limited to 9 tons in LEO, giving less than 4 tons in GEO, while the Shuttled-Centaur system was announced as 6 tons in GEO. Mission studies have looked at the possibility of building a large electric OTV, launched in LEO, and able to bring 9 ton payloads to GEO, spiralling up during 100 days, leaving its payload in GEO, spiralling down to LEO, waiting there for another payload, etc., during a total life in orbit of the order of 7-10 years. The level of thrust needed (10-20 Newtons) raised the question of the power generator because 200-400 kWe were necessary. This power level, associated with a life-time of 10 years was clearly in the middle of the domain of space nuclear generators; this was the beginning of the ERATO studies in France with CEA (ERATO is from the initials of the French expression meaning ‘electric nuclear OTV for orbital transfer'). A couple of years after this approach, the situation was the following: CEA had studied the feasibility and development of the generator, and CNES the size and cost of the vehicle. Then it was possible to assess the economic aspects of this type of mission. This economic assessment finished, we learned that the system was able to divide by a factor of two the orbital launch cost of each kilogram of payload into GEO, but only with an OTV used at its full capacity. The full capacity of the system was 16.4 tons per year at this time, and is now probably of the order of 33 tons/year with the actual version of Ariane 5. The GEO annual world launch market is much lower than this capacity. This is due to the fact that GEO satellites are mainly telecommunications and meteorological systems, and both have seen tremendous progress in their performance for a very small increase in their mass, thanks to new electronic components, new modulation processes, and better receivers on earth. Another aspect is the continuous increase in the life-time of communications satellites, associated with the limitation in the electromagnetic spectrum, giving less and less of an increase in the payload traffic towards GEO. The same can be said for meteorological satellites, which are less numerous. Because of this we cannot actually predict the time when GEO traffic would be sufficient to provide economic arguments in favour of electric propulsion OTVs spiralling between LEO and GEO. The same arguments emerge if we look at ambitious GEO applications such as solar power systems. One day perhaps, in the far future .... Space-based Radars Space-based radars have proved their efficiency for both civilian and military earth surface surveillance missions. Actually, radar observation of the earth with low spatial resolution (10/20 metres) does not need more than 3 kilowatts of electricity, thanks to progress in solid state electronics. We thus concentrate our studies on high resolution radar missions, typically of the order of one metre. The actually foreseen electronic components give us the possibility of building a synthetic aperture radar flying at over 1200 km altitude (very safe nuclear orbit) with an image resolution of about one metre with only 20 kWe on board, just over the inferior power limit for nuclear generators. This sounds perfect, as the nuclear generator is better than its solar counterpart in all aspects for that application. But one problem emerged at the end of our study:
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