A space elevator is a type of surface-to-orbit transport, utilising a solid link to the surface of a planet and a counterweight placed beyond geosynchronous orbit at the other end. It would necessarily be at least 45,000 km (28,000 mi) high, and would, except in special circumstances, need to be constructed at the Earth's equator.
Timeframe for building the first one
Liftport ambitiously claims one could be built by 2031, through the use of automated robotic builders using buckytubes (buckminsterfullene) as a building material. (A buckytube is a molecule made of long chains of covalently bonded carbon atoms.) Buckytube towers would be able to easily withstand airplane impacts, or even indirect nuclear attacks.
Consequences of building one
It could take a while, but once the first orbit-reaching structure is created, there will be more to follow. The convenience of reaching space by climbing rather than launching independently will bring the cost of space travel down to affordability. The political and economic reasons for building such towers will be pursuasive to many countries and groups in the years after they become technologically feasible. We’d better get used to these towers, because we’ll be seeing a lot of them!