polar orbit

A polar orbit is one of the possible Low Earth Orbits. Unlike other classic orbits (that orbit in an east-west direction), the polar orbit travels north-south and over the poles. Each orbit is about 100 minutes long and covers the entire globe in 14 days.

As a satellite in a polar orbit... well, as it orbits, the earth spins under it from east to west. The only east-west component of the statelite's movement is from the spining of the earth and is not part of the movement of the satelite. This gives the satellite access to the entire surface of the earth - one strip at a time. This orbit is classically used by remote sensing satellites and spy satellites.

The altitude for polar orbits around Earth is classically 600 miles up though may go down to 300 or lower. However, the lower the orbit, the faster air friction takes a its toll upon the satellite and the shorter the lifespan.

The polar orbit is avoided by manned spacecraft because flying through or past the area above the magnetic poles poses a significant radiation hazard.

Not all polar orbits travel exactly over the geographic poles (fixed with respect to the stars). Because the Earth does not orbit the sun on the same plane as it rotates, other orbits become interesting. A slightly inclined polar orbit will keep the satellite in one plane with respect to the sun. This can be important in cases where tracking information about the magnetic fields within the magnetosphere. In this case, there is a difference in the magnetosphere in the sun (daytime) and out of the sun (nightime). Thus, the orbit of the MAGSAT was chosen to orbit at the dawn/dusk plane to reduce this difference.

       ||    <- rotation
    ,-'""`-.
  ,'  \     `.
 /     |      \
(      |       )  <--- 
| Night\ Day   |  <--- Sun
(       |      )  <--- (slightly askew)
 \      |     /
  `.     \  ,'
    `-.,.,-'
       ||

Likewise, some spy satellites use a noon/midnight inclined orbit to get the highest contrast. These orbits are called sun-synchronous orbits. A sun-synchronous orbit need not be on the noon/midnight orbit but this is a common orbit to find spy satelites in - it becomes harder to detect them when you have to look into the sun. On the flip side, orbits that given an afternoon or morning orbit will show shadows of buildings giving height information.

Achieving a polar orbit requires a significantly higher amount of propellant than a classical low orbit. This is in part because the polar orbit moves perpendicular to the rotation of earth. While a satellite or spacecraft being launched from earth gets a push from the rotation, the polar orbiter cannot take advantage of this "free ride".