The Kepler Space Telescope, launched March 6, 2009, was designed to search the Milky Way for exoplanets. Using the transit method, Kepler ultimately observed over 500,00 stars and contributed to the discovery of almost 2,700 exoplanets.
Though Kepler was originally intended for a 3.5-year mission, NASA announced on Nov. 14, 2012, that the space telescope had successfully completed its primary mission and would be embarking on an extended mission. When a second gyroscope (also referred to as a reaction wheel) failed in May 2013 – the first having failed in July 2012 – that mission was in question, however, as it left the spacecraft with limited control for pointing. But engineers were able to craft a workaround using the momentum of the Sun’s light pressure to keep Kepler evenly and steadily directed, and the extended mission using this methodology, dubbed K2, was formally approved in May 2014. Kepler continued to function for a further four years, and was retired in October 2018 when it ran out of fuel. TESS (the Transiting Exoplanet Survey Satellite) has since inherited its planet-hunting mantle.
