Space elevator
A space elevator is a theoretical transportation device for cheaply sending people and cargo into orbit. If one can be built, a space elevator would make it possible to go to outer space without inefficient and often single use rockets, lowering the average cost to launch from the current average of $10,000 to $25,000 per kilogram to as little as $100 per kilo, thus making widespread civilian space travel and colonization economically feasible. The elevator would consist of a cable or tower stretching to a mid or geostationary orbit, with two tethers, one at the surface and the other at a space station. Some kind of climbing vehicle would go up and down it. No known building material is strong enough and with a low enough density to build a space elevator, but a material like carbon nanotubes could be it.