What if the Vietnam War ended five years earlier and the “space race” with Soviet Union continued in earnest for decades to come with billions of dollars in military funding for developing new systems including establishing a permanent Moon base and crewed missions to Mars? That's one of the intriguing story lines pursued in Apple TV's fascinating alternate history series “For All Mankind.” Today using a nuclear reactor to power a space ship may seem like pure science fiction, but in fact, the United States did in depth development and testing of nuclear powered space flight engines starting in the 1950s before the project was terminated in 1973 and it's still in the running as a viable system for future crewed inter planetary space exploration missions. The chief advantage of nuclear propulsion over traditional chemical systems is that nuclear engines are twice as efficient in generating thrust carrying an equivalent weight of fuel, with corresponding advatages in payload and range. This progress report ”Nuclear Propulsion in Space” was issued by NASA and Nuclear Energy Commission in 1968 and covers R&D programs going back to Los Alamos in the early 1950s, through full power testing at Jackass Flats in Nevada and later at a facility near Sacramento California in the 1960s. Testing was supervised by “The Space Nuclear Propulsion Office” (SNPO), primarily as the “NERVA” program. (“Nuclear Engines for Rocket Vehicle Applications”) You'll see everything from the theory of nuclear propulsion and its potential for space exploration, through the evolution of successive systems, to full power testing shown in vivid color. But Nerva was terminated in 1973 before a nuclear rocket system could be tested due to competition for funding during the Vietnam War and opposition from President Nixon who favored other projects. The Saturn rocket was adequate for remaining manned Moon missions through Apollo 16 in1972. The space shuttle was developed as an economical alternative for near earth missions. But after NERVA was canceled, theoretical study of Nuclear propulsion continued and interest was revived in 1983 when the Strategic Defense Initiative ("Star Wars") identified missions that could benefit from nuclear rockets that are more powerful than traditional chemical systems and new exploratory funding was allocated. In 2013 the Marshall Space Flight Center (MSFC) in Huntsville, Alabama studied using nuclear thermal rocket (NTR) engines for crewed interplanetary travel from Earth orbit to Mars orbit, and back. Since NTRs are at least twice as efficient as the most advanced chemical engines, they allow quicker transfer times and increased cargo capacity. The shorter flight duration to Mars, estimated at 34 months with NTR engines, compared to 89 months using chemical engines, would reduce crew exposure to potentially harmful and difficult to shield cosmic rays. On 22 May 2019, Congress approved $125 million in new funding for the development of nuclear thermal propulsion rockets.