A major Japanese construction company, Obayashi Corporation, has announced plans to build a space elevator within 40 years, allowing people to be transported to space stations above the earth.
The proposed design (see below) features a 96,000-kilometre-long cable connecting an earth-based spaceport to a counterweight in orbit, roughly a quarter of the way to the moon. An elevator car would then transport people along the cable between the spaceport and a research station 36,000km above the earth – a journey that would take roughly 7.5 days.
But how realistic a vision is this? And what would a space elevator cable even be made of?
The space elevator concept first entered public consciousness through Arthur C. Clarke’s 1978 science fiction novel Fountains of Paradise. In the novel, an elevator made of a strong filament or cable was connected to a geosynchronous satellite to transport materials to and from Earth.
Literary fantasies aside, the reality is that the type of cable proposed by Obayashi Corp. can and will be made. Whether it works or not is another story.
Obayashi Corp’s proposed space elevator will utilise a material discovered in 1991 by Japanese physicist Sumio Iijima – carbon nanotubes (CNTs). CNTs are essentially single tubes (single-walled carbon nanotubes) or multiple concentric tubes (multi-walled carbon nanotubes) comprising sheets of graphene. CNTs are one of the strongest materials ever created, stronger than diamond, Kevlar, or even spider’s silk.
As such, CNTs would be the ideal material for creating a space elevator cable. Of course there are several issues that need to be addressed before this lofty goal can be realised.
The first, and most fundamental challenge, is that nanotubes can only be grown to around 20 centimetres in length, at present. So the first challenge is tethering these tubes together to form ropes, very much like the method used to make wool yarn.
CSIRO scientists have started to work on making fabrics out of sub-microscopic carbon nanotubes yarns, but the length of the CNTs being used – one to 300 microns, where 1 micron is one-millionth of a metre – make this a real challenge.
Nonetheless the technique of twisting nanotubes into a self-locking yarn has been highly successful.
So while we are close to making reasonably long nanotube yarns there is one intrinsic flaw: the self-locking mechanism in CNT yarns is held together by only van der Waals interactions – weak intermolecular forces between the nanotubes.
While the accumulation of these interactions along the body of a nanotube makes for quite strong bonding within the yarn, these forces are much weaker than if the tubes were “welded” to one another.
The ideal situation would be to fabricate an individual carbon nanotube 96,000 kilometres long. But this is unlikely: a CNT of this length, when stretched out, could wrap around the earth’s equator more than twice – hardly manageable in a lab.
Alternatively, scientists could chemically weld (via intramolecular covalent bonding) individual nanotubes, preferably end-on-end with a bonding structure similar, if not identical to, the structure of the carbon nanotubes themselves.
Current techniques can easily attach one nanotube to another or attach nanotubes to a variety of other materials, including nanoparticles, DNA, polymers and proteins. These nanocomposites have a broad range of applications from sensors to solar cells.
But in the context of a space elevator cable, the nanotechnologists’ ambition of end-on-end carbon nanotube attachment is still elusive. Achieving this ambition may prove crucial before anyone will feel confident about being lifted into space by elevator.
Clearly without a strong enough material the entire idea of a space elevator is just an intellectual exercise. Happily there is a whole world of scientists out there working hard to turn our dreams into reality.