News on the exoplanet front has been pretty excellent as of late. NASA's Kepler space telescope has found 132 confirmed exoplanets and a further 2,740 candidates. But the winning streak was broken over the weekend when Kepler suffered a critical hardware malfunction. The failure is certainly putting a damper on the mission, but it doesn’t mean the mission is over--or that we won’t find more exoplanets from Kepler. It just means the mission might be about to change.
Kepler doesn’t find exoplanets directly. It doesn’t look at a star, see the telltale dip in light, and declare “planet, ho!” Rather, Kepler looks at starlight, focussing on one specific star to gather data about the light coming from that star. The information is then sent down to Earth where astronomers “read” it, looking specifically for dips in the star’s brightness, a telltale sign that a planet is crossing between the star and Kepler. The regularity of these dips tell astronomers about the planet’s orbit, and the change of light with every dip gives information on its size and possible type.
Artwork of the Kepler spacecaft showing it's field of view (via)
The hard part for Kepler is staying still enough to capture subtle changes in a star’s light across incredibly large distances--we’re talking lightyears here. If the telescope moves at all, a subtle dip will be lost. To stay steady, Kepler relies on four reaction wheels driven by electric motors powered by the spacecraft’s own power supply. The wheels spin, imparting a physical change on the spacecraft’s orientation (what's know as "attitude") based on the principle of angular momentum transfer and Newton’s Third Law, which states that for every action there is an equal and opposite reaction. The computer knows just where to point Kepler and the wheels spin to get it in the right orientation. And it needs three of these reaction wheels for a pinpoint orientation. Just like pilots use three axes to orient themselves in flight--pitch (nose up and down), yaw (turning left and right), and roll (tilting wings) axes--so does Kepler.
Reaction wheels are common. Mars orbiters use them. The Cassini spacecraft currently orbiting the Saturnian system uses them. And NASA knows they fail. That’s why Kepler launched with four. If one failed, there was a backup. And one did fail. Last July, wheel No. 2 went offline. But the spare was brought online and Kepler's stargazing business proceeded as usual.
What Kepler data--exoplanet Kepler 5-b, specifically--looks like (via)
And here’s where the problem is. Now wheel No. 4 is stuck leaving Kepler with only two axis for orientation. That's not enough to lock on to a target.
The trouble started on Sunday. Kepler’s computer brain noticed it was drifting from its programmed position and immediately went into safe mode. When the Kepler team did their regularly scheduled check-in with the telescope on Tuesday they noticed the problem immediately. They soon realized one of the remaining wheels was stuck. At the moment, the team suspects the culprit is "a structural failure of the wheel bearing."
But the mission isn’t over. Aside from the stuck wheel, the spacecraft is stable and healthy, which means engineers can keep it in safe mode while they work the problem and figure out what the hell to do next. They need to run a series of tests to find out what’s good on the spacecraft and what else, if anything, might be malfunctioning as a result of the stuck wheel. They’re planning to try and unstick the wheel by running it backwards and then back and forth, like a car rocking out of a snowbank. They might try bringing wheel No. 2 back online; it’s a long shot but there’s a chance, somehow, that when they turn it back on after eight months' rest it will spring back to life.
The team needs to see exactly what they’re working with before writing off the mission entirely. Once’s they’ve taken stock of the situation, they’ll start considering their options. If it looks like the wheel is damaged beyond repair and Kepler indeed won’t be able to lock on to one point in the sky for detailed images ever again, they might find a secondary use for the telescope. If everything else is working, it would be in NASA’s best interest to use all the systems they can while they can. But the team can’t even speculate on what those other applications might be before running the necessary tests.
An artist's concept of potential habitable exoplanets as of July 2012 (via)
And unfortunately sending a crew to replace the wheel isn’t an option. Even if we still had the space shuttles up and running, Kepler orbits about 40 million miles away from the Earth; the International Space Station, the shuttle’s main target in its life, is only 200 miles away.
In the meantime, while engineers figure out how to move forward with Kepler, the hunt for exoplanets is still on. There’s a backlog of data downloaded from Kepler, terabytes of data, that the team is confident will keep them and interested citizen scientists busy for at least a decade.
As we wait for news on Kepler fate, it’s worth remembering that Kepler’s been working on borrowed time since its three-and-a-half year mission ended in November. And the mission will never be for naught. Not only has the telescope changed the way we look at the Universe--before it launched on March 7, 2009, we didn’t know there were planets around other stars in the sky. The lessons we've learned from mission will help scientists design better missions in the future. TESS, the Transiting Exoplanet Survey Satellite, JWST, the James Webb Space Telescope, and future exoplanet hunters will be better designed to hone in on the right kind of exoplanets.
And the team remains not only optimistic but thrilled at the mission as it’s happened so far. Kepler was born decades ago as an idea few thought would work and now it’s a worldwide favorite. No matter what happens now, Kepler will always be a success.