Earth orbital flight is achieved by launching vertically and then tilting the trajectory so that flight is parallel to the Earth's surface at the time that orbital velocity at the desired altitude is reached. At this precise point, the rocket engine is cut off. A spacecraft attached to the final-stage rocket is then in free-fall about the Earth, the centrifugal pull on the spacecraft being equal to the Earth's pull of gravity. At an altitude of 200 kilometres, Earth orbital velocity is about 29,000 kilometres per hour. Since this 200-kilometre altitude is above most of the atmosphere, aerodynamic drag is not great, and the spacecraft will continue to orbit for an extended time.

The length of time required for the satellite to make one complete revolution is known as the period of orbit. At 200 kilometres this is about 90 minutes. At higher altitudes than this above the Earth, the velocity of a satellite decreases and the orbital period increases.

For example, at an altitude of 1,730 kilometres the orbital velocity is 25,400 kilometres per hour and the period is two hours. At 35,700 kilometres the velocity is 11,300 kilometres per hour and the period 24 hours. Because this particular period is equal to the time the Earth rotates once, such a satellite travels at the same angular velocity as the surface of the Earth and appears to be stationary in the sky. This particular orbit, called geostationary (or geosynchronous), has special value to communications and meteorological satellites. Finally, the Moon travels in an orbit of about 386,000 kilometres. Lunar velocity about the Earth is approximately 3,700 kilometres per hour, with a period of about 28 days.

The above applies only to circular orbit, often ideal, but difficult to achieve. Usually a satellite's orbit is an ellipse with a perigee altitude and an apogee altitude. (Perigee and apogee are the points of the orbit nearest and farthest from the body orbited.) Orbits may be made more nearly circular if thrust is available by reducing the velocity at perigee (apogee is lowered) or by increasing the velocity at apogee (perigee is raised). Rocket power in such instances is applied along the axis of flight paths.

In projecting a satellite into Earth orbit, the launch vehicle is tilted after lift-off in an easterly direction. Launching to the east is done to take advantage of the Earth's eastward surface velocity. This rotational surface velocity is about 450 metres per second at the Equator and 400 metres per second at the latitude of Cape Canaveral, Florida. It would be possible to launch a satellite on a westerly orbit, but an additional velocity of 600 metres per second would be required to achieve an orbit of the same altitude compared with an easterly orbit.

If the satellite is launched in a northerly or southerly direction, a polar orbit is obtained. The easterly surface velocity launch advantage is lost, but there are other advantages. As the Earth turns on its polar axis, the satellite travels over all parts of the globe every few revolutions. This ground track, the path around the Earth directly under the satellite, thus varies according to the orbit, which is chosen according to the desired characteristics of a particular mission. Ground-tracking and data-receiving stations must be on or near the ground track.

Excerpt from the Encyclopedia Britannica without permission.