Everyone knows about how difficult is to get to space and explore the universe. Approximately 2/3 of the missions to Mars were a failure, and even if Mars is so close to Earth, universally size speaking, it takes 6-7 months to get there. It is a fact that the common propellants are pretty inefficient for the exploration of the universe since the weight of a rocket ready to be launched consists approximately 20% structure and 80% propellant!! There are many other different kinds of propulsion that either they are way more efficient than that but they don't provide enough power to lift a rocket from the Earth, or vice versa. However, nuclear propulsion provides an extraordinary efficiency compared to the common propellants, plus a great power/thrust to keep this spacecraft travelling. It is said and allready studied (since NASA has allready constructed a nuclear rocket) that nuclear propulsion is the key to the exploration of the universe. But one would ask, so why are they holding back? They are holding back for two reasons: 1) for environmental issues and 2) this could enable the use of military nuclear spacecrafts... What are your opinions on that matter? Should they or shouldn't they?