Carbon nanotubes are often assumed to be the sole viable material for the construction of an Earth-based space elevator. However, carbon nanotubes in the lengths required for construction of a space elevator remain a laboratory curiosity. Is there an alternative to carbon nanotubes for building a space elevator?
The next strongest material is zylon. Zylon is a polymer chain of carbon and other atoms. It has a breaking length of 384 kilomometers, fifty percent greater than kevlar. Zylon also has an advantage over carbon nanotubes in that it is being currently produced in mass quantities at relatively low cost.
Unfortunately, when we plug the numbers for zylon into the basic space elevator equation, we get a taper ratio of a million to one. That means we’ll need at least ten million ton space elevator cable to lift a ten ton cable crawler. A mass of ten million tons would seem to rule out zylon as a space elevator construction material.
Nonetheless, if we’re going to put people into space, a space elevator is safer and cheaper than rockets, and even a ten million ton space elevator is better than none at all. So let’s look at three methods of moving large amounts of material in space and how they can be repurposed for provisioning a zylon space elevator.
Method #1: The Space Gun
Orbital space factories would operate in conditions of zero gravity and high vacuum. The manufacture of many products, such as fiber optics and microelectronics, could benefit from such conditions. But how do we supply the orbital factories with raw materials? If we use conventional rockets, the high launch cost will make space-based manufacturing unprofitable.
An electromagnetic ‘rail gun’ could change that. Built on the slopes of a mountain near the equator, the rail gun — or rather, ‘space gun’ — could accelerate payloads up to a muzzle velocity sufficient to achieve orbit. Once the capital cost of construction is absorbed, the cost of shooting payloads into orbit would be only the cost of the electricity. Once in orbit, the payloads of raw materials would be retrieved by robots and processed in factories. The finished products would be packed inside inexpensive ceramic re-entry shields for a soft landing in the ocean for retrieval.
An earth-based space gun could not be used to put humans into space. Even if it were several kilometers long, it would need to accelerate payloads at thousands of gees in order to reach the required velocity. This is a force that would easily squash humans.
A space gun could, however, be used to launch payloads of zylon cable that will be used to build a space elevator. If — when — space manufacturing takes off, the space gun may eventually have a launch capacity big enough to supply the required amount of zylon in just a few years of operation.
Method #2: Lunar Mining
As we continue to scour the surface of the Earth for metals, we are starting to consider mining metals and minerals from the Moon as well. Teleoperated robots on the Moon could prospect and mine for gold, platinum, palladium and other rare metals. This material could be sent back to Earth by utilizing a lunar space elevator. At the high tip of the elevator, we would then drop the payloads into Earth’s gravity well. Again we would use inexpensive ceramic shells for friction braking in Earth’s atmosphere and retrieval in the ocean.
Thanks to the Moon’s lower gravity, a lunar space elevator would be much shorter than an Earth-based space elevator and require no taper at all. A starter lunar elevator could be packed inside a single rocket payload. As lunar mining operations grow, the lunar elevator’s lift capacity can be increased with the use of zylon cable fabricated on the Moon from materials taken from the Moon.
With a big enough lunar elevator, we could consider mining the Moon for not-so-rare metals like lithium and silver. Once the lunar space elevator’s lift capacity is increased to hundreds of thousands of tons annually, we can use lunar resources to fabricate our ten million ton Earth-based space elevator.
Method #3: Asteroid Capture
Asteroids with diameters measured in tens of meters cross the orbit of the Earth every year. It’s statistically inevitable that one of them will impact our planet, and given the increasingly dense population of the Earth, a city could be struck with the kinetic energy release of an atomic bomb. Being able to protect ourselves from asteroid impact is seen by many people as a key priority for national and international space programs.
We need the technology to protect ourselves from asteroid impact. We need to develop robots and space tugs that can intercept asteroids and divert them from their paths. And before we can do the real thing, we need to practice. We need to use robots and space tugs to intercept and capture a mini-asteroid and put it in orbit.
Even these very small asteroids can easily weigh a million tons or more. Catch a few of them, and you’ve got enough material to build a zylon space elevator.
Seeing the Big Space Elevator Picture
As the Earth becomes increasingly overpopulated, global pollution and wars over resources threaten the survival of the human species. A viable system of space elevators could offer space colonization as an alternative to rationing, regimentation, and global conflict.
Space elevators offer a way to economically transport large numbers of people from Earth to colonies in space. Building space elevators that mass in the millions of tons each would be a tremendous engineering challenge, but the cost would be far less having to rebuild civilization after a global thermonuclear war fought over resources and territory.
We can launch the zylon cable from Earth with space guns, from the Moon with a lunar space elevator, or we can mine the material from captured asteroids. With zylon as the construction material for space elevators, we presently have the technology to affordably colonize other worlds in space.