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Polar Orbit
A low altitude polar orbit is widely used for monitoring the Earth because each day, as the Earth rotates below it, the entire surface is covered. Typically, a satellite in such an orbit moves in a near circle about 600 miles above ground (some go lower but don't last as long, because of air friction) and each orbit takes about 100 minutes. Many spacecraft use such orbits, e.g. the US Air Force surveillance satellites of the DMSP series, or the series of French Earth-resources spacecraft SPOT. The space shuttle avoids polar orbits, because flying through the aurora exposes astronauts to radiation and creates other problems. But for studying the aurora, Birkeland currents, polar rain and other phenomena related to the distant magnetosphere, such orbits are very useful. To achieve a polar orbit requires more energy, thus more propellant, than does an orbit of low inclination. A polar orbit cannot take advantage of the "free ride" provided by the Earth's rotation, and thus the launch vehicle must provide all of the energy for attaining orbital speed. |
A Polar orbit is a
particular type of Low Earth Orbit. The only difference is that a
satellite in polar orbit travels a north south direction, rather than
the more common east west direction.