all human beings were the most tremendous nuclear explosions which ever occurred. That ought to be of some philosophical concern to the antinuclear activists. Regardless of philosophical irrationalities, however, if chemical energy is not good enough these days, we have to turn to anything (regardless of cost, convenience, or environmental effects) other than nuclear energy. One of the most frequently extolled alternatives is solar energy. Solar energy has a big problem. Although it is a free and long-lasting source of energy, it is not very concentrated by the time it reaches the Earth’s orbit. It is entirely too diffuse to achieve any reasonable degree of acceleration in a propulsive system. Solar energy, of course, is also a form of nuclear energy. In this case, the source of the energy is nuclear fusion reactions deep inside the sun. The products of these reactions are processed in many ways throughout the layers of the sun and eventually reach us in the form of diffuse thermal radiation. The world runs completely on nuclear energy, whether anyone likes it or not. Turn out the nuclear fires in the sun, and we all die. Had they not existed in the distant past, the chemical and biological reactions which formed our present chemical fuel supplies would not have occurred. We do not argue over whether we will use nuclear energy in propulsion. Rather, the question is, dare we package it and carry it directly in our vehicles, or must we always use it only in reactors which are 150-million km away, deep in the sun? Solar energy can be collected, concentrated, and used to heat the working fluid for a rocket. Unfortunately, once again, the maximum temperature which can be generated by this process is limited. This time the limitation is not the basic chemical energy packaging restrictions, but the fact that the sun has a finite angular diameter. This limits the maximum temperature achievable. Were the sun a point source, we could do much better. Even so, the maximum rocket exhaust velocity achievable is comparable and perhaps slightly higher than solid core nuclear rockets. It is possible that such rockets should be more seriously considered than they have been, particu-
RkJQdWJsaXNoZXIy MTU5NjU0Mg==