due to the gravitational force. If the velocity is too low, the satellite will fall back to Earth. If the velocity is too
high, the satellite will move into an orbit farther away from Earth. If the velocity becomes great enough, the
satellite will reach escape velocity and will sail off into space. For a satellite relatively close to Earth, the
required orbital speed is about 18,000 miles per hour. A vehicle placed in orbit as far away as the moon
(roughly 210,000 miles) would need only to have an orbital speed of about 2,412 miles per hour. Table 1-1
gives the relationship between altitude and orbital speed.
Table 1-1. Altitude versus orbital speed.
4. A satellite launched into a circular orbit parallel with and 22,300 miles over the equator is termed
synchronous, fixed, or stationary. This satellite will take about 24 hours to complete one revolution, the same
time the Earth takes for one full rotation on its axis. Therefore, the satellite remains more or less fixed over one
spot on the Earth. To a ground observer, the satellite would appear to stand still. The time required for a
satellite to complete one orbit is called the period.
5. It is virtually impossible to launch a satellite into an exactly circular orbit because the Earth is not precisely
circular itself. Usually the satellite path will be an elliptical orbit. The perigee may be only a hundred or so
miles, while the apogee may be several thousand miles in some of the more exaggerated ellipses. As the
orbiting satellite approaches the perigee, it speeds up; as it approaches the apogee, it slows down. These
principles are illustrated in Figure 1-4.
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