We have invented it, it's just that there are some political (and PR) obstacles to developing and using it: http://www.islandone.org/Propulsion/ProjectOrion.htm

We have invented it, it's just that there are some political (and PR) obstacles to developing and using it: http://www.islandone.org/Propulsion/ProjectOrion.html http://www.islandone.org/Propulsion/ProjectOrion.html "Our motto was 'Mars by 1965, Saturn by 1970'", recalls Dyson. Orion would have been more akin to the rocket ships of science fiction than to the cramped capsules of Gagarin and Glenn. One hundred and fifty people could have lived aboard in relative comfort; the useful payload would have been measured in thousands of tons. Orion would have been built like a battleship, with no need for the excruciating weight-saving measures adopted by chemically-propelled spacecraft.