The two key problems are that conventional rockets spend most of their fuel simply to accelerate the fuel they carry with them, and that today’s rocket fuel is hopelessly inefficient—the fraction of its mass turned into energy isn’t much better than the 0.00000005% for gasoline that we saw in table 6.1. One obvious improvement is to switch to more efficient fuel. For example, Freeman Dyson and others worked on NASA’s Project Orion, which aimed to explode about 300,000 nuclear bombs during 10 days to reach about 3% of the speed of light with a spaceship large enough to carry humans to another
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