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Interstellar travel can be achieved through nuclear pulse propulsion, fusion rockets, laser-powered light sails, and antimatter-based propulsion. Antimatter and light sails are the fastest and most promising methods. Nuclear pulse propulsion was within reach in the 1950s and 1960s but is now prohibited. Slowing down at the destination is a challenge, and light sails may require nuclear bombs, fusion, or antimatter reactors. The Bussard ramjet is impractical due to drag on the scoop.
Because the stars are so distant, the proposed means of interstellar travel involve very long journeys or very rapid means of getting there. We will focus on the fast means, because nobody likes to wait a long time to get to their destination.
There are four possible means of interstellar travel that are taken seriously. They are nuclear pulse propulsion, one of the first proposed fusion rockets, laser-powered light sails, and antimatter-based propulsion. Of these, antimatter propulsion and light sails look the most promising as they are the fastest. Antimatter or light sails could reach speeds similar to that of light, exploiting the relativistic effect of time dilation, which could make interstellar travel to Alpha Centauri, four light-years away, seem less than a year.
The remaining methods of interstellar travel, nuclear pulse propulsion and fusion rockets, are limited to speeds of about 10% that of light at most, making them less attractive. However, nuclear pulse propulsion is interesting because it has been within our reach since 1945. Had an aggressive nuclear space program been pursued in the 1950s and 1960s, mankind probably could have sent manned expeditions to the outer solar system in the 1970s. However, nuclear explosions are prohibited under current international law, and the Outer Space Treaty prohibits the deployment of nuclear weapons in space.
One of the biggest requirements for interstellar travel is being able to slow down once you reach your destination star system. At speeds approaching the speed of light, the braking process must begin about halfway through the journey. One of the most talked about methods of interstellar travel – light sails accelerated by huge space lasers – would have to pack a few nuclear bombs, a fusion or antimatter reactor, to ensure they have a means to slow down once they reach their destination. For probes, flybys may be sufficient, but for serious colonization efforts, the ability to restrain is a must.
One of the dead but famous ideas for interstellar spacecraft is the Bussard ramjet, which would suck up and then burn interstellar hydrogen with a giant scoop, allowing it to go around carrying its own fuel. However, later calculations found that the scoop would be impractical: the drag on it would exceed its thrust.
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