Although it is possible, by a suitable choice of frequencies and routes, to provide telephony circuits between any two points or regions of the earth for a large part of the time, long-distance communication is hampered by the peculiarities of the ionosphere, and there are even occasions when it can be impossible. A true broadcast service, giving constant field strength at all times over the whole globe would be invaluable, not to say indispensable, in a world society. The service area of a television station, even on a very good site, is only about a 100 miles across. To cover a small country such as Great Britain would require a network of transmitters, connected by coaxial lines, waveguides, or VHF relay links. A recent theoretical study has shown that such a system would require repeaters at intervals of 50 miles or less. The chapter also discuss certain fundamental laws of rocket propulsion and astronautics.