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#29.     Polar Orbiting Satellites


  (Files in red–history)

           Index

26. Polar Caps

26H. Birkeland, 1895

27. Aurora from Space

28. Aurora Origin

28a. Plus and Minus

29. Low Polar Orbit

30. Magnetic Storms

30a. Chicago Aurora

31. Space Weather

32. Magnetic planets

33. Cosmic Rays

34. Energetic Particles
        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

    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.

    The orbital plane of a satellite in space is best defined by its perpendicular direction (its "normal"), defined by distant stars, whose direction on the celestial sphere is fixed. Because Earth orbits the Sun, the angle between the Sun's direction and the orbit's normal slowly changes, increasing by 360° in the course of each year. (The closed path annually taken by the Sun among the stars is known as the ecliptic, and 12 constellations along it, named by the ancients, form the zodiac.) If the normal to the orbit of the polar satellite points at the Sun now, three months from now the Sun's motion across the sky will make the normal perpendicular to the Sun's direction.

        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, a factor that strongly depends on the orientation of the orbit relative to the Sun's direction. By placing the satellite in a sun-synchronous orbit near the dawn-dusk plane (90 degrees to the noon-midnight plane described earlier), not only was the interference kept small, but because the orbit's orientation relative to the Sun did not change, the disturbance also stayed more or less the same throughout the mission.

    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.


Questions from Users:
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Universal Time and Magnetic Local Time
                  ***     What are "geomagnetic conjugate points"?

Next Stop: #30.  Magnetic Storms

Updated 25 November 2001 Last updated 12 January 2014
Re-formatted 3-13-2006