牧 謙一郎

Martian Moons eXploration (MMX) is a mission to Martian moons under development in JAXA with international partners to be launched in 2024. This paper introduces the system definition and the latest status of MMX program. “How was water delivered to rocky planets and enabled the habitability of the solar system?” This is the key question to which MMX is going to answer in the context of our minor body exploration strategy preceded by Hayabusa and Hayabusa2. Solar system formation theories suggest that small bodies as comets and asteroids were delivery capsules of water, volatiles, organic compounds etc. from outside of the snow line to entitle the rocky planet region to be habitable. Mars was at the gateway position to witness the process, which naturally leads us to explore two Martian moons, Phobos and Deimos, to answer to the key question. The goal of MMX is to reveal the origin of the Martian moons, and then to make a progress in our understanding of planetary system formation and of primordial material transport around the border between the inner- and the outer-part of the early solar system. The mission is to survey two Martian moons, and return samples from one of them, Phobos. In view of the launch in 2024, the phase-A study was completed in February, 2020. The mission definition, mission scenario, system definition, critical technologies and programmatic framework are introduced int this paper.

A solar power satellite (SPS) is a renewable energy system that converts the sun's energy into electricity in space and transmits it to Earth using microwaves. The SPS concept, first proposed in 1968 in the United States, has recently started attracting increased public attention as a promising energy system that can be used to resolve global environmental and energy problems. One of the most challenging technologies for the SPS is microwave power transmission from the geostationary orbit to the ground. The technologies for microwave power transmission have been studied for more than 40 years since the initial demonstrations in the 1960s; however, for SPS application, considerable research, especially on high-efficiency power conversion between direct current (dc) and radio frequency (RF) and on high-accuracy microwave beam control over a long range, is still needed. This paper introduces the concept of SPS and presents the technologies and issues associated with microwave power transmission from space to ground. Current research status and the future development prospects for microwave power transmission toward commercial SPS use are also described.

We are developing a phased-array antenna system as a bread board model (BBM) for space experiments using a small scientific satellite toward the solar power satellite (SPS). The purposes of the space experiments are to demonstrate a precise directional control of wireless power transmission (WPT) technology from space to the ground and to clarify the propagation characteristics of the microwave power in the ionosphere. A phased-array antenna possesses a potential for precise beam directional control and beam forming by controlling an amplitude and phase of the microwave from each antenna element. We designed and fabricated the BBM in order to evaluate an effect of amplitude and phase error of the microwave circuits produced at the time of the manufacturing and generated by temperature fluctuation on the microwave beam control. Copyright © 2013 by the International Astronautical Federation. All rights reserved.

The commercial life of Solar Power Satellite (SPS) is usually considered to be 30-40 years. However the disposal plan after expiration of its life has not been well studied so far. This paper describes the life analysis of SPS by evaluating the radiation and debris (meteoroid) environment, and proposes a replacement scenario at the end of life to prevent generating space junk. The SPS basic model (tethered-SPS) is used for this study. If we set an allowable degradation level at 15-20 % for the commercial life 5-10 % by radiation degradation, 5 % by hyper-velocity impact loss, and 5% by spontaneous electrical failure, 40 years life is expected in the following assumptions (1) photovoltaic cells with high radiation-resistance (5-10 % degradation at 2.5 × 1015/cm2 (1 MeV electron equivalent fluences)), (2) redundant tether wires (tape tether) of more than 15 mm wide, and (3) modularized structure for the power generation/transmission panel with a module size of 0.5m × 0.5m, beyond that the impact damage does not propagate. In the end of life scenario, new units of SPS are transported to the geostationary orbit from the ground and are exchanged for degraded units, and then the degraded units are transported to the ground for refurbishment. The replacement operation starts near the end of life and is completed in a year in the same way as the initial construction. The replacement operation does not generate any junk in the orbit. This scenario is heavily dependent on the space transportation system between the ground and the orbit, consisting of reusable launch vehicle (RLV) and orbit transfer vehicle (OTV). Copyright © 2013 by the International Astronautical Federation. All rights reserved.

A terahertz (THz) beam steering method is demonstrated by applying the characteristic of grating lobe (GL) radiation from a linear array antenna and the interference of femtosecond optical pulses. A photoconductive device is illuminated by two femtosecond laser beams combined at an angle of less than 0.5 degrees. Considering the interference pattern as a THz point source array, THz GL radiation is generated through the superposition of radiation emitted from all point sources and steered by varying the interval of the interference pattern. The THz beam direction could be changed by 20 degrees at 0.93THz by varying the relative incidence angle of the pump beams by 0.033 degrees. (C) 2012 Optical Society of America

Construction of the SSPS (Space Solar Power Systems) requires 4 major system-level technologies power generation, wireless power transmission, large space structure, and space transportation, each in an extremely large scale. All technologies except wireless power transmission have been already put into practical use in a certain scale. Wireless power transmission has been demonstrated at the 10 kW level in the field experiments. The major problem associated with the SSPS is to apply the technologies to the huge system at GW level in power, km level in size, and several ten thousands of tons in weight. Also it is requested to make its power price be competitive with that of existing power generation systems on the ground. This paper reviews the current status of each system-level technology and evaluates the technology development scenario required for the commercial SSPS to be realized in the mid-2030's. Copyright © (2012) by the International Astronautical Federation.

We have investigated the characteristics of a terahertz (THz) beam steering method based on a combination of difference-frequency generation (DFG) with the principle of the phased array antenna. In the DFG of THz radiation from a nonlinear optical crystal pumped by optical beams, the phase front of the THz radiation is indirectly tilted by adjusting the relative incidence angle between the pump beams to the crystal. A magnification of the steering angle with a factor of 193 is demonstrated as the most important effect provided by the method. The effect allows the use of a high-speed optical deflector for adjusting the incidence angle, accelerating the steering more than a hundred times compared with mechanical methods. The phase mismatching between the THz radiation and the pump beams as well as the refraction at the crystal surface limit the steering angle of the THz radiation to 56 degrees, full width at half maximum.

We present a THz beam steering method that is based on the generation of grating lobe(GL) radiation from linear array antenna and interference principle of femtosecond laser beam. THz GL radiation with the bandwidth of 110GHz was generated from a strip-line photoconductive antenna. As the pump beam two femtosecond laser beams were combined with finite angle and produced an interference pattern. We demonstrated that the THz GL radiation can be steered by adjusting the relative incidence angle between two pump beams. The direction of the THz beam was changed by 20 degrees when the incidence angle of one pump beam was only varied by 0.1 degrees.

The generation of terahertz (THz) radiation is demonstrated by using the spatial dispersion of an femtosecond optical pulse. The overlap of two dispersed beams with a spatial shift allows us to generate a narrowband radiation on the basis of photo mixing. Additionally the lateral shift of the beam can tune the frequency of the THz radiation.

Femtosecond optical pulse train is produced by the reflections on the step mirror which consists of two D-shape mirrors. The THz radiation is generated from the photoconductive antenna pumped by the pulse train and the frequency tuning is achieved by simply moving one of the D-shape mirrors in order to vary the step difference.

Terahertz (THz) beam steering is demonstrated based on the phased-array antenna principle without using actual phase shifters. The THz radiation angle from lithium niobate crystal can be varied by adjusting the incidence angle of the pump beam. This technique has advantage of high speed, wide angular range compared to conventional steering methods.

Terahertz (THz) beam steering by using difference frequency mixing and phased array antenna principle was demonstrated. The THz radiation was generated by pumping with spatially dispersed beams produced from an ultrafast laser. And the THz beam could be steered by tilting one of the incident pump beams so as to change their relative phase relation. The steering range was 187 times as wide as the tilting angle. In addition, both the direction and the frequency of the THz beam were simultaneously and independently controlled. The frequency tuning was achieved from 0.3 to 1.7 THz.

We report on an actively controlled enhancement cavity for terahertz generation using optical rectification in lithium niobate. By recycling the transmitted fundamental laser power from a Ti:Sapphire laser an increased pump power is available for the nonlinear process. Therefore a much higher THz signal is obtained in relation to single-pass geometry. Terahertz intensities generated by different emitters are compared with Cherenkov-type generation in the enhancement cavity.

We introduce several types of terahertz- (THz) wave parametric sources. THz-waves can be generated by optical parametric processes based on laser light scattering from the polariton mode of nonlinear crystals. Using parametric oscillation of MgO-doped LiNbO3 crystal pumped by a nanosecond Q-switched Nd:YAG laser, we have realized broadband sources as well as coherent (narrow band) and widely tunable THz-wave sources. The THz-wave Parametric Generator (TPG) generates a broadband THz wave using a simple configuration; the THz-wave Parametric Oscillator (TPO) and the injection seeded THz-wave Parametric Generator (is-TPG) are two sources that generate coherent, widely tunable THz radiation by suitably controlling the idler wave. We report the characteristics of the oscillation and the radiation including linewidth and tunability. Further, we show the recent progress about these THz-wave parametric sources. We developed two new kinds of TPG by using compact pump sources. One TPG includes a flash-lamp-pumped multimode Nd:YAG laser with a top-hat beam profile, that allows generating high energy, broadband THz waves. Fitting in a space as small as 12 cm x 22 cm (including the pump source) this TPG outputs more than 100 pJ/pulse, which is about 100 times higher than the best results previously reported for TPG. The other has a potential to be a narrow-linewidth injection-seeded TPG, based on an laser-diode-pumed single-mode microchip Nd:YAG laser. The pump laser linewidth is below 0.009 nm and its size is 105x30x32 mm(3). This allowed us to achieve a narrow-linewidth compact injection-seeded terahertz-wave parametric generator.

We report on the efficient Cherenkov-type terahertz; (THz) generation using lithium niobate crystals and a high repetition rate femtosecond laser. An actively length controlled synchronously pumped ring resonator acts as an enhancement cavity for the near infrared pump and therefore recycles the available pump power towards a higher terahertz output. A seven times higher THz field amplitude was measured with electro-optical sampling.

We report on the generation of broadband terahertz (THz) pulses using Cherenkov-type generation in magnesium oxide doped lithium niobate (MgO:LN). A ring enhancement cavity for the femtosecond pump source led to an increased conversion efficiency for the THz radiation. A silicon prism coupler (PC) attached to the emitting surface of the crystal was used as an output coupler.

Formerly, microwave emission due to a hypervelocity impact to a metal had been found. The same experiments were carried out for various kinds of material: metal, ceramic, brick and rubber. The waveforms are quite different each other. For an aluminum plate, many pulses are observed in addition to almost continuous generation of random noise in 22GHz-band. For an alumina ceramic, most signal power is in the form of random noise with a small number of pulses. In the cases of a red brick and a polyurethane rubber, a small amount of random noise is observed. The generated power for each material is correlated with electric conductivity and density. The result shows especially that the averaged microwave power has strong correlation with the density. The mechanism of the microwave generation is, therefore possibly related with the destruction process of materials.