Why has it not already been developed?
The idea of using atomic bombs as a means of propelling spacecraft is an old one, first put forward in 1946 by Stanislaw Ulam. He proposed using small nuclear explosions to accelerate a spaceship, an idea that underwent serious study in the late 1950s under the name of Project Orion. It's a remarkably simple idea: a rapid series of detonations behind a protective pusher plate would accelerate the spacecraft, with shock absorbers evening out the acceleration effect on the crew.
|Design for a modern Project Orion spacecraft, with a close-up of the crew section|
If one-gee acceleration was achieve the crew would experience normal Earth gravity. Ideally, such acceleration would be maintained until the mid point of the journey, after which the spacecraft would be turned and decelerated at the same rate. This would provide the equivalent of normal gravity for the crew for the entire trip, eliminating the negative health effects of zero gravity.
Such constant acceleration would make interplanetary travel within our solar-system incredibly swift. When Mars is at its closest point to Earth, only 65 million miles away, it would take just two days for the acceleration and deceleration phases at one gee. For Jupiter and Pluto it would take about 6 and 11 days respectively. Those are phenomenally short journey times, all quite achievable with technology available to us now.
|A Project Orion spacecraft leaving Earth orbit, and arriving at Mars two days later|
The use of such a large amount of nuclear detonations means that launching from the surface of a planet is not feasible - the radioactive fallout produced would be unacceptable in all locations, and the electromagnetic pulses would damage sensitive electronics in line of sight with the launch. Such a spacecraft would have to be constructed in orbit and their nuclear detonation drive activated only when a safe distance from Earth had been achieved.
Project Orion had such incredible potential, but the political will was not there, mainly due to concerns about fallout, and that, incredibly, no one in the US government at the time could see a point to having such a massive launch capability. The project died in the early 1960s, with the US government no doubt wanting to put all it's investment into getting a man on the Moon. To have such an incredible lack of vision is quite astounding.
That was not the last of such projects. A decade later the British Interplanetary Society conducted a study under the name of Project Daedalus. The propulsion system studied was a fusion rocket, which would detonate hundreds of pellets of a deuterium/helium-3 mix. Helium-3 is rare on Earth, but fortunately it is relatively abundant on the Moon making that location the ideal one for constructing and launching such a vehicle.
|A Project Daedalus two-stage interstellar spacecraft|
The project focused solely on designing an unmanned interstellar spacecraft that could reach its destination star system, Barnard's Star, within a human lifetime. The design, a two stage system, has the potential to reach a velocity of up to 12 percent the speed of light. Unfortunately this project has remained nothing more than a paper study.
It's disappointing that no spacecraft with a nuclear propulsion system has yet been built. Even a recent project started in 2009, and named Project Icarus, had aims only to design an interstellar spacecraft for a potential mission in the coming centuries, and to motivate scientists in designing such things. There are no concrete plans to build even a test system for launch.
|A Project Icarus design concept for an interstellar spacecraft|
Other means of rapid space travel, such as antimatter warp drives, are likely to remain in the realms of fiction for centuries, even forever. We should get over our moral objections and fears of all things nuclear. We should embrace the raw power and efficiency of such a means of propulsion, and get building and launching such spacecraft now. It may well be the only technical means by which we can ensure the continuation of our species.
If we do that, then within a century or two we could have flourishing colonies and economies on our solar-system's planets, moons and asteroids. With the vast resources that would then be at hand it would only be a matter of century or two more before our first interstellar colonies are established.
We need to just get on with it.