Artificial Satellite

A

Orbit Decay and Reentry

A satellite that orbits within a few hundred miles of Earth’s surface experiences friction from the thin atmosphere that exists at those altitudes. Eventually the satellite’s altitude will decrease until atmospheric friction causes the satellite to plunge earthward out of orbit. The lifetime of a satellite depends on its orbit, the satellite’s orientation in its orbit, and the size, shape, and weight of the satellite. A large, light satellite will probably reenter Earth’s atmosphere sooner than a small, heavy satellite that orbited at the same altitude, because the large satellite has more surface area and experiences more atmospheric friction. At an orbital altitude of 200 km (120 mi), a satellite will likely last from a week to three months. At 300 km (190 mi), a satellite may stay in orbit for two years or more. Satellites that orbit above 1,000 km (620 mi) will stay aloft for thousands of years.

B

Disposal of Satellites

The space around Earth seems boundless, but space operations tend to take place in a limited number of preferred types of orbits. The U.S. Air Force tracks satellites and other objects within these orbits so that other satellites and piloted vehicles can avoid collisions with the objects. Radio interference between satellites can also present spacing problems. Many satellites share a limited area, called the geostationary corridor, where a satellite’s orbit takes it around Earth at the same rate that Earth rotates. Satellites in this area have to maintain certain separation distances, so that the radio signals sent to one satellite do not interfere with the signals sent to nearby satellites. A final rocket thrust is sometimes used to put old satellites into less-desirable orbits to make room for newer satellites.

VI

Satellite Orbits

The defining characteristics of an orbit are its shape, its altitude, and the angle it makes with Earth’s equator. A satellite’s controllers choose an orbit with a particular combination of shape, altitude, and angle that will best serve the satellite’s mission. Most orbits are circular, but some satellites use elliptical orbits—that is, orbits in which the satellite’s distance from Earth is not constant. The altitude of an orbit determines how long the satellite takes to circle Earth and how much of the planet is visible to the satellite at one time. Satellites pass over different ranges of Earth’s latitude depending on the angle of their orbits with respect to the equator. Some satellites orbit along the equator. Satellites that pass over high northern and southern latitudes have orbits that form a large angle to the equator. Some satellites move clockwise around Earth as seen from the North Pole, but most satellites move counterclockwise around Earth.

A

Geostationary Equatorial Orbit

Satellites in geostationary equatorial orbit (GEO) orbit Earth around the equator at a very specific altitude that allows them to complete one orbit in the same amount of time that it takes Earth to rotate once. As a result, these satellites stay above one point on Earth’s equator at all times. The altitude of GEO is about 5.6 times the radius of Earth, or about 35,800 km (about 22,200 mi).

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Direct-broadcast television satellites are in GEO. A few satellites in GEO can provide coverage for the entire Earth, and antennas do not need to track the satellite to receive a signal. Earth-surveillance missions, including military surveillance and weather tracking missions, also use GEO.

B

Low Earth Orbit

A satellite in low Earth orbit (LEO) orbits at an altitude of 2,000 km (1,200 mi) or less. Almost every satellite enters a LEO after it is launched. If a satellite’s mission requires an orbit other than LEO, it uses rockets to move into its final orbit.

A low Earth orbit minimizes the amount of fuel needed. In addition, a satellite in LEO can obtain clearer surveillance images and can avoid the Van Allen radiation belts, which contain harmful high-energy particles. It needs less powerful signals to communicate with Earth than satellites with higher orbits. A signal to or from a low Earth orbit also reaches its destination more quickly, making LEO satellites especially good for transmitting data.