he development have required a large investment. At the time of Arthur C. Clarke's article the most advanced rockets were the V2s, developed by the Germans, and these had never been intended to launch satellites into outer space. For the satellites themselves, methods of powering them had to be developed along with control systems to enable them to be finely positioned. The satellites also had to be made very reliable, as they cannot be serviced once they have been launched. Nowadays all these problems have been mastered and satellites are capable of providing a long reliable service life. 222 Newnes Guide to Radio and Communications Technology Satellite orbits There are a variety of different orbits that can be adopted for satellites. The one that is chosen will depend on factors including its function, and the area it is to serve. In some instances the orbit may be as low as 100 miles (160 km), whereas others may be over 22 000 miles (36 000 km) high. As satellites orbit the earth they are pulled back in by the force of the gravitational field. If they did not have any motion of their own they would fall back to earth, burning up in the upper reaches of the atmosphere. Instead the motion of the satellite rotating around the earth has a force associated with it pushing it away from the earth. For any given orbit there is a speed for which these two forces balance one another and the satellite remains in the same orbit, neither gaining height nor losing it. Obviously the lower the orbit, the stronger the gravitational pull, and this means that the satellite must orbit the earth faster to counteract this pull. Further away the gravitational field is less and the satellite velocities are correspondingly less. For a very low orbit of around 100 miles a velocity of about 17500 miles per hour is needed and this means that the satellite will orbit the earth in about 90 minutes. At an altitude of 22 000 Circular orbit Altitude Elliptical orbit Earth Perigee Apogee Ellipse focal point Geocentre Figure 8.1 Satellite orbit