Politics, specifically the PTBT[1]. Otherwise Project Orion[2] would be a reality now. [1] https://en.wikipedia.org/wiki/Partial_Nuclear_Test_Ban_Treaty https:

Politics, specifically the PTBT[1]. Otherwise Project Orion[2] would be a reality now. [1] https://en.wikipedia.org/wiki/Partial_Nuclear_Test_Ban_Treaty https://en.wikipedia.org/wiki/Partial_Nuclear_Test_Ban_Treat... [2] https://en.wikipedia.org/wiki/Project_Orion_(nuclear_propulsion) https://en.wikipedia.org/wiki/Project_Orion_(nuclear_propuls...