WISE

In this animation, the red arrow indicates Earth’s axis of rotation and the blue circle represents the path of WISE’s orbit around Earth. As Earth orbits the Sun its axis stays pointed in the same direction in space. However, in order for WISE to keep its solar panels facing the Sun its orbit needs to rotate (or precess) to keep it aligned with the Earth’s terminator between day and night. Such an orbit is call Sun Synchronous.

Since WISE will observe a strip of the sky 360° around on every orbit and the orbit will precess 180° in 6 months, it will take only 6 months for WISE to observe the entire sky.