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 nonrotating skyhook that is still economically worthwhile is probably around  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 nonrotating skyhook up to about  km in length if we push it  But we dont need to build one that long to make affordable space flight a reality  A  km long nonrotating skyhook will have a lower endpoint velocity of  of orbital velocity for that altitude and an upper endpoint velocity of  of escape velocity for its altitude  Such a skyhook combined with a reusable single stage suborbital 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 dont know how to make it any clearer