Journey through the universe beyond the speed of light [HD]. DKto

Interstellar space travel is manned or unmanned travel between stars. Interstellar travel is much more difficult than interplanetary travel: the distances between the planets in the Solar System are typically measured in standard astronomical units (AU)—whereas the distances between stars are typically hundreds of thousands of AU, and usually expressed in light-years. Because of the vastness of those distances, interstellar travel would require either great speed (some percentage of the speed of light) or huge travel time (lasting from years to millennia).

The required speeds for interstellar travel in a human lifespan are far beyond what current methods of spacecraft propulsion can provide. The energy required to propel a spacecraft to these speeds, regardless of the propulsion system used, is enormous by today's standards of energy production. At these speeds, collisions by the spacecraft with interstellar dust and gas can produce very dangerous effects both to any passengers and the spacecraft itself.

A number of widely differing strategies have been proposed to deal with these problems, ranging from giant arks that would carry entire societies and ecosystems very slowly, to microscopic space probes. Many different propulsion systems have been proposed to give spacecraft the required speeds: these range from different forms of nuclear propulsion, to beamed energy methods that would require megascale engineering projects, to methods based on speculative physics.