Kepler, a prolific hunter for other Earths, is suddenly in trouble

The planet-hunting Kepler spacecraft’s dramatic, some would say romantic, quest for an Earth-like exoplanet orbiting a star in its habitable zone has suddenly been put on hold, NASA officials said Tuesday, while engineers try to figure out what caused the craft to lose its ability to point itself at its distant targets.

The spacecraft is essentially hibernating, NASA said at a hastily called briefing late Wednesday afternoon, having put itself into "safe mode" on Sunday after sensing it was pointing in the wrong direction.

Controllers discovered the glitch Tuesday during their twice-a-week touch-base with the craft, currently some 40 million miles away in an Earth-like orbit around the sun.

When the craft is in safe mode, thrusters control its orientation, rather than the fast-spinning “reaction wheels” that normally provide stability. The craft uses these wheels to maintain the exquisite pointing precision needed to relentlessly stare at stars long enough to detect the telltale dimming imposed by a planet as its orbit carries it in front of the star.

As controllers tried to restart the craft's reaction wheels Tuesday, one of the wheels woke up, then balked. This left the craft with two functioning wheels. It needs three to resume observing the patch of sky that for four years it has scanned for Earth-like planets in Earth-like orbits around sun-like stars.

While the malfunction is serious, NASA officials were not ready Wednesday to declare the mission over.

"The loss of the reaction wheel is not good news," said Charles Sobeck, Kepler's deputy project manager at NASA's Ames Research Center in Mountain View, Calif.

The goal now is to place the craft in an operating mode that reduces the use of its thrusters in order to preserve fuel and "take the time to figure out what to do next," he said.

Kepler was launched in March 2009 as a kind of planetary census taker. The mission's aim is to observe the same 170,000 stars in a hunt for rocky planets orbiting in their stars' habitable zones – roughly defined as a distance that leaves a planet's surface not too hot or not too cold, but just right for liquid water to persist on its surface. Liquid water is a key prerequisite for organic life.

To date, Kepler has found Earth-mass planets. And it has found larger, super-Earths orbiting in their star's habitable zones. The Kepler team has yet to uncover its ultimate planets. But after bagging more than 2,700 planet-candidates so far, finding the first "just right" extra-solar planet isn't far off, says William Borucki, the mission's lead scientist.

"I'm absolutely delighted that we've got all this data," he said at the briefing. "The mission was designed for four years. It operated four years. It gave us excellent data for four years. On the other hand, I would have been even happier if it continues another four years."

The discoveries the additional data would have yielded "would have been in some sense frosting on the cake," he acknowledged. "But we have an excellent cake."

If Kepler can't be revived, analyzing the 2,700 planet-candidates in the queue so far will keep the team busy for at least two more years, he noted.

But engineers have some tricks up their sleeves they want to try before they give up and NASA declares the mission finished.

Reaction wheels spin constantly at high rates of speed, so engineers anticipate that they will fail at some point.

One approach would be to command the balky wheel to rotate back and forth to see if whatever is preventing it from operating can be dislodged. If the reaction wheel can't be revived, the team could try to reactivate the wheel they took out of service last year.

If that doesn't work, the team will examine other science missions the craft might be able to perform that don't require the level of precision pointing that planet-hunting does. That means relying on the thrusters more frequently to orient the craft, so remaining fuel supplies become a limiting factor.

If the requisite number of reaction wheels can't return to service, and even if researchers can cobble together other astronomical observations the craft could undertake, that does not guarantee that NASA would extend Kepler’s activities. Any new science plan for Kepler would still have to compete for funding with  other NASA missions being proposed.

Should Wednesday’s announcement truly herald Kepler's imminent demise, the mission's impact will be felt for a long time, researchers say. The mission has been nothing if not an object lesson that if you give smart people the right tools, they can wield those tools in ways few imagined when the tools were first built.

For instance, the craft has measured light from the individual stars in its target patch of sky with enough sensitivity to track the smallest variations in starlight.

Indeed, the craft has delivered such high-quality data on the stars themselves that the mission has been a gold mine for astrophysicists studying star structure and behavior.

Apart from its own discoveries, Kepler data and the operating experience gained in running it for four years can inform future planet-hunting missions.

In 2017, NASA plans to launch another planet-hunting craft, called TESS, that will focus its gaze on the 500,000 stars nearest the sun. These are close enough to allow space-based telescopes such as the James Webb Telescope, currently planned for launch in 2018, to study the atmospheres of planets TESS discovers.

"Kepler's been such a wonderful story," said John Grunsfeld, associate administrator for NASA's Science Mission Directorate and a former shuttle astronaut who helped perform the last set of repairs to the Hubble Space Telescope before the space-shuttle program ended.

"I wouldn't call Kepler down and out just yet," he said.