To launch an object into space, we must loft it above the Earth's atmosphere and at such a speed that it can overcome the inexorable pull of gravity. The speed we must give it is no less than 28,000 km/h, and we must direct it so that it travels parallel to the ground. Then, travelling above the atmosphere, parallel to the ground and at a speed of 28,000 km/h, the object will continue to circle endlessly round and round the Earth. It will become an artificial satellite of the Earth, following a precise path, or orbit through space. The minimum speed a satellite must have to stay in orbit, 28,000 km/h, is termed the orbital velocity. The time it takes a satellite to complete each orbit is called its orbital period. For a satellite in orbit about 250 km high, the orbital period is about 90 minutes. The period increases with increasing height: at 15,000 km high it is 9 hours, and at 35,900 km high it is 24 hours. Working satellites are launched into orbits at different heights above the Earth and in different planes relative to the Earth's axis, depending on their purpose. Many weather satellites, for example, are launched into orbits about 850 km high in a plane that takes them over the North and South Poles. In this polar orbit, they can regularly scan every part of the Earth as it spins beneath them. Many communications satellites, however, are launched into orbits 35,900 km high above the Equator. In this particular equatorial orbit they can view almost a whole hemisphere, but more important they appear fixed in the sky. This is because their orbital period is just 24 hours. So they circle the Earth once in the same time (one day) that the Earth spins round once on its axis. In other words they keep pace with the Earth and appear stationary in the sky. Such an orbit is called a geostationary, or 24-hour orbit. It is sometimes called a Clarke orbit after the distinguished British space writer, Arthur C Clarke, who first suggested this type of orbit for space communications. The majority of satellites are launched into more or less circular orbits, but some are notably elliptical. The Soviet Molniya communication satellites are a case in point. They are launched into highly elliptical, or eccentric orbits that take them as low as 600 km and as high as 39,000 km. All satellite orbits are in theory elliptical, a circle being mathematically a special kind of ellipse. And quoted orbital characteristics of every satellite always include the lowest and highest points in its orbit. The lowest point is called the perigee ('nearest to Earth'), and the highest, the apogee ('farthest from Earth'). Also usually quoted are the orbital period and the inclination. The inclination is the angle of the plane of the orbit relative to the plane of the Earth's Equator. So for satellites in an equatorial orbit, the inclination is close to 0 degrees; for satellites in a polar orbit, the inclination is close to 90 degrees.
