The concept of Solar Power Satellite (SPS) is to utilize the space surrounding the earth to tap energy for human society. SPS generates electric power in space using the solar radiation energy and converts it to the microwave power beam to the receiving antenna on the ground. The microwave power is converted to the commercial utility power. SPS has a great potentiality for the large-scale clean energy system to replace the fossil energy plants. To realize the SPS concept, scientific research is widely required for the associated technologies, environmental issues, and social aspects. This special issue is devoted to report the current status of research for microwave power transmission, high voltage solar array, ultra-large antennas, microwave-ionosphere interactions, and construction and structure. This status report will hopefully be a help in promoting the SPS-related research in the field of vacuum science and technology.

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  This paper describes the wireless power transmission via microwave (MPT) system for the Space Solar Power Station/Satellite (SPS). The SPS will be hugest space system and we need high efficient, huge, high accurate, light weight and inexpensive phased array for the SPS. Now we do not have any commercial MPT system on ground. Therefore, we have to consider the roadmap of the MPT system from the ground application to the SPS. We propose three step application of the MPT as follows;
1^st Step: Weak Power Wireless Power Transmission (Ubiquitous Power Source, RF-ID, Energy Harvesting) under Present Radio Wave Configuration or High Power Wireless Power Transmission in Closed Area
2^nd Step: High Power Wireless Power Transmission to/from Moving Target/Source with Phased Array under New Radio Wave Configuration (Special Frequency for WPT) on Ground
3^rd (Final) Step: Wireless Power Transmission in Space (SPS)

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  In order to manage gigantic power of SSPS (Space Solar Power System) efficiently, use of extreme high voltage is necessary unless we minimize the transmission distance by employing a sandwich type of generator/transmitter panel. For 1 GW-class SSPS, the power transmission/generation voltage of 10 kV is necessary to keep the cable mass below a few hundred tons. To realize high voltage photovoltaic power generation, arcing caused by environmental interaction must be overcome. Three issues are reviewed. The first is degradation of solar cell performance due to repetitive arcing resulted from surface charging in GEO. The second is sustained arc phenomenon that may lead to catastrophic loss of significant portion of the total power. The third is micrometeoroid or space debris impact and resulting sustained arc. A design of high voltage solar array is proposed to prevent solar cell degradation due to repeated trigger arcs and sustained arc.

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  Two kinds of ultra-large antennas are required for SPS. The one is called a spacetenna which transmits an enormous amount of a microwave, and the other is called a rectenna which receives and rectifies the microwave. As all microwave power in a transmitted beam has to be received by the rectenna, the beam width should be quite narrow.
  Accordingly, the diameters of both antennas are quite large: For example, in the Reference System of NASA, the spacetenna and rectenna are, 1 km and 3 km in diameter, respectively. Moreover on the spacetenna, several requirements are imposed for space use. In order to construct an ultra-large antenna, several schemes were proposed, and technical issues were clarified. Novel technologies have been studied and proposed in Japan so far. Launch, assembling and testing of ultra-large antennas are also important.
  This paper describes the peculiarities of these antennas in comparison with conventional large antennas. Relevant technologies will be also presented.

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  In the present paper, we reviewed some linear and non-linear interactions which possibly occur when intense microwave beam emitted from Solar Power Satellite (SPS) propagates through the ionosphere down to the ground. In the linear regime, effects are discussed about the Faraday rotation, refraction, scintillation, and particle collision on the microwave beam propagation through the ionosphere. As to the non-linear interactions, three-wave coupling, self-focusing of microwave beam, and plasma perturbation by the Ponderomotive force are briefly reviewed. Since the theoretical analysis of these nonlinear phenomena is generally difficult, numerical simulations play an important role for the understanding of the interactions. In the present paper, we particularly focused on the three-wave coupling and the density perturbation by the Ponderomotive force by showing some simulation results obtained in computer experiments with plasma particle model.

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  Construction methods, vibration reduction techniques and attitude control of space solar power satellites (SSPS) proposed so far are introduced. The SSPS that generate more than 1GW electric power, which corresponds to one nuclear power plant, require some squire kilometers. First, construction methods and various types of SSPS such as SPS2000, JAXA reference models of 2004/2005, USEF/JAXA (former ISAS) model, Tokyo Institute of Technology model, AIST model of Japan, Russian model and MIT model of U.S.A. are discussed. Second, vibration analysis and reduction technique using tethers conducted in Hokkaido University is presented. Finally, attitude control using tethers researched in Tokyo Metropolitan University is introduced.

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  Brief history of tribology experiments in space is described. Tribological performance in a high vacuum was a great concern in early stage of space development, and it was urgent task to verify whether the actual space environment can be simulated using ground-based vacuum facilities from tribological viewpoint. Some friction tests as well as bearing tests were carried out on board satellites in 1960s, and the conclusion was that the results of space experiments was very similar to those of ground-based comparison experiments.
  In 1980s, however, Space Shuttle flown in Low Earth Orbit (LEO) revealed a new problem; oxidization by active atomic oxygen, which is more than 90% constituent in the environment. Some types of solid lubricants might be oxidized, and thus lost tribological effectiveness. Efforts to clarify the effect of LEO environment on solid lubricants, as well as to simulate LEO environment properly, have been devoted including space experiments. Recent tribology-related space experiments are briefly reviewed.

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  The gaseous environment surrounding spacecraft has been revealed to differ greatly from a natural gaseous environment. The former is dominated by surface outgassing effects and is strongly influenced by spacecraft activities such as maneuvering. For this study, an atomic oxygen monitor (AOM), which was intended to measure the atomic oxygen environment that a satellite encounters, was mounted on Engineering Test Satellite-VII (ETS-VII). However, the AOM was unable to monitor atomic oxygen because the pressure around the sensor was higher than expected; moreover, and the resolution was degraded by effects of outgassing. The initial pressure was of 10^-4 Pa-order. Therefore, the purpose was switched, and data acquisition such as measurement of the vacuum environment of the satellite and seasonal variations were done. Results of measurements during one year and eight months showed fluctuation within 10^-5-10^-4 Pa during the satellite eclipse time.

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  The material-exposure experiment was performed on the International Space Station using the Micro-Particles Capturer and Space Environment Exposure Device (MPAC & SEED) developed by the Japan Aerospace Exploration Agency. The experiment was executed on the exterior of the Russian Service Module (SM) of the International Space Station. The SM/MPAC & SEED consists of the MPAC, which captures the space debris, and the SEED, which exposes the polymeric material, the paints, the adhesive, the bearing, and the compound material. This paper is focused on space dust environment, and the results of the MPAC experiment are described.

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  Effects of a low earth orbit (LEO) space environment on tribological characteristics of a solid lubricant film were evaluated. The tested lubricant, which has been used for space applications, was a bonded molybdenum disulfide (MoS₂) film with organic binder. Three sets of the specimen were exposed to a real LEO space environment for about 1, 2 and 3 years aboard the Service Module of the International Space Station (ISS) by the Space Environment Exposure Device (SEED) experiment. In this paper, results of friction tests in vacuum and surface analyses for the specimens exposed for 1 and 2 years are reported. The results are also compared to those of the specimens irradiated individually with atomic oxygen (AO), ultraviolet rays (UV) and electron beam (EB) on the ground.

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  Change in such properties were shown as friction, surface roughness, structure and so on of lubricative coating films of TiN, Cu, Cu/BN that have been effected by exposure to orbit environment for a year, two years and three years with a Russian Service Module/Space Environment Exposure Device (SM/SEED) prepared by JAXA on the International Space Station (ISS). It was found that substrates with the exposure to orbit for a year generally decrease friction coefficient and can keep almost constant even after bakeout under a high vacuum as well as in an atmospheric pressure due to some kind of modification of lubricant layer by the exposure. Surface analysis with XPS showed the formation of Si based contamination and carbon layer and this might have an influence on the change in friction behavior.

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  The pressure dependence of the electron stimulated desorption (ESD) ions were investigated by the quadrupole mass spectrometer with a Bessel-box type energy analyzer. The mass spectra of the ESD ions were measured under introduction of H₂O, CO, O₂ and CO₂. The ESD ion peaks for those gasses were observed at O⁺ (m/z=16). The mass spectra of the gas phase ions were also measured and compared with those of ESD ions during the pumping down process. The origin of the main species in ESD ions, O⁺, was the adsorbed H₂O on the grid surfaces.

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