24 March 2014
Project Orion was an inter-planetary space craft capable of carrying thousands of tons and hundreds of people to Mars and the outer satellites in human survivable time scales (a trip to the nearest star, 4 light years away, would take 130 years). It used the force from thousands of hydrogen bombs to propel itself, achieving a level of efficiency impossible for chemical rockets.
Project Orion had to be a military project because it involved cheap, low-yield, directional hydrogen bombs. Being a military project it had to be developed from a tool for space exploration to the ultimate planet killing weapon. Being a weapon it frightened a lot of people who didn't want to see something that destructive exist. And so it was cancelled.
It would take 1000 H-bombs to get to orbit, producing enough fallout to kill 1 person somewhere on the Earth every time it was launched from the middle of the Pacific Ocean. It is also the only off the shelf technology available to get us to Mars or intercept asteroids in weeks instead of years.