image courtesy NASA/Kepler
In May 2013, after four years of very successful planet-hunting, the Kepler space telescope suddenly went into safe mode. Engineers found that Reaction Wheel #4 had failed. Combined with the earlier failure of another reaction wheel, this meant that 2 out of 4 of its reaction wheels were out of commission. Kepler needed a minimum of three wheels to maintain its high precision pointing accuracy and continue its original mission. Could Kepler continue to perform science in this degraded state?
Reaction wheels are gyroscopes which keep spacecraft stable by rapidly spinning to maintain the spacecraft’s position. Using reaction wheels offers the spacecraft's operators very fine control over the spacecraft's position and movements without having to use up precious fuel to fire thrusters. In the case of the Hubble Space Telescope, failed reaction wheels were replaced on service missions by space shuttle astronauts. However, Kepler is much to far away from Earth to allow such a servicing mission. Once the reaction wheels fail, they cannot be replaced by a visiting astronaut.
Image of Kepler, showing the locations of various components, including the reaction wheels.
Credit: Ball Aerospace/NASA
Kepler engineers figured out a way to keep the craft pointed in one spot for months at a time by using its last two reaction wheels plus the pressure from the Sun's radiation. This is actually one of the oldest tricks in the spacecraft operator's book: NASA had used the Sun's radiation pressure to help stabilize the Mariner 10 probe on it's mission to study Venus and Mercury, and the Japanese space agency (JAXA) used radiation pressure to steady the Hayabusa probe when its reaction wheels failed as well. However, due to constraints with this new solar-wind balancing method, Kepler's operators could no longer keep the spacecraft pointed at its original target.
In June of 2014, the K2 mission became fully operational. K2 repurposed the Kepler spacecraft and its incredible light sensitivity to survey the sky near the galactic equator using its excellent photometer. Due to the way Kepler needs to orient itself to use the solar wind to help remain on target, this survey will focus on one section of the sky for several months before moving to its next target. K2 will study the light from many kinds of targets, including supernova, variable stars, and of course, exoplanet transits.
New exoplanet discoveries have already been made in its new observing fields, and other interesting observations of light curves from supernova and variable stars have been recorded. A guest observer program allows outside organizations to suggest targets for the K2 mission, and all the data will be publicly available for download on the K2 data archive site. There promises to be many more suprising discoveries from Kepler's many new fields as K2 keeps its steady eye on our universe!
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