Brent Kearney

Many people are unaware of the project to build an elevator to space. It is exactly what it sounds like: a project to build a lift for people, equipment, energy — whatever — that extends 100,000km, from Earth’s surface into space. There is a plethora of information about the project available. The NASA link in the first sentence is a good place to start, followed by a Google search.

The biggest technical problem with the project has been to create a material that is strong enough for the cable. The cable will link a platform in geostationary orbit to one that floats in the ocean. It needs to withstand estimated forces of 62 gigapascals. Thats 62 billion Pascals. A "Pascal", for those of you who fell asleep in physics class, is a measurement of pressure equal to the force of 1 Newton per square metre. A "Newton" is the amount of force required to accelerate 1 kg of mass at a rate of 1 metre per second per second. 62GPa is a lot of pressure; 10GPa is the pressure required to form diamonds out of carbon. The strongest steels have a tensile strength of 1.2GPa. So this cable needs to be stronger than any known natural materials.

The proposed material for use in the space elevator cable is carbon nanotubes. Carbon nanotubes are a relatively recent invention, and are said to have a theoretical tensile strength of 100GPa. Laboratory tests have shown a strength of 63GPa, and improvements are being made all of the time. However, a recent study by Nicola Pugno of the Polytechnic of Turin, Italy, reported by the prestigious journal, Nature, takes a critical view of the feasibility of a carbon-nanotube cable for use in the space elevator project.

The crux of the skepticism rests on the inevitability of manufacturing defects in carbon nanotube cables. Small defects at the atomic level dramatically reduce the tensile strength of carbon nanotube fibers by as much as 30%. According to his calculations, a 100,000km cable would have so many defects that its strength would be reduced by 70%, leaving a theoretical maximum tensile strength of 30GPa. Keep in mind that a 100,000km cable could wrap around the Earth 2.5 times!

I have no doubt that Pugno’s calculations are accurate, but the results do not support the conclusion that the project is infeasible. It only means that with current technology (2006-05), it cannot be done. Its a good thing that its not scheduled for completion next week! One recent development in nanotechnology is inorganic nanotubes, which use materials other than carbon to produce nanotubes. Inorganic nanotubes also produce remarkable materials with surprising properties, and there have been more than 50 varieties of them reported. The cable might end up being produced using carbon nanotubes with a more refined manufacturing process, one that eliminates the defects that degrade its strength. Or, perhaps a strong enough cable could be produced using matter other than carbon. All that Pugno has accomplished is to point out that a space elevator cable won’t be manufactured today.