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Kepler: Back from the Dead

This is a guest post from my graduate student, Andrew Vanderburg. Over the past few months, Andrew has been focusing his attention on data collected by the crippled Kepler Space Telescope.  After being disabled in May of last year by the failure of the second of two reaction wheels used to point itself, Kepler has been given new life thanks to some brilliant work done by Ball Aerospace and the Kepler team. Here's my previous post about the K2 Extended Mission. 

Below, Andrew describes his work, which is documented in a recently accepted paper available here and which has recently been incorporated into the Kepler team's guest observer tools. Also, be sure to check out his website, where you can access corrected K2 data from an engineering test conducted in February on this interface. Who knows, you might even find a transiting planet!

Since its launch in 2009, the Kepler Space Telescope has revolutionized the field of exoplanetary science with the discovery of thousands of planet candidates, many of which are smaller than the Earth. Kepler’s science operations were prematurely halted, however, when the spacecraft was disabled in May of 2013 by the failure of the second of four reaction wheels used to point and stabilize the telescope. Because Kepler’s scientific punch came from its high precision enabled by its fine pointing control, many people assumed that Kepler’s exoplanet discovering days were over. 

Fortunately, the Kepler team and Ball Aerospace thought otherwise. Over the next six months after the failure of Kepler’s second reaction wheel, they devised a way to control Kepler with only two reaction wheels, balancing the spacecraft against the constant stream of photons and particles being ejected from the Sun, and correcting any imbalances with very precise burns of Kepler’s thrusters. Their brilliant work has led to the new (and recently approved) K2 mission, in which Kepler looks in new fields, moving every 75 days to look at a completely new set of stars, to search for new planets.

Graphic from the Kepler/K2 team describing the K2 mission strategy

One of the biggest uncertainties about the K2 mission was: “How well can Kepler measure photometry in this new operating mode?” If Kepler’s worsened ability to point itself degrades the quality of its data, it may be harder for the K2 mission to accomplish its goals of finding exoplanets in new environments and around different types of stars. When the Kepler team released data from a 9 day engineering test of the new operation mode taken in February, we attempted to answer that question. 

After four years of being spoiled by ultra-high-quality photometry from Kepler, our first look at the K2 data came as a bit of a shock. Unlike the pristine Kepler data, K2 data (shown below compared to Kepler in the first plot) had wild jagged features contaminating the light curve, which made it hard to see all but the deepest planet transits. In order to continue searching for small planets in the K2 mission, something would have to be done to improve the quality of the photometry.
Comparison of Kepler (bottom) and K2 (top). Raw K2 data is much noisier than Kepler data.
Fortunately, it turned that there was a way to improve the quality of K2 data. The additional noise in the data was caused by the spacecraft moving back and forth ever so slightly as it rolled due to a slight imbalance between the spacecraft and the Solar wind. Every six hours or so, Kepler’s thrusters fired to bring the telescope back to its original position. We found that even though raw K2 photometry was noisy, it was noisy in a predictable and consistent way, which meant there was a way to improve it. 

We did this by comparing the star’s brightness measured by Kepler at every position during its roll to other measurements taken nearby. When we corrected K2 data using other measurements taken nearby, we found that the quality of data was greatly improved, as we show in the image below. Overall we were able to improve raw K2 data by a factor of 2-5, and got back to within at least factor of 2 of Kepler -- for stars of a particular brightness between 12th and 13th magnitude, K2 performed with 35% of Kepler’s precision. K2 should be able to continue hunting for small exoplanets and doing impactful science even without two of its reaction wheels. 


Correcting the light curve based on the motion of the spacecraft substantially improves K2 data.

We have released our processed K2 data to the community and built a web interface to easily view and explore it. We encourage the community to take a look, explore, and learn the quirks of K2 data before the real science begins with data from the first K2 campaign fields. To learn more about our technique, download our paper describing the technique here.

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