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K2 – Kepler’s (Proposed) 2nd Exoplanet Mission


K2 mountain, the 2nd highest peak in the world Image credit: Wikipedia (

In May, Kepler lost its 2nd reaction wheel halting continuing observations of the Kepler field and its original exoplanet mission.  Later this summer, NASA announced that attempts to revive the broken reaction wheels had failed, and that was the official end to the observations of the Kepler field with the 30 ppm (parts per million) precision obtained with the 3-wheel pointing. Other than the bum leg, the spacecraft and imager were in good condition. NASA issued a call for white papers soliciting ideas for potential use cases for a 2-wheeled Kepler.

It was announced two weeks ago at the 2nd Kepler Science Conference, that the Kepler team has a plan to return Kepler to exoplanet hunting that they will be proposing to NASA to give the go ahead and fund. They are dubbing this new 2-wheeled mission for Kepler ‘K2’. You might be inclined to call ‘K2’  Zombie Kepler, but in reality Kepler hasn’t gone anywhere. After the 2nd reaction wheel failure, the spacecraft has just been resting while NASA and Ball Aerospace engineers have been working on ways they could use the remaining two reaction wheels and thrusters to do exoplanet science. In the past few weeks Kepler has been taking engineering data to test stability and photometric precision in this K2 pointing/observing scheme.

Here’s how K2 works. With the loss of the 3rd reaction wheel, Kepler lost fine tuning in one of three spatial directions. If Kepler is  pointed keeping the Sun in the X-Y plane, there’s a pointing ridge where they can balance the spacecraft and use the remaining wheels and thrusters to keep pointing. That means fields have to be in the ecliptic (plane of the Earth’s orbit). With that pointing  very little changes to the current Kepler team data pipeline are needed to produce light curves with the same 29.4 minute cadence. The photometric precision is predicted to be better than 300 ppm (measured from preliminary engineering runs and testing). So there is  a loss of sensitivity, 3-wheeled Kepler had 30 ppm photometric precision. Still Kepler can detect giant planets and for both bright and small stars, Kepler can detect rocky planets.

Kepler will not be able to stare at any one field for very long. The fields on the ecliptic would change as the Kepler orbits the Sun. Each field would get ~40 days worth of observations with some craft pointings able to extend the baseline to ~70-80 days.  Also the number of pixels per star needs to be increased significantly, and since Kepler has a limited memory on board to store all of this data, the number of stars observed needs to significantly decreased from the over 160,000 stars  monitored when observing the Kepler field. 10,000-20,000 stars would be monitored in K2. The K2 data, like that in the Kepler extended mission, would be available to the public and scientific community after it was downloaded and reduced.

The exciting prospect is that Kepler would observe different populations of stars than the Kepler field, which will be interesting to see how the frequencies of planets compares to the Kepler field.  Not to forget, the prospect of having many many more new planets/planet candidates  to characterize and study. There will be also be observing of cooler M dwarf stars where the habitable zone (the goldilocks region where water might exist on a rocky planet’s surface) is close to the star, and thus Kepler will  find many more rocky planets in the habitable zone to further study.  Also bright stars will be targeted which will enable ground-based follow-up with the radial velocity technique, which for gas giant planets can actually measure masses and confirm these planets. Also there’s a wealth of stellar astrophysics (and even potentially microlensing monitoring ) that could be done with K2, including observing stars in open clusters (conglomeration of stars loosely bound together that were all formed from the same molecular cloud) where we know their ages.

K2 is a mission concept at this point. The Kepler team is working hard, and has achieved or on track to finish software upgrades needed to enable Kepler to point and track on ecliptic fields for K2. Test data is starting to coming down from the spacecraft. The Kepler scientists and engineers are analyzing the data and assessing the data quality from K2-like observing.  In the next few weeks the Kepler team will propose K2 to NASA, in December NASA will decide if K2 is viable and then give the go ahead for the Kepler team to propose for K2 in the senior review in April, where Kepler as well as other NASA missions will be examined and funding will be decided. Let us hope that K2 gets the full go ahead  with (cross our fingers) observations starting some time in 2014.

The prospects for K2 are exciting, and I hope the missions gets the green light.  I think the place for Planet Hunters in the K2 era is interesting.  I think there will be a niche for Planet Hunters, especially with the short time span on each field, identifying single transits will be important for follow-up of the planetary systems discovered. There will be new eclipsing binaries monitored, and the prospect for more circumbinary planets which is also where I can see Planet Hunters contributing. Plus don’t forget the unexpected discoveries waiting to be found, as we’ve learned from the Kepler field data. So bring on K2!

Want to know more about the proposed K2 mission?  Watch the 2nd Kepler Science Conference talks by Charlie Sobeck and Steve Howell detailing the specifics about K2.

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