The Kepler Spacecraft and Its Legacy

Kepler has generated an abundance of data since its launch ten years ago. Although originally designed to observe 160,000 stars across one patch of sky, the photometer has since been used to uncover thousands of planets.

Discovering extrasolar planets was done through measuring their slight drop in brightness when they passed in front of their star. With its 372.5-day orbital period and trailing Earth, occultations or distortion from atmospheric interference was avoided.

The Mission

Kepler conducted an expansive survey of our Milky Way region in search of Earth-like planets in or near their stars’ habitable zones (HZs), and estimated how many stars might harbor such planets. Its sole scientific instrument was a photometer which continuously measured brightness from 150,000 main sequence stars within its field of view.

The photometer observed tiny dips in starlight caused when planets pass in front of their host stars and block some of its light. To keep its thrusters working effectively and to move in new fields of view, the spacecraft required three gallons of hydrazine fuel for thruster powering.

Engineers devised an ingenious solution when two reaction wheels of K1 telescope failed in 2013: they rebooted it as K2, continuing its original mission of planet hunting while broadening its focus to include objects like supermassive black holes at the heart of galaxies and distant supernovae.


Kepler’s photometer was designed to detect small dips in brightness caused by planets passing in front of their host stars and blocking some or all of their light, using 42 charge coupled device (CCD) image sensors covering an 83 day field in space.

The spacecraft targeted a patch of the Milky Way galaxy that was far enough away that sunlight wouldn’t disrupt measurements taken with a photometer, such as Cygnus, Lyra and Draco constellations located outside our solar system’s orbital plane.

Over time, one of the four reaction wheels keeping the spacecraft pointed toward its intended patch of sky failed; however, with help from three functioning reaction wheels and an effective observing strategy the mission continued until May 2013 when another failed causing it to begin tumbling and point at different spots across the sky.


Kepler left behind an incredible legacy when it completed its long mission, including over 2,600 planet discoveries beyond our Solar System – some which could offer potential habitats for life.

Sensitive to variations in brightness of stars, its telescope detects transits of exoplanets as small dips in starlight when an exoplanet passes in front of its parent star. Furthermore, this telescope mapped asteroid clusters, studied sound waves inside stars, and tracked supernova explosions.

Kepler stands out as an exception among space missions exploring Solar System bodies, which typically end in controlled impacts with the objects they explore. Kepler was designed with an extended lifespan in mind; its orbit will continue producing scientific observations for years to come. Indeed, so important has been Kepler’s legacy that NASA recently launched the Transiting Exoplanet Survey Satellite (TESS), to build upon it and search all stars within 100 light-years of Earth with TESS, unlike Kepler.


Kepler revolutionized how astronomers view our solar system. Scientists now understand that planets are common even among Sun-like stars, and that there may be Earth-like planets in other star systems.

Astronomers have discovered that planet systems containing planets smaller than Neptune (about four times larger than Earth) are far more common than expected, and more likely to harbor habitable environments than previously expected. In February 2014, using a technique called “verification by multiplicity,” they released 715 confirmed planets all at once for announcement – the highest total ever released all at once!

After experiencing an anomaly that caused one of its reaction wheels to stop functioning in May 2013, NASA engineers determined it no longer viable to repair the spacecraft. Since then, engineers have placed it into a stable point rest state and continue monitoring by NASA; subsequent missions will build on what the mission accomplished such as Transiting Exoplanet Survey Satellite (TESS), while James Webb Space Telescope will explore space in another manner altogether.

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