#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 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.

Sun-synchronous Orbits

The Earth is not an exact sphere but bulges slightly at its equator. Any orbit passing exactly above the geographic poles is symmetrically affected by the bulge and its plane stays fixed relative to the stars.

Relative to the Sun, however, the orbital plane will slowly rotate. The reason is that the Earth itself orbits the Sun, so that the Sun's position in the sky, relative to the distant stars, slowly rotates around the Earth, one circuit per year. (The 12 constellations through which the Sun passes on that journey were named by the ancients and are known as the zodiac.) If the orbital plane of the polar satellite points at the Sun now, in six months' time the Sun's motion across the sky would make it point 180 degrees from the Sun (picture below).

An inclined orbit, whose northermost point is not the north pole but falls short by (say) 1000 km, will be affected asymmetrically by the Earth's bulge, and as a result its orbital plane would slowly rotate around the Earth's axis. With a suitable inclination, about 8 degrees off the polar orbit, that motion matches the slow motion of the Sun across the sky. If the satellite then starts near a noon-midnight orbit, it will always pass near noon and near midnight. A noon-midnight "Sun-synchronous" orbit was actually used by some DMSP satellites.

A different choice was made for MAGSAT, orbited 1979-80 to survey the Earth's own magnetic field near its surface. Magnetic fields from the magnetosphere are a disturbing factor in such a mission, and by placing the satellite in a Sun-synchronous orbit near the dawn-dusk plane (90 degrees to the noon-midnight plane described earlier), the interference was kept small and relatively steady.

On the other hand, the Dynamics Explorer (DE) mission of 1981 used two polar spacecraft, one in a low orbit to intercept the aurora (among other things) and a second one in an elongated orbit to observe auroral acceleration and also to take pictures of the entire auroral oval from a distance. To ensure the best chance for the two spacecraft to intercept the same auroral electron beam at different altitudes, it was decided that both orbits would always share the same plane. They were therefore made to pass over the geographical poles: with any other choice the Earth's bulge would have rotated the planes at different rates and they would have soon drifted apart.

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Last updated March 13, 1999