There are many ways to get around space, but most of them are pretty slow. This is why, even when launching at an optimal time, a spacecraft leaving Earth requires about six months to reach orbit around Mars.
For decades, many rocket scientists have looked to a propulsion system powered by a nuclear reactor as the fastest practical means of getting to Mars and other places in the Solar System more quickly.
Wernher von Braun, the German engineer who defected to the United States after World War II, recognized the potential of nuclear thermal propulsion even before his Saturn V rocket landed humans on the Moon with chemical propulsion. Eventually, this led to a project called NERVA, which stood for Nuclear Engine for Rocket Vehicle Application. It was eventually canceled to help pay for the space shuttle.
NASA has supported efforts to develop a nuclear thermal engine in fits and starts since. The basic idea is pretty simple—a nuclear reactor heats a propellant such as liquid hydrogen, and it expands through a rocket nozzle and provides thrust. No such rocket engine has ever flown, however, and at present, NASA is more interested in developing nuclear energy for surface power on other worlds than working on propulsion.
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