The loss of a flywheel-like device to ensure precise pointing means that NASA's planet-hunting telescope has perhaps made its last observations.

Kepler in space

An artist's impression of the Kepler spacecraft, which is basically a wide-field photometer fed by a 37-inch (0.95-m) telescope.

NASA / Ames Research Center

After rocketing into deep space and passing its initial tests, the Kepler telescope began its search for planets around other stars on May 12, 2009.

Engineers designed the spacecraft to last four years, and apparently the warranty has expired. Last Sunday, four years to the day after science observations began, a component critical to pointing the spacecraft failed. Kepler placed itself in safe mode, a kind of electronic hibernation, and chances are not good that it can resume its work.

As deputy project manager Charles Sobeck explained during a NASA briefing today, ground controllers learned yesterday that one of the craft's four reaction-control wheels (RCWs) had failed — the same one that showed worrying symptoms of excess friction back in January. At that time engineers shut it down for 10 days in the hope that "rest" would extends its life.

But the problem continued, and even though the spacecraft resumed science operations the mission personnel knew they were on borrowed time. "We've been anticipating this for a while, unfortunately," notes John Grunsfeld, the ex-astronaut who now heads NASA's Science Mission Directorate.

Kepler's primary mirror in cleanroom

The honeycombed blank for Kepler's primary mirror, 36 inches (95 cm) in aperture. The spacecraft's telescope feeds a detector array that will watch an estimated 100,000 stars in a field covering 105 square degrees.

Ball Aerospace

On paper, NASA's Kepler spacecraft has done exactly what it was supposed to do. The task is conceptually simple: use a big telescope (95 cm in aperture) to stare at roughly 150,000 solar-type stars all at once. If a star's brightness displayed periodic dims, chances are good that the brief dips are caused by a planet crossing in front of the star's disk.

RCWs are essentially precision flywheels that maintain the telescope's rock-solid pointing on the target field, a generous 115 square degrees near the Cygnus-Lyra border. Kepler needs three wheels to maintain attitude, but its only spare was lost last July. Now, with a second failure, only two RCWs are working, and observations have ground to a halt. Unless engineers come up with some mechanical magic — and it might take months to exhaust all the possible options — Kepler's planet-hunting days are over.

Kepler's celestial targets

The fields of view of Kepler's 42 detectors sample a rich area of the Milky Way where mission scientists hope to monitor the light from 100,000 stars to distances of 3,000 light-years. Click here to see a larger version.

NASA / Carter Roberts

In the meantime, the spacecraft has been placed in a fuel-saving mode that relies on brief occasional thruster firings to keep its solar-cell arrays pointed toward the Sun and its antenna toward Earth. The spacecraft travels around the Sun in an Earth-trailing orbit and is currently some 40 million miles (65 million km) away. "Kepler's not in a place where I can go rescue it," laments Grunsfeld, a member of two Space Shuttle servicing missions to the Hubble Space Telescope.

"I am just devastated. My hands are trembling, and my heart is aching," exoplanet hunter Geoffrey Marcy (University of California, Berkeley) said after hearing the news. But sifting through the existing observations will continue, he adds. "We will be able to detect Earth-size planets just inward of the habitable zone, and also planets a bit larger than Earth within the habitable zone. We will be working seven days a week, day, evenings, and weekends to extract the Earths from the existing data. Still I'm so sad."

One person who clearly hasn't written off Kepler is William Borucki, who championed the mission concept for decades before NASA finally gave it a green light and tapped him to be its principal investigator. "There's a reasonable possibility that we can mitigate the problem," he insists. "I don't want to be a pessimist here."

Even if the spacecraft stops sending observations back to Earth, the team still has plenty to chew on. Borucki estimates that it will take two years to comb through all the archived data, and buried in those are almost certainly transits from exoplanets with years-long orbits around their host stars. These are precisely the kinds of worlds, situated in "habitable zones" with moderate temperatures, that would be most conducive to life.

Kepler's planet-candidate tally

The distribution of candidate planets discovered in Kepler data, as of January 7, 2013. Since its previous release in February 2012, the Kepler census has increased by 20% and now totals 2,740 potential planets orbiting 2,036 stars. Click here for a larger version and a more complete explanation.


Just last month astronomers announced Kepler's discovery of a five-planet system that includes a "hot Mars" and four "super-Earths", two of which have temperatures that might be right for liquid water.

"We're really pretty positive that we'll find earthlike planets in habitable zones around stars like the Sun," Borucki said today. "The most interesting and exciting discoveries are coming in the next two years — the mission is not over."

As I noted in mid-2011, Kepler ran into an unexpected complication because its target stars proved "noisier" (less stable in brightness) than predicted. This meant that the spacecraft would have to amass brightness data for more than more than 3½ years — the $600 million mission's planned duration — to coax out the very slight dimmings (less than 0.01%) produced by transits of Earth-size worlds. A longer timeline became crucial to achieving the mission's goal of finding habitable, Earth-size worlds, and in 2012 NASA managers obliged by approving a four-year mission extension.

So far, Kepler has identified more than 2,700 candidate planets using this transit method. Follow-up observations show that 130 of these really are planets (as opposed to false alarms of some kind), and statistically most all of the remaining candidates will likewise be confirmed. Another major achievement, though less well known, is the revolution that Kepler has triggered in the nature of the stars themselves.




Image of Bruce


May 16, 2013 at 8:25 am

This is sad, though not unexpected news. IMO Kepler has been one of the most interesting/important NASA missions since the Apollo Moon landings. And Kepler was just beginning to generate a long enough baseline of data to begin finding planets in earthlike orbits too. What a bummer. A commenter (Mark) at another site (APOD) called this process of Kepler’s reaction wheel failures “succumbing to entropy”. I feel like I am too. I need to go take a pain reliever.

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May 17, 2013 at 5:50 pm

2 thoughts:

1) Replacement mission.

2) Suggestion for NASA - The capability to get to equipment (like we did for Hubble) in this type of solar orbit, or at earth's trojan points, might be worth the effort. And I'd be glad to pay a little more Fed tax to make it so, in addition to the near earth asteroid missions, and the potential impactor projects.

And thanks for all your efforts to date.

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May 21, 2013 at 6:04 pm

Good ideas, Roger. Let me suggest a third: future craft should have some minor rocket capability. Then they could slowly return near Earth, and the expensive air-breathers (us) could have a shorter and cheaper mission.

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May 22, 2013 at 2:19 pm

Feeling better now, thanks to forward thinking. So what’s next, after Kepler? I couldn’t find any evidence that NASA has any Kepler2 type missions as Roger and Richard discussed, but I did discover an upcoming MIT – NASA mission called TESS (Transiting Exoplanet Survey Satellite) that has been scheduled for launch in 2017. This mission has several advantages over Kepler: It is designed to be a whole sky survey equipped with four wide field telescopes. It is to investigate, not a measly 156,000 dim and distant suns, but a whopping 2 million stars brighter than 12th magnitude, so its looking at virtually all the closest stars. It’s claimed that TESS will be able to detect light dips down to only 40 parts per million, which is less than half of what an earth-sized planet would dim a sun-sized star by. Funding for this mission has been largely from the private sector too, so NASA is getting a bargain. Good news of glad tidings, I’d say.

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Robert Simpson

May 27, 2013 at 9:52 am

The mirror blank in the second photo looks a lot larger than "36.

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