> are you sure? i thought it was four (in a tetrahedral arrangement), but they were all constantly in use, and one failure could be tolerated. Source: http://k

> are you sure? i thought it was four (in a tetrahedral arrangement), but they were all constantly in use, and one failure could be tolerated. Source: http://kepler.nasa.gov/mission/faq/ http://kepler.nasa.gov/mission/faq/ Quote: "Only 3 reaction wheels are needed to control the 3 degrees of freedom of rotation of spacecraft. But Kepler was provided with 4 reaction wheels, one extra for redundancy in case a wheel fails."