6.2.3 On-Orbit Construction Effects In the case of constructing a large satellite, most important effect is the space debris problem. Man made space debris comes from spent rocket stages, inactive/deactivated space crafts, rocket/spacecraft fragments, spacecraft/rocket separation hardware, solid rocket motor exhaust particles and spacecraft covering/thermal insulation. At the end of 1990, about 6,500 manmade objects were in orbit around the Earth, weighing in total about 2,000 ton. But, this space objects data is not complete, space scientists believe there are many more smaller objects. Figure 6.7 presents a space objects flux data. It includes both natural objects (meteoroid) and artificial objects. Figure 6.7 On the environmental point of view, solar power project makes two problems concerning with space debris. The first one is the impacts of space debris and a large satellite. For debris sizes less than about 0.01 cm, surface pitting and erosion are the primary effects. Because of the very large flux, the cumulative effect of individual particles colliding with a satellite might become significant. For debris larger than about 0.1 cm, structural damage to the satellite becomes significant. In the case of examination of Long Duration Exposure Facility (LDEF), 29.37 m2 thick aluminum plates revealed 606 craters that were 0.5 mm in diameter or larger in 5.8 years exposure on LEO (lower than 480 km altitude). The flux becomes 3.6 /m2/year. In the case of solar power satellite 2000 (1000 km altitude), assuming the life time of the satellite is 10 years, only 80% of solar cells can survive. We have to develop some effective debris removal or protection system. The second one is making space debris. Man-made debris are increasing, and according normal traffic model, the mass of them might become twice every 10 years. When we construct a large satellite like Solar Power Satellite, we have to launch many rockets. Additionally, when a satellite is constructed on orbit, much exhaust matter may be created. J. A. Angelo Jr. and T. E. Albert suggest that their 5 GW solar power satellite system will make 2.5 times space debris in maximum. Thus, our big project will make many space debris. Man-made space debris are not so much now, but if they are increasing this pace, quite serious situation will come out. The danger of Extra Vehicle Activity (EVA) will be increased and the accident of impact between inactive and active satellites might happen. But, more important situation is the final stage of such a large satellite. The satellite on low Earth orbit (LEO) will be fall into the atmosphere after its life. Such a large satellite cannot bum out in the atmosphere, and almost of it must fall on the ground. In the worst case, it falls on a big city keeping on its size. At that time, many buildings are destroyed, the city is burning up and millions of people will be dead. It cannot compare with the case of a small meteoroid. It will be the biggest and unprecedented disaster in our history. We have to worry about its final stage. One solution of it is drawing it to high altitude orbit for not falling into atmosphere. In such a case, space debris increase on the new orbit of the
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