Interstellar spacecraft, or spacecraft capable of making trips between solar systems, would need to be considerably more advanced than the interplanetary craft that mankind has already constructed. While the distance between planets in this solar system can be measured in light-minutes or light-hours, distances between star systems are typically dozens of light years. Because solar systems are so small relative to the vast expanses of empty space they are embedded within, high-precision navigation and propulsion systems will be necessary for an interstellar spacecraft to reach its target successfully.
One advantage of interstellar travel is the near-vacuum of space; once you get an object moving, there is little resistance to slow it down. However, with contemporary chemical rocket technology, accelerating any object to a speed above many thousandths of the speed of light would be tremendously difficult. To make an interstellar spacecraft, it has been acknowledged that solid fuel chemical rockets will be insufficient and that new methods of propulsion must be devised.
The other major technical hurdle in creating an interstellar spacecraft is making sure that its occupants stay comfortable and psychologically fit throughout the trip, which could take hundreds or thousands of years. An interstellar spacecraft would likely have to serve as an autonomous space colony, perhaps housing generations of individuals and the means for their survival and prosperity. Proposed alternatives include suspended animation systems or humans genetically engineered to live longer, eat less, or be resistant to boredom.
One of the first concrete proposals for an interstellar spacecraft comes from Project Orion, which was born in 1958 at General Atomics in San Diego. The project, believed by several atomic scientists to be practical with today's technology, entails a ship fitted with a massive shock absorber, propelled forward by the explosions of thermonuclear bombs jettisoned out the rear of the craft. The bombs would vaporize part of the blast plate (some form of plastic was judged to be optimal), which would serve as a propellant to provide thrust. Many dozens of scientists and engineers were serious about building an Orion spacecraft in the 60s and using it to make trips around the solar system in the 70s and 80s, but the project was shelved due to the political sensitivity of nuclear weapons.
Another, more advanced form of proposed interstellar spacecraft is the Bussard ramjet. This ramjet would consist of a giant scoop at the front of the craft, designed to intake interstellar hydrogen much in the way a jet engine intakes air. The nuclei of the hydrogen would be fused in proton-proton reactions, providing energy for thrust. Although the amount of free-floating hydrogen in interstellar spaces is diffuse, much hydrogen could be harvested if the craft were moving at near the speed of light, as its design demands. It has been calculated that such a ship might move at some 16% of the speed of light.
Yet another proposed spacecraft is the solar sail. Because even sunlight exerts some pressure on surfaces, it might be used to push a payload attached to a gigantic solar sail. Such a sail would need to be many kilometers wide, but perhaps only a few atoms thick, depending on the material used. Graphene, a particularly strong and thin arrangement of carbon atoms, might be one potential candidate. Rolled up tightly, a payload consisting of only a few hundred tons might be sufficient to deploy a solar sail capable of accelerating a payload to a substantial fraction of the speed of light. The only problem is slowing down once you reach your target star system, a task that might be assigned to nuclear retro-rockets.
Many other types of interstellar spacecraft have been considered, both in a serious context and in science fiction. Spacecraft that use wormholes or the "bending" of spacetime are popularly discussed, although such designs typically require either stupendous amounts of energy (about on the order of what the sun releases in a couple minutes) or the use of negative matter, an exotic form of matter which may or may not actually exist. The creation of ships which use actual propellants to provide thrust is far more realistic, though their creation is probably decades away, given our current technology. Considering the truly radical consequences of interstellar travel and colonization, a few decades hardly seems a long time to wait.