satellite. Moreover, if we keep our mind on the final stage, we cannot avoid that an unexpected situation make it destroy and a part of it falls into atmosphere. Another effect of constructing a large satellite is breaking the Van Allen belts. The Van Allens belts surround the Earth and are formed by charged particles. The particles move between the north pole and south pole very quickly. The solar power satellite is very large structure. Such a large orbiting space structure disturbs particles forming the Van Allen belts. But the Van Allen belts are spread 108 - 109 km2 and the area of Solar Power Satellite is about 10 km2. So the ratio of disturbing area is about IO"7. It will be negligible. Crew Health and Safety Concerns Radiation One of the most hazardous aspects of space activities on living systems is radiation. Unlike most of the dangers involved with solar power satellite construction, space radiation is initially painless. It is the extended term effects that are the chief cause of concern for this project It has been demonstrated that astronauts have a greater likelihood of contracting cancer years after their mission than the general public . [Peterson and Nachtwey, 1990] With increased EVA required for construction of a large solar power satellite, more astronauts will be exposed to greater amounts of space radiation than ever before and thus radiation is a major concern. Non-ionizing radiation is a general term for electromagnetic radiation (EMR) from wavelengths of 10'5 cm (3xl09 MHz) to 106 cm (0.03 MHz) including: part of the ultraviolet spectrum, visible light, infrared, microwave and radio wave radiation. Its effects on human health are described in a previous section. Transmitting power from a solar power satellite to other sites in space or to the Earth “may produce potentials for EMR exposure above current terrestrial levels” and this should be evaluated both in space and on Earth as part of the ongoing health and safety concerns [Vanderploeg, 1992], Ionizing Radiation refers to high-energy particles and photons which can induce electrons to separate from their parent molecule when in close proximity to the molecule. Thus, primary ionizing radiation (with energies of megaelectron volts - MeV) produces millions of electrons (with energies of electron volts - eV). These secondary electrons can disrupt cell processes by breaking up molecules such as DNA or by creating reactive chemical radicals [Letaw, 1992], There are three primary sources of ionizing radiation in space. Galactic Cosmic Radiation (GCR) - originates outside the solar system; composed chiefly of protons, alpha particles (helium nuclei) and a small amount of heavier nuclei. These particles penetrate deeply into matter. Energy levels typically average 300 - 3000 MeV. Trapped Radiation - electrons and protons of the solar wind which are geomagneticly trapped in the Van Allen Radiation Belts or of the South Atlantic Anomaly (SAA). The Van Allen Belts consist of a higher belt, mainly of electrons, at about 10000 km above the Earth and a lower belt at 800 - 1200 km. Energies are typically 30 - 300 MeV. The SSA is a discontinuity of the Earth's geomagnetic field in the Southern Hemisphere where the inner Van Allen Belt dips down to lower altitudes.. Solar Energetic Particles - particles, mostly protons, emitted from the sun during solar flares. Solar flares follow an 11 year cycle of activity. Energies are typically 10 - 100 MeV [all energy values taken from Letaw, 1992], From this knowledge, it would be suggested that if construction is carried out in Low Earth Orbit (LEO) care is taken to avoid the SAA as much as possible and absolutely no EVA be carried out in this area. An orbital inclination of 28.5° and 500 km altitude will pass through the SAA for six orbits, then miss it for nine orbits. A spacecraft in LEO is protected from the outer belt trapped electrons, galactic cosmic radiation and normal solar flare radiation [Newman, 1992] that would pose safety problems for construction in a higher Earth or Geostationary Earth Orbit (GEO). It is therefore proposed that on-orbit construction be performed in LEO and the structure(s) boosted to a higher orbit if necessary, with a minimum of EVA at higher orbits. Radiation Dose Concept Absorbed dose is the amount of radiation energy deposited in tissue. It can be expressed in units of the Gray (Gy) or radiation absorbed dose (rad). 1 Gy = 1 Joule/kilogram
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