1. Excellent summaries of the problem are given by Sheperd, L. A., Realities of Space Travel, (London, Putnam, 1957); Hoerner, S. V., Science, 137, 18 (1962); and Pierce, J. R., Proc. Inst. Radio Eng., 47, 1053 (1959), where further references are to be found.
2. Marx, G., Astronaut. Acta, 19, 131 (1963).