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Classes of Orbits

Although a satellite can orbit, or go around, a celestial body in an infinite number of ways, some orbits are more useful to people than others. Two main types of orbits exist: closed and open orbits. Open orbits follow mathematical shapes such as a parabola or hyperbola, deep curves that never reconnect. These are common to some comets and spacecraft as they move past celestial bodies, rounding them without completely circling them. The closed orbits of man-made satellites around the Earth follow circular or oval orbits that vary in angle, shape and distance from the Earth's surface.

Geosynchronous Orbit
- A satellite in geosynchronous orbit, also known as geostationary or geo orbit, circles the Earth at the equator at the same speed the Earth is rotating. In effect, the satellite appears stationary in the sky when viewed from the Earth's surface and therefore maintains a consistent view of the same location. At about 22,000 miles or 36,000 km from the planet's surface, the geosynchronous orbit is considered a high Earth orbit. A satellite following this kind of path can monitor most of a hemisphere at a time, but never gains a clear view of polar activities.
Low Earth Orbit
- Convenient for visiting satellites via space shuttle, low Earth orbits are appropriate for the International Space Station and Hubble Telescope. As the name implies, a low Earth orbit moves closer to the Earth's surface than other orbits do. However, this proximity subjects a satellite to friction from the Earth's atmosphere, which decays the orbit over time. The satellite slows, making it more vulnerable to the pull of gravity dragging it toward the Earth. A satellite in this orbit also circles the Earth too quickly -- one orbit completed every 90 minutes -- to allow study of one location on the planet for very long.
Polar Orbits
- A specific type of low Earth orbit, a polar orbit path crosses very near the north and south poles in a pattern that appears perpendicular to the equator. As the Earth rotates inside the orbit path, seeming to follow the line of its equator like a globe as the satellites follow the poles, these satellites can view nearly every part of the Earth's surface. As with other low Earth orbits, one polar orbit takes about 90 minutes to complete, so the satellite does not view one location for very long; the orbit is most useful for tasks such as measuring temperatures in the atmosphere or ozone in the stratosphere. It also fills in the monitoring gaps left by geosynchronous orbits, which do not offer clear views of the north and south poles.
Highly Elliptical Orbits
- The highly elliptical orbit is most appropriate for a system of satellites, such as the global positioning system used for mapping and navigational purposes. Following the path of this oval-shaped orbit, a satellite will move close to Earth on one end of its ellipse, then further away from Earth on the other end. At the end nearest the Earth, the satellite moves very quickly, while it seems to almost hover at the far end, allowing more leisurely study of a location. A system of satellites would follow different elliptical orbit paths so that together they would allow study of all sections of the Earth.

Astronomy