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  • Writer's pictureSven Piper

Kepler Space Telescope

Updated: 7 days ago

Artistic impression of NASA's planet-hunting Kepler spacecraft / Credit: NASA Ames/JPL-Caltech/T Pyle

The Kepler Space Telescope from NASA was designed to discover Earth-like planets orbiting other stars. The telescope is named after the German astronomer Johannes Kepler (1571-1630), who is best known for Kepler's laws of planetary motion.

Until now, hundreds of extrasolar planets have been discovered. Nevertheless, most of these planets are Jupiter-like gas giants. The detection of terrestrial planets, especially those in the habitable zone of their stars where liquid water and possibly life might exist, is a big challenge. With its space photometer, the telescope was able to monitor fluctuations in the brightness of more than 100,000 stars over a period of 3.5 years to discover small terrestrial planets like Earth or Mars. This is because every small planet changes the brightness of its star when it crosses in front of the star, blocking some of the starlight. This is also known as a Transit. The telescope continuously points at a single star field in the Cygnus-Lyra region.

Kepler was a mission under NASA's Discovery Program (Mission no. 10) of low-cost and was launched on 6 March 2009 with a Delta II rocket. It was named in honor of Johannes Kepler because he was the first person to correctly describe the motions of planets about the Sun. The mission also used Kepler’s third law to determine the size of planetary orbits from the periods discovered by observing repeated transits.

The scientific objectives of the Kepler Mission were:

  • Determine how many terrestrial and larger planets there are in or near the habitable zone of a wide variety of spectral types of stars;

  • Determine the range of sizes and shapes of the orbits of these planets;

  • Estimate how many planets there are in multiple-star systems;

  • Determine the range of orbit size, brightness, size, mass, and density of short-period giant planets;

  • Identify additional members of each discovered planetary system using other techniques;

  • Determine the properties of those stars that harbor planetary systems. [1]

In January 2006, the project was delayed eight months due to budget cuts and consolidation at NASA. It was delayed again by 4 months in March 2006 due to fiscal problems. At this time, the high-gain antenna was changed from a gimballed design to one fixed to the frame of the spacecraft to reduce cost and complexity, at the cost of one observation day per month. [2]

Alan Gloud from the University of California, Berkeley, told us that there was an additional reason. "It reduced the risk of failure of the arm to extend the antenna after launch."


Kepler is not in an Earth orbit but in an Earth-trailing solar orbit so that Earth does not affect the observations and the photometer was not influenced by stray light from Earth. This orbit also avoids gravitational perturbations and torques inherent in an Earth orbit, allowing for a more stable viewing platform.

Missions Details

The spacecraft has a mass of 1,039 kilograms, a primary mirror of 1.4 meter diameter, a 0.95 meter photometer or light meter and a 105 square degrees field of view. Ball Aerospace & Technologies Corp. was responsible for developing the Kepler flight system whereas Corning Incorporated provided the mirror blanks of fused silica for the primary and corrector plate. Brashear LP figured the primary and corrector plate for the Schmidt telescope (the main instrument). The English company E2V delivered the CCDs.

Mission Results:

  • Planets outnumber the stars: Kepler has proven there are more planets than stars in our galaxy.

  • Small planets are common: Kepler has shown us our galaxy is teeming with terrestrial-size worlds; the most recent analysis of Kepler’s discoveries concludes that 20-50% of the stars in the sky are likely to have small, possibly rocky planets similar in size to Earth within the habitable zone of their parent stars.

  • Planets are diverse: Kepler has discovered a diversity of planet types, opening our eyes to new possibilities.

  • Solar systems are diverse too: Kepler found systems with considerably more planets — up to eight — orbiting close to their parent stars.

  • New insights revealed about stars: Besides launching us into the golden age of exoplanets, Kepler has reinvigorated the study of stars. Kepler observed more than a half million stars over the course of its nine years in operation.

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