BI I am not biased spamy incorrect or defiant CNTs are not required to build all space tethers only those that are as long as an Earth surface to geostationary orbit space elevator There are space tethers in orbit right this very minute that are made with existing materials Even the Boeing report that you have quoted says they are possible with existing materials Is there an upper limit to the length that a practical space tether can be built with existing materials Yes there is The overall upper length limit for a non rotating skyhook that is still economically worthwhile is probably around 5 000 km To the best of my knowledge no one has gone to the trouble of actually determining that upper limit simply because by the time we build one that long there will be better materials that will allow us to build an even longer one A good example of this is suspension bridges About the longest you can build a practical suspension bridge using steel cable is a tad over one mile They have since made longer ones using carbon fiber cables But even carbon fiber cables are not strong enough to build a suspension bridge across the Strait of Gibraltar It is all a matter of degree With current materials we can build a non rotating skyhook up to about 5 000 km in length if we push it But we don t need to build one that long to make affordable space flight a reality A 2 200 km long non rotating skyhook will have a lower endpoint velocity of 80 of orbital velocity for that altitude and an upper endpoint velocity of 91 of escape velocity for its altitude Such a skyhook combined with a reusable single stage sub orbital launch vehicle and the Orion spacecraft would give us affordable access to Earth orbit and all of near Earth space Space tourism asteroid mining satellite solar power stations and space colonies would all become possible with such a system This is the truth I don t know how to make it any clearer