Once a satellite enters coasting flight in orbit, it behaves according to the same rules that govern natural satellites. The low point of an elliptical earth orbit is called perigee, the high point is apogee. In a lunar orbit, the low point is perilune, the high point is apolune. In a solar orbit, the corresponding points are perihelion and aphelion.
In general, satellites in an earth orbit with perigee of less than 200 miles stay in orbit for a month or less, many just for a day or so, because of the retarding effect of air present at that altitude. Satellites with perigees of more than 200 miles can remain in orbit for months or years. Various types of orbits are chosen for different missions. Some are circular; others are highly elongated.
To achieve a perfectly circular orbit directly, the satellite must be traveling exactly parallel to the earth's surface as it enters coasting flight. The speed must exactly match the circular orbital speed for the altitude of the satellite. A small deviation in either speed or direction will cause the satellite to enter an elliptical orbit; the greater the deviation, the more elongated the ellipse.
To make an orbit more circular, rocket engineseither in the final stage of the launch vehicle or built into the satellite itselfare fired to accelerate the satellite along a tangent to the orbit at apogee. This action brings the perigee to a higher altitude on the satellite's next pass. A circular orbit is achieved when the perigee is brought up to the altitude of the orbit's apogee.
To elongate an orbit, the rockets are fired when the satellite is at perigee; the apogee is raised without affecting the perigee, causing the satellite to travel in a more elongated ellipse. A circular orbit can be changed to an elongated one by accelerating the satellite at any point along the orbit; this point becomes the perigee.
If the satellite is accelerated to more than 6.95 miles per second (11.18 km/s), or about 25,000 miles per hour (40,200 km/hthe escape velocity of the earth), it will leave orbit around the earth and travel into space, never to return.
A satellite in a circular orbit at an altitude of approximately 22,240 miles (35,790 km) completes one orbit in the same time that the earth completes one rotation. Such a satellite is called a geosynchronous satellite. A geosynchronous satellite in orbit directly over the Equator appears to remain motionless in the sky and is referred to as a geostationary satellite.
Satellites designed to obtain images of cloud patterns or of the earth's surface are often placed in a polar or near-polar orbit. In this type of orbit the satellites can pass above virtually all areas of the earth. A polar or near-polar orbit can be chosen in which the satellite passes over any given area of the earth at the same local time every day. The advantage of such an orbit, called a sun-synchronous orbit, is that the shadows on the earth and the illumination of the clouds in all the photographs that the satellite obtains of a particular region are more or less constant.
Lunar or Planetary Orbits
Orbits around the moon and planets other than earth are difficult to achieve. Among the factors that must be accurately determined are the positions of the earth and the target body, the distance between them, and their relative motions. The strength of the gravitational attraction of the target body must be taken into account because it determines the orbital speeds around that body. A spacecraft approaching a body at a speed greater than the body's escape velocity will pass by without becoming a satellite unless sufficiently slowed at the proper time.
Some satellites have been placed in solar orbit to study solar phenomena. Others, such as probes sent to acquire data about Venus and Mars, have become solar satellites after passing near their target planets. The probes sent past Jupiter and Saturn, however, are not solar satellites; they have achieved a speed great enough to escape the gravitational pull of the sun.