Farewell, Kepler: NASA’s planet-hunting probe runs out of gas, but mission goes on

Kepler telescope
Stylized artwork shows NASA’s Kepler space telescope among planetary systems. (NASA Illustration / Wendy Stenzel / Daniel Rutter)

NASA’s Kepler planet-hunting telescope now belongs to the ages, with its fuel completely spent and its instruments shut down — but the planet quest continues, thanks to a treasure trove of downloaded data as well as a new generation of robotic planet-hunters.

Space agency officials declared the end of spacecraft operations today, nine and a half years after the car-sized probe was launched. The hydrazine fuel ran out about two weeks ago, signaled by a sharp drop in pressure readings for the propulsion system.

“In the end, we didn’t have a drop of fuel left over for anything else,” Charlie Sobeck, project system engineer at NASA’s Ames Research Center, said during a teleconference.

For years, Kepler stared at a fixed area of the sky bridging the constellations Lyra and Cygnus to monitor about 150,000 stars for signs of planets. The probe detected distant worlds by watching for the telltale dimming of starlight as a planet passed over an alien sun’s disk.

Bill Borucki, the mission’s retired principal investigator, compared the task to “trying to detect a flea crawling across a car headlight when the car was 100 miles away.”

Kepler’s originally planned $640 million mission went far longer than anticipated, thanks in part to a spacecraft-saving fix that was made in 2013 when a crucial part of the probe’s fine-pointing system went out of commission. “That was an amazing diving catch,” said Paul Hertz, astrophysics division director at NASA Headquarters.

Mission planners reworked Kepler’s mode of operation to point at other parts of the sky, expanding its list of targets to 500,000 stars. Sobeck said the extra five years of operation cost roughly $10 million a year — which is a bargain by space probe standards.

As of today, Kepler has detected 2,681 confirmed planets, plus 2,899 other candidates yet to be confirmed, said Kepler project scientist Jessie Dotson.

That database is large enough to allow for a statistical analysis of planetary systems. Those systems range from Kepler-233, whose parent star may be merely 5 million to 10 million years old, to Kepler-444, whose planets may be more than twice Earth’s 4.5 billion-year age.

“We’re really seeing planets at a wide variety of stellar ages,” Dotson said.

One of the big conclusions is that there may be well more planets than stars in the Milky Way, thanks to the number of multiple-planet systems found by Kepler. Depending on assumptions, the mission identified somewhere around two to 12 potentially habitable rocky planets. When scientists factored those finds into statistical formulas that take Kepler’s limitations into account, they concluded that 20 to 50 percent of the Milky Way’s stars may have rocky planets in habitable zones.

There’s certain to be more to come: Much of the data downloaded from the spacecraft has yet to be fully analyzed.

“The Kepler spacecraft may now be retired, but the Kepler data will continue to yield scientific discoveries for years to come,” Hertz said. “And the Kepler mission has paved the way for future exoplanet-studying missions.”

NASA’s Transiting Exoplanet Survey Satellite, or TESS, was launched in April and started sending back science data this summer. TESS is designed to survey about 200,000 stars across a wide stretch of sky in our celestial neighborhood, and identify prospects for further study.

“NASA is handing off the mantle of planet hunter from the Kepler space telescope to TESS,” Hertz declared.

Planetary exploration is going through a wider-ranging changing of the guard: For example, NASA’s Dawn mission to the dwarf planet Ceres is ending, due to the same empty-tank issue that Kepler faced. And recent glitches with the 28-year-old Hubble Space Telescope and the 19-year-old Chandra X-Ray Observatory have signaled that those grand-scale telescopes are past their prime, mechanically if not scientifically.

When it comes to the planet quest, the next big thing on the horizon is the James Webb Space Telescope, which is currently due for launch in 2021 and may be able to look for signs of life in the atmospheres of alien planets.

Even more planet-hunting telescopes, such as LUVOIR and HabEx, are on the radar screen for the decade ahead.

Meanwhile, Kepler is destined to have all its systems turned off within the next week or two, and will then drift into its own orbit around the sun. That orbit isn’t expected to come any closer than a million miles to Earth, but future space travelers could well put it on their itinerary — if for no other reason than to leave behind an epitaph.

What should that epitaph say? “Basically, Kepler opened the gate for mankind’s exploration of the cosmos,” Borucki said.

Dotson gave Kepler a more personal tribute. “I’ve always felt like it was the little spacecraft that could,” she said. ““It always did everything we asked of it, and sometimes more. That’s a great thing to have in a spacecraft.”

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