prototype Powersat could also, for example, act as a test bed for an SDG system. Another possible power generator option would involve the use of nuclear systems such as those like the US SP-100, the French ERATO concepts.[12] and the former Soviet thermionic generators such as TOPAZ.[13] Nuclear systems have many benefits, including: • They do not depend on sunlight and, therefore, can provide power during eclipse, • They eliminate the need for complex solar tracking systems typical of solar array or SDG designs, • They have very low drag coefficients and, therefore, will not be as severely affected by drag. The use of nuclear systems, of course, poses significant environmental and safety concerns, although the Powersat approach would ensure the nuclear source remained well away from manned space station activities. In addition, their development costs are probably higher than more conventional alternatives. Even though their potential is enormous, it might not be appropriate at this stage to make any assumptions over their future availability. Summary To summarise, it would seem that a co-orbiting Powersat might be technically attractive if: • Its operations were effectively transparent with respect to the receiving station's power subsystem. • It could save significant propellant, and • Battery upload mass per year could also be saved if a solar dynamic or nuclear system was used instead of solar arrays.
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