The energy requirements make •••••••• travel very difficult.
possible ") identified some breakthroughs that are needed for interstellar travel to be possible.
nuclear Many different spacecraft propulsion systems have been proposed to give spacecraft the required speeds, including nuclear propulsion, beam-powered propulsion, and methods based on speculative physics.
interstellar Right on!
high Although a high density interstellar medium may cause difficulties for many interstellar travel concepts, interstellar ramjets, and some proposed concepts for decelerating interstellar spacecraft, would actually benefit from a denser interstellar medium.