The Kepler space telescope
NASA telescope that discovered thousands of exoplanets.
Named after the astronomer Johannes Kepler, the Kepler space telescope was launched by NASA in 2009 with the primary goal of discovering Earth-sized planets orbiting other stars. It was part of NASA’s Discovery Program, a series of relatively low-cost science missions. The spacecraft was placed into an Earth-trailing heliocentric orbit, and its sole scientific instrument was a photometer that continuously monitored the brightness of roughly 150,000 main sequence stars in a fixed field of view. By detecting the periodic dimming caused when a planet crosses in front of its host star—a method that only works for orbits seen edge-on from Earth—Kepler aimed to estimate how many of the Milky Way’s billions of stars host Earth-size planets in or near habitable zones. The principal investigator was William J. Borucki. Construction and initial operations were managed by NASA’s Jet Propulsion Laboratory, with Ball Aerospace building the flight system. The project faced delays in 2006 due to budget cuts and fiscal problems, and the high-gain antenna was redesigned to be fixed to the spacecraft frame to reduce cost. The Ames Research Center handled ground system development and mission operations from December 2009. The initial planned lifetime was three and a half years, but greater-than-expected noise from stars and the spacecraft required more time. In 2012, one of four reaction wheels failed; a second failed in May 2013, halting science data collection. NASA abandoned repair efforts in August 2013 but approved a modified mission called K2 in May 2014, which used the remaining two wheels and thrusters to hunt for planets around smaller, dimmer red dwarfs. Over its nine-and-a-half-year service, Kepler observed 530,506 stars and detected 2,778 confirmed planets as of June 2023. Its data also helped scientists study supernovae by providing half-hourly light curves. The telescope was retired on October 30, 2018, after its reaction control system fuel was depleted.
- retired
- October 30, 2018
- field
- Astronomy, exoplanet detection
- agency
- NASA
- principal_investigator
- William J. Borucki
Lore & Background
The Kepler space telescope was developed under NASA's Discovery Program, with construction managed by the Jet Propulsion Laboratory and Ball Aerospace building the flight system. The telescope was placed into an Earth-trailing heliocentric orbit and began surveying a fixed field of view in the Milky Way. After launch, the Ames Research Center handled mission operations and data analysis. The initial planned lifetime was three and a half years, but noise from stars and the spacecraft required additional time. In 2012, one of four reaction wheels failed, and a second failed in May 2013, disabling science data collection. NASA announced on August 15, 2013, that it would not repair the wheels, but the mission was modified into the K2 'Second Light' extension, approved on May 16, 2014, to detect planets around red dwarfs. Kepler's camera had a 94.6-megapixel focal plane made of 42 CCDs, the largest camera system launched into space at the time. Its primary mirror, 1.4 meters in diameter, was made of ultra-low expansion glass and coated with a reflective silver layer. The photometer's performance fell short of design goals, with a median combined differential photometric precision of 29 ppm instead of the target 20 ppm, partly due to higher-than-expected stellar variability.
Reader's Guide
The Kepler space telescope fundamentally changed humanity's understanding of exoplanets by providing the first large-scale census of Earth-sized planets in the Milky Way. Its data enabled estimates that up to 40 billion rocky Earth-size exoplanets may orbit in habitable zones of Sun-like stars and red dwarfs. Kepler also contributed to supernova studies by collecting half-hour light curves. The mission's legacy continues through the TESS telescope, launched in 2018, and through the K2 extension that allowed continued planet hunting after reaction wheel failures. Kepler's discoveries shifted the scientific consensus from uncertainty about planetary systems to a view that planets are common around stars.
Did You Know?
- Kepler was named after Johannes Kepler, who was convinced that planetary systems did not exist around any of the stars.
- The telescope's primary mirror was 1.4 meters in diameter and had a mass only 14% that of a solid mirror of the same size.
- Kepler's camera had a total resolution of 94.6 megapixels, the largest camera system launched into space at the time.
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