to be thickest there and taper 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 would need to be made of a material with a high tensile strength/density ratio. For example, the Edwards space elevator design assumes a cable material with a tensile strength of at least 100 gigapascals. Since Edwards consistently assumed the density of his carbon nanotube cable to be 1300 kg/m3, that implies a specific strength of 77 megapascal/(kg/m3). 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 of its own weight at sea level to reach a geostationary altitude of 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 (0.2–0.3 MPa/(kg/m3)). Modern fiber materials such as kevlar, fiberglass and carbon/graphite fiber have breaking lengths of 100–400 km (1.0–4.0 MPa/(kg/m3)). Nanoengineered materials such as carbon nanotubes and, more recently discovered, graphene ribbons (perfect two-dimensional sheets of carbon) are expected to have breaking lengths of 5000–6000 km (50–60 MPa/(kg/m3)), and also are able to conduct electrical power. For a space elevator on Earth, with its comparatively high gravity, the cable material would need to be stronger and lighter than currently available materials. For this reason, there has been a focus on the development of new materials that meet the demanding specific strength requirement. For high specific strength, carbon has advantages because it is only the sixth element in the periodic table. Carbon has comparatively few of the protons and neutrons which contribute most of the dead weight of any material. Most of the interatomic bonding forces of any element are contributed by only the outer few electrons. For carbon, the strength and stability of those bonds is high compared to the mass of the atom. The challenge in using carbon nanotubes remains to extend to macroscopic sizes the production of such material that are still perfect on the microscopic scale (as microscopic defects are most responsible for material weakness). As of 2014, carbon nanotube technology allowed growing tubes up to a few tenths of meters. In 2014, diamond nanothreads were first synthesized. Since they have strength properties