The Kepler spacecraft

Spacecraft

A spacecraft or spaceship is a craft or machine designed for spaceflight. Spacecraft are used for a variety of purposes, including communications, earth observation, meteorology, navigation, planetary exploration and transportation of humans and cargo....

 is an American space observatory

Space observatory

A space observatory is any instrument in outer space which is used for observation of distant planets, galaxies, and other outer space objects...

, the space-based portion of NASA

NASA

The National Aeronautics and Space Administration is the agency of the United States government that is responsible for the nation's civilian space program and for aeronautics and aerospace research...

's Kepler Mission to discover Earth-like planets

Terrestrial planet

A terrestrial planet, telluric planet or rocky planet is a planet that is composed primarily of silicate rocks or metals. Within the Solar System, the terrestrial planets are the inner planets closest to the Sun...

 orbiting other stars. The spacecraft is named in honor of the 17th-century German astronomer Johannes Kepler

Johannes Kepler

Johannes Kepler was a German mathematician, astronomer and astrologer. A key figure in the 17th century scientific revolution, he is best known for his eponymous laws of planetary motion, codified by later astronomers, based on his works Astronomia nova, Harmonices Mundi, and Epitome of Copernican...

. The spacecraft was launched on March 7, 2009, with a planned mission lifetime of at least 3.5 years.

The Kepler mission is "specifically designed to survey a portion of our region of the Milky Way galaxy to discover dozens of Earth-size planets in or near the habitable zone

Habitable zone

In astronomy and astrobiology, a habitable zone is an umbrella term for regions that are considered favourable to life. The concept is inferred from the empirical study of conditions favourable for Life on Earth...

 and determine how many of the billions of stars in our galaxy have such planets." Kepler's only instrument is a photometer

Photometer

In its widest sense, a photometer is an instrument for measuring light intensity or optical properties of solutions or surfaces. Photometers are used to measure:*Illuminance*Irradiance*Light absorption*Scattering of light*Reflection of light*Fluorescence...

 that continuously monitors the brightness of over 145,000 main sequence stars in a fixed field of view.