A polar orbit is an orbit with an inclination of 90 degrees. A spacecraft following such an orbit has access to virtually every point on Earth's (or some other planet's) surface, since the planet effectively rotates beneath it. This capability is especially useful for mapping or surveillance missions. An orbit at another inclination covers a smaller portion of the Earth, omitting areas around the poles. A polar orbit covers the entire globe every 14 days.
|A polar orbit.
Placing a satellite into terrestrial polar orbit demands more energy, and therefore more propellant, than does achieving a direct orbit of low inclination. In the latter case, the launch normally takes place near the equator, where the rotational speed of the surface contributes a significant part of the final speed needed for orbit. Since a polar orbit is not able to take advantage of the free ride provided by Earth's rotation, the launch vehicle must provide all of the energy for attaining orbital speed.