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Kepler is a space observatory launched by NASA to discover Earth-like planets orbiting other stars. The spacecraft, named after the Renaissance astronomer Johannes Kepler, was launched on March 7, 2009.

Designed to survey a & portion of our > region of the Milky Way to discover dozens of Earth-size extrasolar planets in or near the habitable zone and < estimate how many of the billions of stars in our galaxy have such planets, Kepler's sole instrument is a photometer that continually monitors the brightness of over 145,000 main sequence stars in a fixed field of view. This data is transmitted to Earth, then analyzed to detect periodic dimming caused by extrasolar planets that cross in front of their host star.

Kepler is part of NASA's Discovery Program of relatively low-cost, focused primary science missions. The telescope's construction and initial operation were managed by NASA's Jet Propulsion Laboratory, with Ball Aerospace responsible for developing the Kepler flight system. The Ames Research Center is responsible for the ground system development, mission operations since December 2009, and science data analysis. The initial planned lifetime was 3.5 years, but greater-than-expected noise in the data, from both the stars and the spacecraft, meant additional time was needed to fulfill all mission goals. Initially, in 2012, the mission was expected to last until 2016, but this would only have been possible if all remaining reaction wheels used for pointing the spacecraft remained reliable. On May 11, 2013, a second of four reaction wheels failed, disabling the collection of science data and threatening the continuation of the mission.

, Kepler and its follow-up observations had found 977 confirmed exoplanets in more than 400 stellar systems, along with a further 3,277 unconfirmed planet candidates. In November 2013, astronomers reported, based on Kepler space mission data, that there could be as many as 40 billion Earth-sized planets orbiting in the habitable zones of sun-like stars and red dwarf stars within the Milky Way Galaxy. 11 billion of these estimated planets may be orbiting sun-like stars. The nearest such planet may be 12 light-years away, according to the scientists.

On August 15, 2013, NASA announced that they had given up trying to fix the two failed reaction wheels. This meant the current mission needed to be modified, but it did not necessarily mean the end of planet-hunting. NASA had asked the space science community to propose alternative mission plans "potentially including an exoplanet search, using the remaining two good reaction wheels and thrusters". On November 18, 2013, the K2 (also named "Second Light") plan proposal, which would include utilizing the disabled Kepler in a way that could detect habitable planets around smaller, dimmer red dwarf stars, was reported. On May 16, 2014, NASA announced the approval of extending the Kepler mission to the K2 mission.

Spacecraft
The spacecraft has a mass of 1039 kg and contains a 1.4 m primary mirror feeding an aperture of 0.95 m – at the time of its launch this was the largest mirror on any telescope outside Earth orbit. The spacecraft has a 115 deg2 (about 12-degree diameter) field of view (FOV), roughly equivalent to the size of one's fist held at arm's length. Of this, 105 deg2 is of science quality, with less than 11% vignetting. The photometer has a soft focus to provide excellent photometry, rather than sharp images. The mission goal is a combined differential photometric precision (CDPP) of 20 ppm for a m(V)=12 solar-like star for a 6.5-hour integration, though the observations so far have fallen short of this objective (see mission status). An Earth-like transit produces a brightness change of 84 ppm and lasts for thirteen hours when it crosses the center of the star.

Camera
The focal plane of the spacecraft's camera is made up of 42 CCDs at 2200x1024 pixels, which made it at the time the largest camera yet launched into space, possessing a total resolution of 95 megapixels. The array is cooled by heat pipes connected to an external radiator. The CCDs are read out every six seconds (to limit saturation) and co-added on board for 58.89 seconds for short cadence targets, and 1765.5 seconds (29.4 minutes) for long cadence targets. Due to the larger bandwidth requirements for the former, these are limited in number to 512 compared to ~160,000 for long cadence. However, even though at launch Kepler had the highest data rate of any NASA mission, the 29-minute sums of all 95 million pixels constitute more data than can be stored and sent back to Earth. Therefore the science team has pre-selected the relevant pixels associated with each star of interest, amounting to about 6 percent of the pixels (5.4 megapixels). The data from these pixels is then requantized, compressed and stored, along with other auxiliary data, in the on-board 16 gigabyte solid-state recorder. Data that is stored and downlinked includes science stars, p-mode stars, smear, black level, background and full field-of-view images.

Second Light (K2)
In November 2013, a newly proposed mission plan, initially named "K2" (also called "Second Light"), was presented for consideration. K2 would involve using Kepler's remaining capability, photometric precision of about 300 parts per million, compared with about 20 parts per million earlier, to collect data for the study of "supernova explosions, star formation and solar-system bodies such as asteroids and comets, ... " and for finding and studying more exoplanets. In this proposed mission plan, Kepler would search a much larger area in the plane of earth's orbit around the sun.

In early 2014, the spacecraft underwent successful testing for the K2 mission. From March to May 2014, data from a new field called Field 0 was collected as a testing run. On May 16, 2014, NASA announced the approval of extending the Kepler mission to the K2 mission. Kepler's photometric precision for the K2 mission is estimated to be 50 ppm on a magnitude 12 star for a 6.5 hour integration. In February 2014, photometric precision for the K2 mission using two-wheel, fine-point precision operations was measured as 44 ppm on magnitude 12 stars for a 6.5 hour integration. The analysis of these measurements posted by NASA stated that the measurements "suggest" the K2 photometric precision "approaches" that of the Kepler archive of three-wheel, fine-point precision data.

Field 1 of the K2 mission is set towards the Leo-Virgo region of the sky and Field 2 is towards the "head" area of Scorpius and includes two globular clusters: Messier 4 and Messier 80 and part of the Scorpius–Centaurus Association which is only about 11 million years old and at a distance of 380-470 light years with probably over 1000 members.

Data releases
The Kepler team originally promised to release data within one year of observations. However, this plan was changed after launch, with data being scheduled for release up to three years after its collection. This resulted in considerable criticism,    leading the Kepler science team to release the third quarter of their data one year and nine months after collection. The data through September 2010 (quarters 4, 5, and 6) was made public in January 2012.