Actually making this kind of super-strong material, right here on Good Ole' Planet Earth, is quite well understood. These days, the use of such materials is limited to more prosaic purposes, such as the tension support cables of cable-stay bridges. Here, the economic benefits are compelling: the savings in weight and the reduction of the number of separate cables more than makes up for the cost of the materials themselves, even if they are hundreds of times more expensive than steel, tonne for tonne.

Cable-stay bridges are often confused with suspension bridges, perhaps because both are held up with towers and cables. A suspension bridge - like the Golden Gate Bridge in San Francisco - has a pair of towers which together support a thick cable (in SF, thick enough to easily walk on) in a loop forming a graceful inverted arch. The roadbed itself is held up by supplementary cables dangling at frequent intervals from the main support cable.

The problem with suspension bridges is that the cable itself is a significant fraction of the weight of the entire bridge. A cable-stay bridge dispenses with the looping cable and the hanging supports, and replaces it with straight cables directly from the support towers to the roadbed itself. Of course, this comes at a cost: the mathematics to work out the positioning and tensions in each of the cables is prodigious, not to mention the complexity of the simulations to determine how the bridge would move - all bridges move, all the time - under various weather conditions up to and including full-scale hurricanes and tornadoes.

Modern analysis techniques and computer simulations can deal with these complexities, but we still run up against the fundamental problems inherent in the design. The support cables need to be very strong in tension, much more so that for suspension bridges. Traditionally high-tensile steel is used, but the restricted strength of such materials limits the maximum size of the crossing that can be constructed.

Until recently, that is. We are just beginning to be able to get hold of carbon-nanotubes reinforced materials in industrial quantities capable of being used for large-scale civil engineering projects. Such fabrics are somewhat lighter and considerably stronger than any steel, which make them perfect for large cable-stay bridges. And it is of course exactly this material which would make the kind of space elevator I described earlier feasible.

Now, ordinary carbon fibre reinforced plastics have been around for quite some time - the material has long been used in high-performance cars, for example - but in order to be an effective building material for crossings of any kind, including large-scale bridges, the carbon reinforcements need to be huge linear molecules. The general structure is a single molecule composed of millions of carbon atoms; the hexagonal nature of carbon bonds means that the fibres are naturally hollow at the atomic level, hence the term "nanotubes". These single molecule fibres can be grouped together to form exceptionally and uniformly strong cables and, just as importantly, state-of-the-art manufacturing plants can make these available in quantities upwards of a thousand tonnes.

The colossal strength of individual fibres is difficult to describe. Imagine a thread no more than a millimetre across supporting three ordinary family cars, or ten elephants on one another’s backs, all standing on a film no thicker than paper. Despite their strength, the fibres are incredibly flexible. When I visited the first manufacturing plant for carbon nanotubes, I saw one of the first samples of the support cables for the new bridge. It was thinner than my little finger - a steel cable of the same strength would be as thick as my thigh - but, as part of the demonstration, it was formed into a series of complex knots as easily as a Boy Scout playing with a piece of string.

Part 2 Part 4