope

The Kepler space telescope can no longer search for planets orbiting other stars. After 9 1/2 years, NASA’s premier exoplanet-hunter is out of gas.

NASA officials announced the mission’s end at a news conference on October 30.

“Because of fuel exhaustion, the Kepler spacecraft has reached the end of its service life,” said Charlie Sobeck. He is a project system engineer. Sobeck works at NASA’s Ames Research Center in Moffett Field, Calif. “While this is a sad event, we are by no means unhappy with this remarkable machine.”

Kepler’s discoveries have forever changed the way astronomers think about planets in other solar systems. Known as exoplanets, only about 350 were known to exist before Kepler launched in 2009. And nearly all of these were the size of Jupiter or larger.

As of this week, there are now more than 3,800 known exoplanets. And Kepler discovered 2,720 of them. The spacecraft found planets in all shapes, sizes and family structures. For instance, it found seven planets orbiting one star. Some planets had two suns. Other planets orbited their star at jaunty angles. And five that Kepler found orbiting one star were more than twice as old as Earth. “These planets formed at the beginning of the formation of our galaxy,” says William Borucki. “Imagine what life might be like on such planets.” Until he retired in 2015, this astronomer was Kepler’s principal investigator.

Astronomers also have used Kepler’s exoplanet tally to predict that every one of the hundreds of billions of stars in the Milky Way should have at least one planet. And scientists suspect that billions of these planets might have the right size and temperatures to support life.

Second death notice for the spacecraft

Kepler was declared dead once before. In 2013, the telescope lost use of a second of its four reaction wheels. These devices helped to keep the telescope pointed steadily at a selected patch of the sky. That consistent pointing was crucial for Kepler’s planet-hunting strategy. It worked by spotting a slight dip in stars’ light as planets cross in front of them. Five years ago, this seemed to mark the end of Kepler.

But engineers soon revived the telescope. They arranged for it to work in a new observing mode. It now used the pressure of sunlight on Kepler’s solar panels to keep it pointing straight.

“I always felt like it was the little spacecraft that could,” said astronomer Jessie Dotson. She’s a Kepler project scientist at NASA Ames. “It always did everything we asked of it,” she said, “and sometimes more. That’s a great thing to have in a spacecraft.”

Kepler’s official demise came two weeks ago. That’s when the telescope’s fuel pressure dropped by three-quarters in a matter of hours, Sobeck said. Before it shut down, NASA had Kepler send all of its remaining data back to Earth. “In the end,” he observed, “we didn’t have a drop of fuel left.”

Kepler’s legacy lives on, however. TESS, or the Transiting Exoplanet Survey Satellite, launched this past spring. And this planet-hunting telescope has already spotted some exoplanets.

For its final acts, the Kepler team will remotely turn off the telescope’s radio transmitters. The team will also turn off protective systems that might allow those transmitters to be turned back on. “The spacecraft will then be left on its own to drift away in a safe and stable orbit around the sun,” Sobeck said.