Issue 12 - October 2013
By Steve Howell and Nick Gautier In May 2013, the Kepler spacecraft suffered its second failure of a momentum wheel. Kepler uses momentum wheels to point itself accurately and stably to obtain the extremely precise brightness measurements of stars that have allowed the spacecraft to detect small planets around the target stars in its exoplanet survey. Kepler started its mission with four operational momentum wheels and requires three operational wheels to maintain stable pointing. With the failure of a second wheel on Kepler, the existing pointing-system software can no longer control the spacecraft as before, thus ending the ability to search for Earth-size planets orbiting Sun-like stars. For now, science data collection has stopped and the Kepler spacecraft has been placed in a fuel-efficient safe mode while we decide what to do next. All of the science data taken before the second wheel failure have been sent back to Earth, giving a full four years of exoplanet survey data. The first three years of survey data have been thoroughly analyzed and searched for planets. Analysis is in progress of the fourth year of data and comprehensive analysis of all four years together continues. We expect to discover many new planets in these data, perhaps even a few Earth-analogs -- Earth-size planets in the habitable zones of stars like the Sun. While the existing pointing-control software cannot control the Kepler spacecraft with only two momentum wheels, it has become clear that modest modifications to the software will allow control of the spacecraft pointing with two wheels. The pointing stability with two wheels, as well as a number of other factors, is not expected to provide sufficient photometric precision and high-impact science benefit to continue to pursue the original exoplanet survey. However, the stability is expected to be good enough to continue to do important and exciting scientific observations. The Kepler Project, Ball Aerospace (which operates the spacecraft), and NASA are actively working on planning an exciting science mission that can be enabled by a two-wheel repurposed Kepler mission. Our plans for a two-wheel science mission will be submitted to NASA review in early 2014. If this initial review by NASA is satisfactory, Kepler will submit a formal proposal for a 2 wheel mission to the Senior Review Panel for Operating Missions asking for funding and approval to execute the mission.