page_title: Kepler space telescope
section_title: Orbit and orientation
hierarchical_section_title: Kepler space telescope / Spacecraft design / Orbit and orientation
caption_attribution_description: Painting of Milky Way galaxy used as background for diagram of Kepler Mission search space.
context_page_description: The Kepler space telescope is a retired space telescope launched by NASA to discover Earth-size planets orbiting other stars. Named after astronomer Johannes Kepler, the spacecraft was launched on March 7, 2009, into an Earth-trailing heliocentric orbit. The principal investigator was William J. Borucki. After nine years of operation, the telescope's reaction control system fuel was depleted, and NASA announced its retirement on October 30, 2018.
  Designed to survey a portion of Earth's region of the Milky Way to discover Earth-size exoplanets in or near habitable zones and estimate how many of the billions of stars in the Milky Way have such planets, Kepler's sole scientific instrument is a photometer that continually monitored the brightness of approximately 150,000 main sequence stars in a fixed field of view. These data are transmitted to Earth, then analyzed to detect periodic dimming caused by exoplanets that cross in front of their host star. Only planets whose orbits are seen edge-on from Earth can be detected. During its over nine and a half years of service, Kepler observed 530,506 stars and detected 2,662 planets.
context_section_description: Kepler orbits the Sun, which avoids Earth occultations, stray light, and gravitational perturbations and torques inherent in an Earth orbit.
  NASA has characterized Kepler's orbit as "Earth-trailing". With an orbital period of 372.5 days, Kepler is slowly falling farther behind Earth (about 16 million miles per annum). As of May 1, 2018, the distance to Kepler from Earth was about 0.917 AU (137 million km).
  This means that after about 26 years Kepler will reach the other side of the Sun and will get back to the neighborhood of the Earth after 51 years.
  Until 2013 the photometer pointed to a field in the northern constellations of Cygnus, Lyra and Draco, which is well out of the ecliptic plane, so that sunlight never enters the photometer as the spacecraft orbits. This is also the direction of the Solar System's motion around the center of the galaxy. Thus, the stars which Kepler observed are roughly the same distance from the galactic center as the Solar System, and also close to the galactic plane. This fact is important if position in the galaxy is related to habitability, as suggested by the Rare Earth hypothesis.
  Orientation is 3-axis stabilised by sensing rotations using fine-guidance sensors located on the instrument focal plane (instead of rate sensing gyroscopes, e.g. as used on Hubble). and using reaction wheels and hydrazine thrusters to control the orientation.
caption: Kepler's search volume, in the context of the Milky Way
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