Unfortunately, the Orion concept is inherently "dirty" because it uses fission fuel. It is also inefficient; this is acceptable only because of the vast amounts of energy available. A much better alternative is fusion, since a fusion rocket would not leave a wake of heavy radioactive ions. The British Interplanetary Society's Daedalus project (61) was a study of an unmanned interstellar probe. It would have been driven by fusion "microexplosions" caused by irradiating fuel pellets with electron beams at pulse rates up to 250 Hz, in a magnetic "combustion chamber". Confinement and shaping of the plasma with a magnetic field would make Daedalus vastly more efficient than Orion. Daedalus would work just as well in the solar system as between the stars, and one can imagine that in 75 to 100 years fusion freighters will be sailing regularly between the planets. An important point is that no one has yet produced controlled fusion energy with electron beams or anything else, while the technology required to build an Orion-type spaceship has existed for over thirty years. Nuclear propulsion will get into space eventually. Orion might be the device that makes possible human occupation and economic exploitation of the solar system.


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