One of the most futuristic and far-fetched is the space elevator....simple enough on paper – extend a 22,000 mile set of cables from a space station in geostationary orbit to a corresponding structure somewhere at Earth's equator. ... The problem...is that the cable system must be constructed from a material far stronger than anything known. Carbon nanotubes have been hypothesized to fill this niche, but they aren't ready yet. … Another potential rocket replacement is StarTram…. Magnetically-levitated spacecraft will be propelled inside a curved tube aimed skyward. All air will be evacuated from the tube in order to eliminate drag....