Earth Satellites

Abstract

Artificial satellites have been circling the Earth since October 4, 1957, when the Soviet satellite Sputnik was launched. Their orbits are as diverse as their missions. Many scientific satellites, especially those launched to study the Earth’s magnetic field, have highly elliptical orbits, so that they can sample both near and far Earth environs. Communications satellites, on the other hand, usually move in circular orbits in the plane of the equator and revolve in the direction of the Earth’s rotation. Ideally, the latter should have an orbital radius r ≈ 42,170 km from the Earth’s center. By Kepler’s third law, such an orbit has an orbital period, in this case known to very high precision. The period is 23^h56^m04^s.0989 (or 0.997 269 663 24^d) of mean solar time, the rotational period of the Earth.¹ A satellite with these properties is called a synchronous satellite; it remains stationary over a particular point on the Earth’s equator, to provide continuous relay of television and telecommunications to a wide area within direct sightlines of the satellite.