Kepler Mission photometer. Cutaway artwork showing the internal workings of the 0.95-metre diameter photometer that is being used by NASA's Kepler Mission to discover Earth-like planets orbiting other stars. The photometer will scan 100,000 stars in the Milky Way for evidence of Earth-sized terrestrial planets that transit the star's path. The mission was launched on March 7th, 2009, and will orbit the Sun for at least 3.5 years. The mission is named in honour of German astronomer Johannes Kepler.