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Nuclear pulse propulsion or external pulsed plasma propulsion, is a hypothetical method of spacecraft propulsion that uses nuclear explosions for thrust. It was first developed as Project ''Orion'' by DARPA, after a suggestion by Stanislaw Ulam in 1947.〔(【引用サイトリンク】title=History of Project Orion )〕 Newer designs using inertial confinement fusion have been the baseline for most post-''Orion'' designs, including Project ''Daedalus'' and Project ''Longshot''. ==Project ''Orion''== (詳細はGeneral Atomics in the late 1950s and early 1960s. The idea of ''Orion'' was to react small directional nuclear explosives against a large steel pusher plate attached to the spacecraft with shock absorbers. Efficient directional explosives maximized the momentum transfer, leading to specific impulses in the range of 6,000 seconds, or about thirteen times that of the Space Shuttle Main Engine. With refinements a theoretical maximum of 100,000 seconds (1 MN·s/kg) might be possible. Thrusts were in the millions of tons, allowing spacecraft larger than 8 × 106 tons to be built with 1958 materials. The reference design was to be constructed of steel using submarine-style construction with a crew of more than 200 and a vehicle takeoff weight of several thousand tons. This low-tech single-stage reference design would reach Mars and back in four weeks from the Earth's surface (compared to 12 months for NASA's current chemically powered reference mission). The same craft could visit Saturn's moons in a seven-month mission (compared to chemically powered missions of about nine years). A number of engineering problems were found and solved over the course of the project, notably related to crew shielding and pusher-plate lifetime. The system appeared to be entirely workable when the project was shut down in 1965, the main reason being given that the Partial Test Ban Treaty made it illegal (however, before the treaty, the US and Soviet Union had already detonated at least nine nuclear bombs, including thermonuclear bombs, in space, i.e., at altitudes over 100 km: see high altitude nuclear explosions). There were also ethical issues with launching such a vehicle within the Earth's magnetosphere: calculations using the now disputed Linear no-threshold model of radiation damage showed that the fallout from each takeoff would kill between 1 and 10 people.〔Dyson, George. Project Orion – The Atomic Spaceship 1957-1965. Penguin. ISBN 0-14-027732-3〕 In a threshold model, such extremely low levels of thinly distributed radiation would have no associated ill-effects, while under hormesis models, such tiny doses would be negligibly beneficial. It should be noted that with the possible use of less efficient clean nuclear bombs for achieving orbit and then more efficient higher yield dirty bombs for travel would bring down the amount of fallout caused from an Earth-based launch by a significant factor. One useful mission for this near-term technology would be to deflect an asteroid that could collide with the Earth, depicted dramatically in the 1998 film ''Deep Impact''. The extremely high performance would permit even a late launch to succeed, and the vehicle could effectively transfer a large amount of kinetic energy to the asteroid by simple impact, and in the event of an imminent asteroid impact a few predicted deaths from fallout would probably not be considered prohibitive. Also, an automated mission would eliminate the most problematic issues of the design: the shock absorbers. Orion is one of very few interstellar space drives that could theoretically be constructed with available technology, as discussed in a 1968 paper, ''Interstellar Transport'' by Freeman Dyson. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Nuclear pulse propulsion」の詳細全文を読む スポンサード リンク
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