The power transmission is another key technology to determine the characteristics of an operational SPS. Microwave is the oldest and a still strong candidate of power transmission, since it is not much influenced by weather condition at a power receiving site. This should be a strong reason for a country of much rain like Japan to prefer a solar power satellite to a ground based solar power station, although the disadvantage to use a large area necessary for the receiving antenna and resulting environmental issues should be resolved. Another option for this is a laser power transmission. This method can solve the problem of large area of a receiving site compared with the microwave method, but the laser beam would be easily absorbed by water droplets of rain and cloud in the atmosphere. The structure design is a main technical problem to determine a system concept combining specific power conversion and power transmission methods. It also heavily depends on the material technology which is related to the problems of orbital transportation. If the structure elements have to be fabricated on the Earth, the structure should be so designed that the packing efficiency is as high as possible. It is also anticipated that a space factory can efficiently fabricate them or even lunar resource processing can produce raw materials. The attitude and position control system of the SPS would be considered as secondary element of the SPS compared with the above mentioned factors, since this can take much options of system design than the other key elements under the same penalty for weight and power efficiency. Problems will arise concerning the choice of the best for a specific concept of the SPS. Environmental and economical issues are the most important criteria for choices of the technical options. Although importance of these problems are emphasized even now, there are few concrete technological bases for these studies to stand on. Especially, potential impact on the Earth environment of space transportation system operation, microwave power transmission and introducing solar energy to the Earth are identified as main problems and often blindly believed as unavoidable negative factors of the SPS. An economical study has a different scale of measure, which depends on a long range prospect of the existing energy supply. As important technological breakthroughs will take place during the long preparation period until operational system will begin to work, the early economical evaluation of SPS would be difficult. EXPERIMENTAL STUDY Considering complexity of the problems, experiments which can be done for the SPS at the present stage do not seem efficient to solve the problems. But there are people who have potential interest in the SPS but do not necessarily understand these problems very well. Such early experiments are good opportunity for these people to be familiar with the SPS through a very simple demonstration of the principle. The following are examples of such experiments which are not dedicated to the SPS study but indirectly related to it as described here. MINIX Experiment (2) The MINIX (Microwave Ionosphere Nonlinear Interaction Experiment) is a sounding rocket experiment. The main objective of this experiment is to study tech-
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