The Kepler uses the ‘transit method’ to find exoplanets. As a planet passes in front of its star from our vantage point, we see a slight dimming of the light coming from that star. If that dimming follows a regular pattern, we have a strong suspicion that an object is orbiting the star and that the object is most likely a planet.
The Kepler telescope monitors a single patch of sky, continuously observing the same 100,000 plus stars. In order to do that job, the Kepler is kept in position by four reaction wheels. Three wheels are necessary to control orientation in each of the three dimensions and the fourth is a redundant spare. Last month, two of the wheels failed, leaving the telescope unable to maintain its precise orientation.
An illustration of the gravitational microlensing technique, showing an Einstein ring.
Credit: Timberlake Studios.