Polar orbit

orbit A polar orbit is an orbit in which a satellite passes above or nearly above both poles of the body (usually a planet such as the Earth, but possibly another body such as the Sun) being orbited on each revolution. It therefore has an inclination of (or very close to) 90 degrees to the equator. Except in the special case of a polar geosynchronous orbit, a satellite in a polar orbit will pass over the equator at a different longitude on each of its orbits.

Polar orbits are often used for earth-mapping, earth observation, and reconnaissance satellites, as well as for some weather satellites. The disadvantage to this orbit is that no one spot on the Earth's surface can be sensed continuously from a satellite in a polar orbit.

It is common for polar orbiting satellites to choose a sun-synchronous orbit: meaning that each successive orbital pass occurs at the same local time of day. This can be particularly important for applications such as remote sensing of the atmospheric temperature, where the most important thing to see may well be changes over time, which you do not want to see aliased onto changes in local time. To keep the same local time on a given pass, it is desirable for the orbit to be as short as possible, which is to say as low as possible. However, very low orbits of a few hundred kilometers would rapidly decay due to drag from the atmosphere. A commonly used altitude is approximately 1000 km; this produces an orbital period of about 100 minutes. The half-orbit on the sun side then takes only 50 minutes, during which local time of day does not greatly vary.

To retain the sun-synchronous orbit as the earth revolves around the sun during the year, the orbit of the satellite must precess at the same rate. Were the satellite to pass exactly over the pole, this would not happen. But because of the earths equatorial bulge, an orbit inclined at a slight angle is subject to a torque which causes precession; it turns out that an angle of about 8 degrees from the pole produces the desired precession in a 100 minute orbit.

A satellite can hover over one polar area a large part of the time, albeit at a large distance, using a polar highly elliptical orbit with its apogee above that area. This is the principle behind a Molniya orbit.