Nuclear Thermal Propulsion (NTP) works by passing a propellant, usually liquid hydrogen, through a nuclear fission reactor core, heating it to extreme temperatures before expelling it through a nozzle. NTP can theoretically achieve specific impulse of 850-1,000 seconds, roughly double that of the best chemical engines, while providing thrust levels high enough for crewed deep-space missions.
NASA and DARPA are jointly developing the DRACO (Demonstration Rocket for Agile Cislunar Operations) program to flight-test an NTP engine by the late 2020s. NTP is widely considered the most promising technology for reducing Mars transit times from 7-9 months to approximately 3-4 months, significantly reducing crew radiation exposure and mission risk.