Image credit: NASA/NASA Ames/Kepler Team
There has been much talk about Kepler’s reaction wheels over the past year, when in July 2012 one of Kepler’s four spinning reaction wheels (wheel 2) failed. Kepler uses these wheels to precisely point the spacecraft so that the stars it is monitoring stay nearly at the same positions on Kepler’s imaging plane in order to achieve the ~30 ppm photometric precision required to detect Earth-sized planets transiting Sun-like stars. Kepler’s thrusters used for coarse adjustments are unable to provide that kind of sensitive nudging.
Kepler only needs 3 reaction wheels to successfully keep pointing for the exoplanet observations, so with 1 out of 4 wheels non-functional, observations could continue. It was known at that point that another reaction wheel (wheel 4) was already acting up in similar ways to the failed wheel 2. It was unknown at that point how long the wheel would last. It could be days or months or years, and NASA was investigating ways and implementing strategies in attempts to prolong the lifetime of Kepler’s remaining working reaction wheels.
In May of this year, after 4 years of light curves, Wheel 4 failed halting the exoplanet observations in the beginning of Quarter 17. (Read Chris’ take on Wheel 4’s failure). Kepler was placed in a configuration to preserve fuel while NASA explored ways of reviving one of the 2 broken wheels. After engineering tests, NASA announced two weeks ago that the failed reaction wheels are unrecoverable. The spacecraft is in an Earth-trailing orbit, not reachable for an astronaut servicing mission. This means the end of Kepler’s exoplanet transit observations, and NASA is exploring alternative observations that Kepler could be used for (like looking for Near Earth Asteroids).
This is the end of the Kepler’s exoplanet transit observations, but this in many ways just the beginning of the mission’s next phase as the focus shifts solely to analysis of the data that has been collected and thinking of new ways of processing the existing observations to find smaller and smaller planets. The mission is far from over. Although there will be no more light curves coming from Kepler, there are still many discoveries yet to be made and science to do. There are still ~ 2 years of worth of Kepler observations already on the ground that the Kepler team and the astronomical community have yet to fully analyze. To fully search and analyze all the Kepler data will take at least another several years, keeping astronomers and citizen scientists busy until the launch of TESS. Kepler has revolutionized the field of exoplanets and will continue to do so for a long time to come.
What does the news about Kepler mean specifically for Planet Hunters? At Planet Hunters, we have only searched a small fraction of the Kepler quarters. In addition new and better data reduction techniques have been implemented by the Kepler team to improve the quality of the Kepler light curves and help reveal planets that may been invisible previously. We plan to search all four years of the newly reprocessed light curves with Planet Hunters. We need your help more than ever. We’ll be serving light curves (with previously unknown planet transits likely lurking in the dataset) needing classifications for a long time to come!
So join us today, as one phase of the Kepler era ends and the next one begins. Help continue the exoplanet search today at http://www.planethunters.org