diameter of 10 km or more for large-scale power transmission, will be a potential problem in densely populated regions. Microwave beaming is challenged by laser as method for wireless power transmission. Laser transmission has the advantage of being able to achieve high power density and it is therefore possible to use much more compact receivers. The drawbacks of laser transmission is the low conversion efficiency, less than 20%, and the attenuation of the laser beam in the atmosphere. For CO2 lasers with a laser wavelength of 10.6 pm the attenuation is 20%. The transmission is also highly dependent on the weather conditions. For receiving the laser beam, there are several possibilities, such as photovoltaics, heat engines, energy exchangers and thermoelectric laser energy converters. For laser to become competitive with microwave transmission it would probably be necessary to greatly improve the efficiency. A breakthrough might be the development of free-electron lasers, which would have an efficiency of 50% or more. For the near future microwave transmission will in most cases remain the preferred method of power beaming, but it is advisable to continue to watch the development of laser technology as it with time may become an increasingly attractive alternative. 4.3.2 Engineering Space Technologies Control of Space Structures Presently, design of space structures have been limited to simple satellite designs and a few very complex design cases such as Space Station Freedom (SSF). In the future, any solar power satellite project will require a very good understanding of the vibration characteristics and the successful control of these unwanted effects. Current work in this area, such as the modeling of the vibration characteristics, use of robust control, and the Reduced Order Model (ROM)ZResidual Mode Filter (RMF) design for large space structure control, have been, theoretically, successful. Research has primarily been limited to mathematical, both theoretic and computational, and terrestrial experiments, although a number of in-space experiments have been proposed and will probably be completed by the time space solar power becomes commercially viable. Space Construction Due to the sheer size of Space Solar Power Program, in-space construction will be necessary. This will be done either with the use of erectable systems such as truss bay assembly, or through the use of large deployable systems. Currently, erectable systems have shown considerable promise in the EASE/ACCESS experiment, but the rescue of the INTELSAT satellite on STS-61B demonstrated that the dynamics of berthing and docking might be an issue. In contracts, other areas in erectable systems such as configuration design and methods of joining have shown their maturity and have been successfully used. System maturity also has been shown in deployable systems. It is quite common in almost every satellite, to see deployable solar panels being used. Unfortunately, complex deployable structures such as antennas have not fared as well. The unsuccessful deployment of the Galileo high gain antenna, a deployable mesh type structure, demonstrated that complex deployable structures still are not up to where they need to be, although they will continue to be examined because of there many advantages. Also, new systems with new techniques and different advantages are being developed almost yearly. With all these choices, all with different advantages and disadvantages, space construction should be ready for the Space Solar Power Program. Resources In the near-term, it is difficult to propose a viable role for on-orbit construction, in-space manufacturing, or non-terrestrial materials within the program. However, several studies have concluded that, longer term, solar power satellites are unfeasible with only Earth-launched material and have suggested the use of non-terrestrial resources to provide material for construction. These resources could include materials indigenous to the moon, asteroids, or even refined material such as empty external tanks brought to orbit by the U.S. Space Shuttle. Analysis of the support required for such a scheme emphasizes technology development and infrastructure on the Moon. A program for developing these technologies is necessary so that we have the capability to realize the larger systems in the long term. An indigenous space resources utilization (ISRU) program can be envisioned which will be evolutionary, justify itself at each step, and demonstrate the necessary technologies for the next step. This can all be done with an eye towards developing technology to support a satellite construction project in the long term.
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