COBE

NASA's Cosmic Background Explorer (COBE) revolutionized study of the cosmic microwave background. It is critical to observe from above the atmosphere, because the blackbody curve peaks in the microwave spectrum, which is absorbed strongly by the atmosphere.

COBE is in an orbit that passes almost over the poles with a period of 103 minutes. Here is a simulated view of the Earth from COBE. Notice how rapidly the view changes (click on the link several times over a period of 5 minutes). This orbit helps COBE map the entire sky while shielding itself from heat noise generated by the Sun and Earth.