#29.     Polar Orbiting Satellites

    Different types of satellite orbits have different uses: while the synchronous orbit is best for communication satellites, Lagrangian point orbits help monitor the solar wind before it reaches Earth. 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 1000 km (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. For instance, although the DMSP spacecraft (above) were designed for military needs, scientists have also equipped them with magnetometers, particle detectors and other instruments, which have provided a great amount of scientific information.

    Many spacecraft use such orbits, e.g. the US Air Force surveillance satellites of the DMSP series (successfully adapted to carry science sensors), or the series of French Earth-resources spacecraft SPOT. The next generation following DMSP, named NPOEES and essentially dedicated to research, also uses such orbits. 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. For instance, although the DMSP spacecraft (above) were designed for military needs, scientists have also equipped them with magnetometers, particle detectors and other instruments, which have provided a great amount of scientific information.

    In fact, although the DMSP mission was originally conceived as a project of the US Air Force, its scientific usefulness has been so widely recognized, that its follow-up will be a joint mission of the USAF, NOAA (National Oceanic and Atmospheric Administration, successor to the US Weather Bureau) and NASA. Known as the National Polar-orbiting Operational Environmental Satellite System or NPOESS ("en-poss") for short, the satellites of that mission, to be launched in the first decade of the 21st century, will carry a sophisticated complement of scientific instruments.

Sun-synchronous Orbits