One of the most well-known tropes of science fiction is the space elevator (the Wiki lists scores of sci-fi, manga, and anime stories that use space elevators as a plot device). At first glance, the reason is obvious: relatively cheap transfer of mass from Earth’s gravity well into space.
The key words, however, are “at first glance”, for even with the great strides in economical spaceflight made by Elon Musk’s SpaceX and his competitors, sending mass from dirtside to orbit is expensive:
When the space shuttle was in operation, it could launch a payload of 27,500 kilograms for $1.5 billion, or $54,500 per kilogram. For a SpaceX Falcon 9, the rocket used to access the ISS, the cost is just $2,720 per kilogram.
On a side note, the current cost of gold at the time of this writing is $57,119/kg, so dirtside-to-orbit transfer of mass via the space shuttle was only slightly less expensive than gold. That, people, is really what grounded the space shuttle program.
So while SpaceX is providing a 95% savings compared to using the space shuttle, it still costs a cool quarter million to send a 200-lb man into space (not counting life support requirements e.g. oxygen, food, and water). It has been estimated that the cost of mass transfer using a space elevator could be as little as $25/kg, so it seems that using a functioning and stable space elevator to transfer mass to orbit would certainly be much cheaper than using chemical rockets, right?
Um, not so much. Ya gotta build the doggone thing first, and a space elevator would likely be the most expensive object ever built by man. Sure, a recent proposal estimates the cost of construction to be about $10B, significantly less than the current $13.5B cost of a modern Ford-class aircraft carrier.
I call BS on that estimation. BS, BS, B-effing-S.
First off, graphene — the material most likely to be used — is wonderful stuff:
Graphene is 200 times stronger than steel by weight.
It is 1,000 times lighter than paper.
It is 98 percent transparent.
It conducts electricity better than any other known material at room temperature.
It can convert light at any wavelength into a current.
And, last but not least, graphene is made from carbon, the fourth most-abundant element in the universe, so we’re not likely to run out
But guess what? There’s graphene, and then there’s graphene. As with most construction materials, ya get what ya pay for. Graphene can be made for as little as $50/kg. Problem is:
The costs of producing graphene have already dropped substantially over the past 20 years from hundreds of thousands of dollars per kilogram to less than $50. However, exploiting the material’s electronic properties places much higher requirements on the crystal quality — grain boundaries, defects and dislocations all disrupt the material’s electronic behavior — so that the price tag for electronics-grade graphene remains high.
Would we really need electronics-grade graphene for the space elevator? Almost certainly, if for no other reason than to minimize those defects and dislocations. How much electronics-grade graphene would we need? Let’s get really optimistic and say the graphene elevator only needs to be a half meter in diameter. However, a space elevator would also need to be about 42,000 kilometers long. The math seems simple — all one need do is solve for the volume of a tube: πr²h (pi times radius-squared times the height of the tube).
The answer is 8,246,700,000,000 cm³. The mass of graphene is 0.763 mg/m².
But does anyone see the problem in those two sentences? The first sentence is a matter of three-dimensional volume. The second sentence is a matter of two-dimensional area. Graphene is a two-dimensional material because it’s one. freaking. atom. thick. How many sheets of one-atom-thick graphene would need to be wrapped around each other to produce a half-meter-thick tube? I don’t effing know!
But I do know that bringing all that graphene to high earth orbit would be insanely expensive, even with the cost of commercial spaceflight being somewhat less than stratospheric.
So the dream of a space elevator is effectively dead, right? For now, yes, but maybe not forever. There’s more than one way to skin a cat.
Most of the cost of a space elevator would be in the manufacture of the graphene, its transport out of Earth’s gravity well, and the assembly of the elevator itself in the vacuum of space. How could those costs be cut using current or near-term technology?
Step 1: Eight trillion cubic centimeters of graphene requires a lot of very pure carbon. We can get that carbon here on Earth, but there’s lots more available in the asteroid belt. In fact, most asteroids are carbonaceous, meaning they have high contents of carbon. Identify a good candidate, stick a few rockets on it, and send it off plunging towards the sun. Not into the sun, of course, but around our neighborhood star using something called the Oberth Maneuver, a type of ‘gravitational slingshot’ which has already been used several times to increase the velocity of NASA probes to the outer planets and beyond.
But the Oberth Maneuver isn’t just for increasing the velocity of probes. Instead of acceleration burns, those rocket engines already on the asteroid can fire at precisely determined times, decelerating the asteroid just enough for it to travel to the outer reaches of Earth’s gravity well, to one of our Lagrange points where they can be parked and remain stationary without posing any danger to Earth. Of course the plan sounds far simpler than it really is, but the fact remains that if we humans really want to do so, it is within our current technology to park asteroids (and all the resources they contain) in permanent orbit around our planet, all for the cost of a few rocket engines, hundreds of bitcoins’ worth of computational power, and (most importantly) a great deal of trust-building diplomacy to reassure most of humanity that no, we’re not about to have another Yucatan Impact.
Step 2: Once the asteroid is in stationary orbit at a Lagrange point, the next stage is the installation of automated mining and construction. This may actually be the most difficult — and most expensive — part of the whole enterprise, one that would probably require pressurized boots on the ground. Fortunately, there are deep-pocketed corporations already working on precisely what would be needed to mine asteroids. The mining is only half of what’s needed, for after the carbonaceous mineral is mined, it must be refined and then used to manufacture electronics-grade graphene.
There is of course a host of problems and issues that must first be resolved, such as what to do with the waste material from the mining operation. The nature of asteroid mining in microgravity may well obviate any attempt to use it as backfill, so it may need to be pulverized and ejected spaceward, probably in a vector either above or below the plane of the solar system where it would pose no threat to any planet (especially our own). Another issue would be how to power the mining, refining, and manufacturing plants. It is not clear whether solar power would be sufficient, but nuclear power certainly could fit the bill. Best of all, once the automated mining, refining, and manufacturing plants are in operation and being regularly maintained, it can continue production as long as it has power.
Step 3: The graphene manufacturing plant would attach the new graphene directly to the growing elevator shaft — picture an ever-lengthening piece of licorice growing out of a machine. This may be the part most trying to the patience of investors and humanity as a whole — and the one most likely to doom it from the beginning — for even in the incredibly-unlikely event that it could pump out a kilometer of graphene shaft each day, it would still take over a century to produce the minimum shaft length of 42,000 kilometers.
That’s a major reality check, isn’t it?
But the fact remains that building a working space elevator without bankrupting our nation is within our grasp. We have the technology and the industrial base to make it happen. All that we really need are the political will and societal patience to see it through.