Interstellar travel is the term used for hypothetical
manned or unmanned travel between
stars. Interstellar travel will be much more difficult than
interplanetary spaceflight; the distances between the
planets in the
Solar System are less than 30
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 a high percentage of the
speed of light, or huge travel time, lasting from decades to millennia or longer.