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Interstellar spacecraft require advanced propulsion and navigation systems due to the vast distances between star systems. Solid-fuel chemical rockets are insufficient, and new methods of propulsion must be devised. The psychological well-being of occupants during the journey is also a challenge. Proposed designs include Project Orion, the Bussard ramjet, and the solar sail. Other designs, such as wormholes and spacetime bending, require huge amounts of energy or exotic matter. Creating realistic propellant-based ships is more feasible, but still decades away.
Interstellar spacecraft, or spacecraft capable of travel between solar systems, are expected to be considerably more advanced than the interplanetary vehicles humanity has already built. 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 compared to the vast expanses of empty space in which they are embedded, high-precision propulsion and navigation systems will be required for an interstellar spacecraft to successfully reach its goal.
One advantage of interstellar travel is the near vacuum of space; once an object moves, there is little resistance to slow it down. However, with modern chemical rocket technology, accelerating any object faster than many thousandths of the speed of light would be tremendously difficult. To make an interstellar spacecraft, it has been recognized 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 its occupants remain comfortable and psychologically fit throughout the journey, which could take hundreds or thousands of years. An interstellar spacecraft would probably 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 genetically engineered humans to live longer, eat less or resist boredom.
One of the first concrete proposals for an interstellar spacecraft comes from Project Orion, born in 1958 at General Atomics in San Diego. The design, believed by several atomic scientists to be practical with today’s technology, involves a ship equipped with a huge shock absorber, propelled forward by the explosions of thermonuclear bombs dropped from the rear of the aircraft. The bombs would vaporize part of the blast plate (a plastic form was found to be optimal), which would serve as a propellant to provide thrust. Many dozen scientists and engineers were serious about building an Orion spacecraft in the 1960s and using it to make trips around the solar system in the 1970s and 1980s, but the project was shelved due to the political sensitivity of weapons nuclear.
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 suck in interstellar hydrogen much like the way a jet engine sucks in air. The hydrogen nuclei would fuse in proton-proton reactions, providing energy for thrust. Although the amount of hydrogen floating in interstellar spaces is widespread, much hydrogen could be collected if the spacecraft moved close to the speed of light, as its design requires. It has been calculated that such a ship could move at about 16% of the speed of light.
Yet another proposed spacecraft is the solar sail. Since sunlight also exerts some pressure on surfaces, it could be used to propel a payload attached to a giant 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, could be a potential candidate. Tightly rolled up, a payload of a few hundred tons could be enough 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 it reaches the target star system, a task that could be assigned to nuclear retrorockets.
Many other types of interstellar spacecraft have been considered, both in a serious context and in science fiction. Spacecraft using wormholes or spacetime “bending” are commonly discussed, although such designs typically require huge amounts of energy (about on the order of what the sun releases in a couple of minutes) or the use of negative matter, an exotic form of matter that may or may not exist. Creating ships that use real propellants to provide thrust is far more realistic, even if their creation is likely decades away given our current technology. Considering the truly radical consequences of interstellar travel and colonization, a few decades doesn’t seem like a very long time to wait.
