A generation ship is a concept for a spacecraft that could cover interstellar distances in the Milky Way galaxy utilizing contemporary propulsion technologies that exist as of 2011. Since star systems are so far apart in space, standard rocketry, solar sail, ion, or other forms of known space-propulsion capabilities like nuclear power lack the sophistication to produce rapid enough final velocities. Using these propulsion methods could take thousands of years to reach nearby stars, and the idea of the generation ship is a work around to this problem. Essentially, a generation ship would be a miniature version of Earth itself with a self-sustaining environment. The crew that set out on such a mission from Earth would have children along the way, and, as these children were born, they would also have children, resulting in a line of distant descendants being alive on the ship when it reached its final destination.
Two fundamental limitations are faced with using a generation ship to reach distant star systems at velocities that are only a small fraction of the speed of light. The first limitation is the development of an ecosystem on the ship itself that could be reliably sustained for hundreds or thousands of years without crucial equipment failures. Such an ecosystem would have to be built in Earth orbit due to the massive size of the structure, or based inside the hollowed out center of a small asteroid. Accelerating such a large mass to even a small fraction of light speed is also problematic. If the generation ship could be accelerated to 1% of light speed over the course of many decades, it would still take more than 450 years to reach Earth's nearest neighboring star, Alpha Centauri.
Equally problematic for generation ship design is building a stable political and economic social structure that could survive intact for hundreds or thousands of years, unlike the history of most social structures that have had shorter lifespans on Earth itself. Many science fiction stories that feature generation ship travel portray societies at the end of the voyage who have lost all technical sophistication and knowledge of their past, believing that their ship is an actual world itself and refusing to leave it for colonization on the intended planet. Other accounts reflect the inhabitants stratifying socially and killing each other off due to racism or other biases before the destination is reached, or forming radical religious or political structures detached from the reality of their circumstances.
Any generation ship would therefore have to have many reliable redundancies built into the life support and navigational systems to avoid catastrophic breakdowns, as well as proven psychological controls for preventing the breakdown of the society over time. Spaceship Earth travel would also require heavy shielding against cosmic rays and other deadly radiation, which increases in intensity as the ship gets closer to the speed of light. Even with these safeguards in place, it is likely that the ship would only be a fraction of the way to its destination, having traveled for a century or so, when advances on Earth create faster propulsion systems for spacecraft that would quickly pass the generation ship by and arrive at the point of interest much sooner.
A possible alternative to multi-generational space travel with developing technology as of 2011 is that of the relativistic rocket. A relativistic rocket would accelerate at a constant level of 1 gravity (G) while on course to its destination, providing an Earth normal gravity effect for the inhabitants and allowing it to reach an eventual velocity of nearly the speed of light. Halfway through its journey, it would have to reverse thrust to de-accelerate at 1 G so that it could slow to a reasonable speed once the destination had been reached and attain orbit.
Propulsion systems that utilized hydrogen gas in space or photonic drives where the fuel supply would be nearly endless would be required, but a constant 1G acceleration spacecraft could reach star systems thousands of light years away in about 30 to 40 years. A 30-year journey to the Orion Nebula, at a distance of 1,565 light years from Earth, could eliminate the severe social tensions and vast resource needs that a generation ship would have. Due to relativistic effects of travel near the speed of light, however, 3,100 years would pass on Earth during the same 30 year span for the residents of the spacecraft. This would make it very likely that the relativistic rocket's inhabitants would arrive at their destination to find that the planet had already been settled by humans thousands of years in the past, perhaps populating and polluting the new natural environment to a state that was no better than the one they left behind on Earth 30 years ago.