NASA resurrects planet-hunting Kepler, replaces broken parts with magical Sun power


Over the past year, we’ve told the sad story of NASA’s Kepler space observatory, and how the failure of two vital components have resulted in the sad and premature retirement of humanity’s best tool for spotting Earth-like planets elsewhere in the universe. Just when all hope seemed lost, though, NASA has approved an ingenious plan to bring Kepler back online — a novel, never-before-seen technique that uses the Sun’s constant bombarding of photons upon Kepler’s solar panels to keep the telescope pointed in the right direction.

How to use sunlight (photon pressure) as Kepler's third reaction wheel

Way back in August, NASA officially gave up on fixing Kepler. Kepler’s primary mission was to focus on a very specific patch of night sky, hunting for planets that orbit some of the 150,000-odd stars in that region. To spot an exoplanet, you need to watch the same star for a long period of time, to monitor every time a planet passes in front of its parent star. To keep a fix on that exact patch of sky, Kepler used four reaction wheels — small flywheels — to precisely position the telescope in all three dimensions. One of these reaction wheels was a redundant backup (you only need three wheels for three dimensions), but unfortunately two of them failed in fairly rapid succession, leaving Kepler — which orbits the Sun at high speed — unable to keep itself focused on a single patch of sky. For the last six months, Kepler has basically been trailing behind Earth, unable to get a fix, and thus unable to provide any scientific data.

The scientific community, of course, was deeply disappointed by Kepler’s premature death just four years into its extended eight-year mission. Kepler’s successor, the Transiting Exoplanet Survey Satellite (TESS), isn’t scheduled to launch until 2017. Now, however, it seems that engineers at Ball Aerospace have devised a novel method of replacing that vital third reaction wheel with sunlight. NASA has already begun testing Ball Aerospace’s technique — dubbed K2 — and the early results are good: Image quality is within 5% of the imagery captured when Kepler was whole.

Kepler's field of view, overlaid on a visible light photo of the Milky Way

Kepler’s original field of view, overlaid on a visible light photo of the Milky Way. This new plan would see Kepler observing four different regions, instead of just one.

This ingenious plan relies on the fact that Kepler has solar panels arranged symmetrically along its long axis. When sunlight from the Sun hits these solar panels, the photons exert a small but significant amount of force (radiation pressure). If Kepler can be oriented so that the photon pressure is evenly distributed (see diagram above), the two working reaction wheels can be used to point the telescope at a fixed point.

This method isn’t without it flaws, though. As Kepler orbits around the Sun, the angle of the sunlight changes, until eventually it hits the side of the spacecraft that isn’t covered in solar panels. To compensate, Bell Aerospace proposes that Kepler’s orbit around the Sun be broken into individual “campaigns,” where it focuses on a single patch of sky for 83 days — and then, after it travels far enough around the Sun, it turns around and looks at another patch of sky. One orbit would be broken into four campaigns of 83 days, with each campaign analyzing a different patch of sky. After a few orbits of the Sun, the Kepler K2 plan would produce enough scientific data to locate some more exoplanets.

Now read: 9 gigapixels, 84 million stars: Peer into the world’s most detailed photo of the Milky Way

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