nobody is able to digest the terabits such a radar would emit during its ten years in orbit. This is not a radar, nor a nuclear generator problem, but a ground computer question: the land mass of the earth only (30% of the earth surface) represents 6 X 1014 pixels to observe, transmit, receive, and adjust by transfer function corrections before user analysis. Each pixel is seen approximately during 2.5 X IO-8 seconds in a classical SAR radar. So it looks impossible actually to digest the total amount of information output of a technically feasible radar of this kind. The only thing you can do is to choose a bit rate limit that you are able to digest, losing the rest of the territory observation. This solution is acceptable because not all the surface of the continents is of interest at the one metre resolution level. Looking at the cost of each bit used, you can immediately imagine a different radar, with an electronically scanning array antenna, able to integrate microwave energy of each pixel during 7-10 times more time, and such a radar would be giving the information you need with only 3 kilowatts. And there, solar panels are better than a nuclear system if you are not too concerned with the military vulnerability aspect. The use of nuclear generators in space for radars have to wait for a time where more ambitious radars will be needed (GEO high resolution radars for example) or a time where the information of each radar will be shared by about 10 advanced countries. We, engineers, are not able to put dates on this kind of future. Manned Space Stations The amount of energy needed onboard manned space stations will certainly grow in the future, and over a certain level of power, nuclear generators will be the best choice. The ‘choice' power level is higher for manned stations than for automatic satellites, because man is more affected by radiation (neutrons and gamma rays) than electronics. This necessitates a thick and heavy shield between the reactor and astronauts. Another aspect of manned space stations is the fact that they are visited continuously by new crews, with contingencies for space rendez-vous approach and docking. So, the volume of space to protect from nuclear radiation is about the total sphere, or safety constraints become difficult to observe. This very heavy shield gives a large mass and launch cost penalty to nuclear generators for space stations, raising the power ‘decision level' very high (around 500 kWe). Another difficulty with space stations is their maximum orbital altitude, limited to 350-400 km. The reason for the limitation in altitude is the natural ‘Van Allen' radiation belts of the earth, or it would be necessary to shield all the station in order to protect men onboard. At such an altitude, the natural life-time of a space object is of the order of 10-15 years; after that, an atmospheric re-entry is the rule. Skylab is a classic example of that. For the safety of the earth population, re-entry of a nuclear reactor is to be avoided before 300 to 1000 years after its end of service; this is the reason for choosing orbits over 800 to 1000 km for nuclear-equipped space missions. This does not mean that nuclear generators will never be used at 350 km altitude, but using them there implies the moral obligation to have, permanently in orbit, ‘rescue' robotic systems able to take care of spent or damaged reactors completely safely, bringing them into higher orbit or returning them safely to the ground, in specially organized plants.
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