We’ve long been studying ways to achieve interstellar travel. In the late-Fifties, NASA had Project Orion. This spacecraft would have used nuclear pulse propulsion – a series of exploding atomic bombs – to propel it the distance needed to reach Mars, Saturn and ultimately another star. The British Interplanetary Society studied the possibility of a spacecraft to the stars in the Seventies, called Project Daedalus. Daedalus had the goal of taking no longer than a human lifetime to reach its destination, requiring a speed greater than the capabilities proposed for Project Orion. It would have needed a very powerful fusion rocket. Then, in the late-Eighties, NASA tried again with Project Longshot, an unmanned probe designed to be built and launched at Space Station Freedom, the predecessor to the International Space Station. Instead of a closed fusion rocket for power, Longshot would have used a nuclear fission reactor.
So what happened to keep these projects concept only? For Orion, the Partial Test Ban Treaty, signed in 1963, rendered it impossible. The treaty banned the test detonation of nuclear weapons, except underground. Both Project Daedalus and Project Longshot require significant advances in nuclear fission technology to be viable. Today we don’t have the ability to get to Proxima Centauri in less than a thousand years, much less a hundred years.