This is certainly foreseen in space under the general theme of ‘orbital intervention', but when it will happen is another question. Again the time-scale of such missions escapes us. Unmanned but Visited Space Stations The problem of nuclear reactor use in visited-only space stations is not different from the preceding one, except if the visited station is orbiting at over 800 km altitude. For the moment, no such missions are foreseen, even to unvisited, but robot-maintained stations. Planetary Exploration Nuclear energy has been used extensively for planetary exploration, in the form of RTGs with Plutonium 238. This kind of mission is clearly the type of space activity where nuclear generators are the only solution, mainly because for each planet except earth, the sun is ‘too far' or ‘too near' for solar cells. The use of RTGs is known to be limited to power levels of less than 1 or 2 kilowatts. So nuclear generators with reactors will be the next probable step. But this step is a giant one. The reason again is ‘radiation', because reactors have to be associated with a heavy shield while RTGs only need a long boom. The power level is not a problem for nuclear reactor generators in the 0-50 kWe range, because the mass of the generator is not very sensitive to the power level. But the minimum generator mass is a real concern (2500 kg) because this is far more than the mass of all probes sent to the planets by men. Sending a costly 2500-kg generator to a distant planet has no meaning if the scientific payload and onboard vehicle systems are not of the same order of magnitude, in order to use efficiently the available power. Planetary probes with a total launch mass of the order of 6-10 tons will certainly be good candidates for nuclear generators, but no launcher is foreseen, until very far into the future, for sending such a mass very far from Earth. The only subject of preliminary conversations is the Mars planetary exploration by huge automatic vehicles, in view of its far-future manned visit, with construction of very large space vehicles in Earth orbit. Our studies are continuing in this direction, and Soviet scientists have expressed to us their interest for preliminary conversations on the subject. Those missions are effectively only possible with large-scale international cooperation, due to the total cost of such civilian scientific ambitious missions. But this future, again, is probably very far away, because no financial provision actually exists for its preparation. ‘Classical' Space Applications Missions We have looked at missions able to use large platforms instead of many satellites, and found that they are very unlikely in LEO, but possible in GEO. Again, the use of nuclear generators in large GEO platforms is able to decrease operational costs only if the number of platforms is sufficient and if platform use of each is shared between many countries. No such activity in space is foreseen in the known future; on the contrary, space telecommunications are going in the opposite direction.
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