Nuclear rockets could travel to Mars in half the time − but designing the reactors that would power them isn't easy 

 Fission technology is well established in power generation and nuclear-powered submarines, and its application to drive or power a rocket could one day give NASA a faster, more powerful alternative to chemically driven rockets.  

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https://www.space.com/nuclear-rockets-could-travel-to-mars-in-half-time-designing-power-reactors-not-easy