Structure: (2) Cable: Carbon nanotubes are one of the candidates for a cable material. A space elevator cable must carry its own weight as well as the additional weight of climbers. The required strength of the cable will vary along its length. This is because at various points it has to carry the weight of the cable below, or provide a downward force to retain the cable and counterweight above. Maximum tension on a space elevator cable is at geosynchronous altitude so the cable must be thickest there and taper carefully as it approaches Earth. Any potential cable design may be characterized by the taper factor - the ratio between the cable's radius at geosynchronous altitude and at the Earth's surface. The cable must be made of a material with a large tensile strength/density ratio. For example, the Edwards space elevator design assumes a cable material with a specific strength of at least 100,000 kN/ (kg/m). This value takes into consideration the entire weight of the space elevator. An untapered space elevator cable would need a material capable of sustaining a length of 4,960 kilometers (3,080 mi) of its own weight at sea level to reach a geostationary altitude of 35,786 km (22,236 mi) without yielding. Therefore, a material with very high strength and lightness is needed. For comparison, metals like titanium, steel or aluminium alloys have breaking lengths of only 20-30 km. Modern fibre materials such as kevlar, fibreglass and carbon/graphite fibre have breaking lengths of 100-400 km. Quartz fibers have an advantage that they can be drawn to a length of hundreds of kilometers even with the present-day technology