rcular and equatorial and its period becomes roughly equal to one 51.3 Spacecraft technology sidereal day; it will, in fact, be approximately geostationary. Low- energy thrusters incorporated in the satellite itself, typically fuelled 51.3.1 Orbits with hydrazine, are then used to move the satellite to the point in longitude at which it is to operate and to correct any remaining By far the most useful orbit for communication satellites is the errors in circularity, period or orbital inclination. geostationary satellite orbit. This is a direct equatorial orbit about The United States Space Transportation System (Space Shuttle) 35800km above the ground, the period of which is the same as the enables the first phase of launching to be carried out by reusable length of the sidereal day, about 23 hours 56 minutes. A satellite in rockets, although this facility has not been available for commercial this orbit, moving in the same direction as the Earth's rotation, launching since 1986. Other reusable launch and upper stage sys- remains stationary as seen from points on the Earth's surface. A items are under development. However, all other launch vehicles in geostationary satellite has line of sight coverage of a great area of current use are expendable. the Earth and, as Clarke noted in 1945, three of them suitably The classic procedure described above can be varied to take located around the Equator could cover almost all the Earth's advantage of the capabilities of particular launch vehicles. 'Strap- surface. Figure 51.1 shows, for example, the coverage provided by on' solid propellant rockets are usually added to the booster to satellites located at 30°W, 150°W and 90°E longitude. increase the payload mass which it can launch. The second stage There are, however, other orbits of interest for satellite communi- rocket may also perform the functions of the perigee motor, allow- cation. The USSR, with a territory which has an exceptionally wide ing the first and second phases of the launch procedure to be fused span in 