By Teodor Teofilov
Updated to include a clarification that the lunar space elevator isn’t a new concept pioneered by the recent student study
“The Earth is the cradle of humanity, but mankind cannot stay in the cradle forever.”
— Konstantin Tsiolkovsky, оne of the fathers of rocketry and cosmonautics, along with Goddard and Oberth
From the Tower of Babel and Jacob’s ladder to the space race of the 20th century, we have strived to reach the heavens. Humanity has looked up at the stars and dreamt of conquering the cosmos. But it’s hard to get to space.
No matter how much we wish for it to be easy and cheap to float out and see our little blue marble in the vast darkness, right now the only way to do this is to be an astronaut or a billionaire. However, not all is lost. There is a concept that could serve as the starting point of our exploration and spread across the stars — the space elevator.
The idea isn’t exactly new. The first mention of a space elevator was in 1895 when a Russian scientist — Konstantin Tsiolkovsky, was inspired by the Eiffel Tower to create a tower that reached all the way to space. His concept never became a reality because there was no substance that could hold the weight of a tower that was over 35,000 kilometers (about 22,000 miles) tall. Like all buildings, Tsiolkovsky’s tower would be under compression, supporting its weight down below.
Building tall has historically been difficult. Before structural steel replaced load-bearing masonry, building high was hard unless you went the way of the pyramids — to have little interior space and just build much wider at the base. The pyramids still stand, because there is one solid heap of stone that is wider at the bottom than the peak.
This is compressive strength — how good a material is at being compressed, like stone being piled on top of it. When each consecutive layer is wider than the one above it, the compression is spread out and lets you build taller. Theoretically there is no limit to how tall you can build if you keep making the base wider, but obviously as we are on a spherical planet there are some limits in this regard. This is basically the way our mountains are so high, but even then Everst, the tallest peak only stands at 8,848 metres, far from the 35,000 kilometers needed for Tsiolkovsky’s tower.
The Great Pyramid of Giza, which is over 145 meters high, was the tallest man-made structure for almost 4,000 years, before arguably being overtaken by the 160 meter tall Lincoln Cathedral in the 14th century. The Lincoln Cathedral only held that position for a short while, because the central spire collapse and was not rebuilt.
The skyscrapers of the 20th and 21st centuries dwarfed these ancient giants. The Burj Khalifa stands at more than 828 meters and is expected to be overtaken by the Kingdom Tower in Jeddah, which was originally planned to reach 1,600 meters, but it looks like it will be one kilometer once its complete.
Since the 1960s the idea has changed from a tower to an elevator, focussing mostly on tensile strength structures, with the weight of the system held up from above by centrifugal forces and a counterweight to keep the cable taught.
|Material||Tensile strength (MPa)||Breaking Length (km)|
|Colossal carbon tube||
Tensile strength means how much tension a material can handle before snapping. Now, relying on tensile strength instead of compressive strength wasn’t really on the table until recently when we discovered super strong materials like carbon nanotubes.
The space elevator needs to be 36,000 kilometers (about 22,000 miles) high and if carbon nanotubes can only do about 6,000 kilometers (3,700 miles) then we have a problem, because we want to carry material up it. There are two reasons that his might not be such a big problem. First, you can taper the tether to be wider at the top than the bottom, which lets it hold a longer length.
Second, the breaking length is how long something can be before breaking while in Earth’s gravity. The breaking length is higher on Mars and the Moon for instance because of the low gravity there. Although we have to work with Earth’s gravity, the cable does not — at least not most of it. The higher you go, the weaker the gravity is.
For example at 5,000 kilometers (3,100 miles) above the surface of our planet, which has a radius of 6,400 kilometers (4,000 miles), the gravity drops to just under a third of what it is on the ground and the breaking length is three times longer there, since everything weighs only a third of what it does down here. At geostationary orbit, gravity is only a couple of percent of what it is on Earth and breaking lengths are almost 50 times longer.
So the fact that the breaking length is lower than the distance of geostationary orbit, which the space elevator needs to reach doesn’t really matter, because it can be addressed by giving it a higher taper ratio — making it wider at the top than the bottom, and the breaking length is increased the further away we get from the surface. In theory any substance could be used for a space elevator if you use a high enough taper ratio.
Building something tens of thousands of kilometers tall, when the tallest building isn’t yet at a kilometer and the tallest mountain isn’t even 10 kilometers (6.2 miles) tall is pretty ludicrous. Although we currently lack the materials to build a space elevator that is anchored to our planet, a recent study showed that it is financially feasible to construct a spaceline from our Moon to geosynchronous orbit around Earth with the materials we currently have. But we will look at this idea later on.
How does a space elevator work?
Before we understand exactly how a space elevator will get us to space, it’s important to look at what an orbit is. Simply put, being in orbit is falling towards something, but moving fast enough to miss it. Imagine throwing a football — it makes an arch through the air and then hits the ground. When in space, you move pretty much the same way thanks to gravity, but if you move fast enough the curvature of the earth makes the hard ground fall away beneath you as fast as gravity pulls you toward it. To enter Earth’s orbit, rockets go up and sideways fast.
A space elevator works differently. It taps into energy from our planet’s rotation to get its cargo going fast. A good visualization would be holding a rope with a rock tied to one end and you spin it. Now there is an ant on your hand that starts climbing along the rope. The and begins to move faster and faster as it ascends towards the rock.
A good analogy of the concept of a space elevator is the game of tetherball in which a rope is attached at one end to a pole and at the other to a ball. The rope in this case is a carbon nanotube tether, the pole is our planet and the ball is the counterweight. The ball is in perpetual spin around the pole, so fast that it keeps the rope taut.
This is the general idea of the space elevator. The counterweight spins around our planet keeping the cable straight and allowing for lifters to ride up and down it.
LiftPort, one of several companies developing plans for a space elevator, proposed the space elevator to be about 100,000 kilometers high (about 62,000 miles). Why so high?
Well, the tether doesn’t end at geostationary orbit, because, even though an object at geostationary doesn’t fall down, any point beneath it would. The reason to extend the tether so far out is to provide an equal pulling force upward, and past geostationary the tether is pulling outward. This will keep the tether taut.
A different solution for a counterweight, which doesn’t require a tether running thousands of kilometers more, would be to have a big space station right over geostationary orbit.
One of the biggest problems is the tether, which will need to be mass manufactured to operate at its maximum tensile strength and have few or no flaws. Getting things into space is extremely expensive and we will have to fly the tether up and spool it down.
The cost of getting a kilogram to space varies between private companies like SpaceX and government agencies like NASA, because of special mission requirements. It costs NASA on average $23,750 per kilogram (≈ 2.2 lbs), where SpaceX’s price per kilogram is around $6,078, with prices in the private space travel sector expected to fall further. Reducing this cost is the reason we want to build this thing.
The whole building process will be extremely expensive but it will be worth it. Because it costs about $1.3 million to send the average human to space, about $40 million for a car and billions for the international space station, a space elevator could do away with the limitation of human spaceflight. If a cheap space elevator cost $20 billion to build, then it will pay for itself after launching only a million tons (about the weight of two international space stations).
The space elevator, once built will only need enough energy to go up and the fast sideways movement comes free with the Earth’s rotation. This makes it less energy demanding than the current method of using rockets to send cargo and people up in space.
Building a space elevator will be without a speck of a doubt the single largest and most expensive structure ever built by humanity. A question we need to answer is how are we going to power the climber. It could be done a number of ways. We could use a nuclear reactor, or beam it power with a super power laser from the ground.
By default the space elevator will have to be on the equator, most likely on a floating platform in the sea, because geostationary orbit is limited to a ring around the equator. Problem is that most countries with a robust space program are north of the equator and would rather have their space ports near them. This can be fixed, because only the end station needs to be above the equator (the center of force of the tether). So you can stick a tether, say in New York, and one on the opposite side of the equator to counterbalance it. It would be best to use at least three and there will have to be a system in place to be able to tweak the position to keep the tension right for each tether.
Can the tether break?
Of course it can. Although it will be made from an extremely tough material, it won’t for example survive sabotage, and as everything man made it could be flawed and break.
A popular idea is that if the tether breaks it will wrap around the planet leaving nothing but carnage in its way. This is exaggerated, because there isn’t much of a reason to build them individually thicker than an arm — you can just add more in parallel if needed, and they won’t be hitting the ground at hypersonic speed.
These cables will most likely be no thicker than large electrical wires and won’t fall down much faster than one of those would — the air will slow them. They will either burn up in the atmosphere if they fell from high enough or slow to normal speeds before hitting. So as long as you aren’t underneath one and you will be fine.
There is also this magical invention that slows things that are falling — the parachute. Although we wouldn’t want to add more weight to our elevator, if we have the ability to manufacture superlight materials with huge tensile strength, as are needed for the tether, we could place them at intervals with charges to sever the cable into smaller bits. In the first place they won’t do much damage when falling, but with parachutes we can minimize it.
It also matters where the tether breaks. If it is anywhere within our atmosphere, the cable will ripple up like a cut threat toward the station and then fall back down, dangling over the atmosphere. All we have to do is to repair it, or if we used the idea to have the tether extend much further than geostationary we could just lower it back to the ground. The cable in the atmosphere will just fall straight down.
There is one problem. The orbital speed around the Earth get lower as you get further away, but for the tether it gets faster the higher up you are. At geostationary the tether is moving at exactly the orbital speed for that height and at the station that is above it the station is moving a bit faster. If the tether is cut below the station, the station would drift off to a higher orbit, so it would need to have thrusters on it to drop to geostationary until the repairs are complete and then boost back up.
Once cut, the individual bits of the tether are going to be moving too slow for orbit and will fall down. This is where the idea of it wrapping around the equator comes from. The higher up, the faster it is moving, so it will fall and wrap around the equator, but as mentioned above the ground damage isn’t something to worry about. The people in the elevator will be in much more danger.
It depends on where the elevator is when the cable snaps. If above 23,000 kilometers (14,000 miles), the people will be okay, because they’ll fall gaining speed and would enter an elliptical orbit. As long as air doesn’t run out, someone can retrieve them. For those below 23,000 kilometers (14,000 miles), its easier as they won’t go into orbit, because the planet will be in the way and if the pods are built for reentry and have parachutes, they should be fine.
A Space Elevator From The Moon
Two graduate students published a study on Aug. 25 on the online research archive arXiv, detailing a system that would be used to provide transportation to and from the moon. Zephyr Penoyre and Emily Sandford contend that it is both technologically and financially feasible to build a lunar space elevator. The idea of a lunar space elevator was first publicly detailed by Jerome Pearson at a conference in 1977 and by Yuri Arsutanov in a separate paper published in 1979.
They call their concept Spaceline and it’s main element is a cable that is anchored to the moon and spans more than 320,000 kilometers (200,000 miles) to a point above the surface of the Earth — around 43,000 kilometers (27,000 miles). It wouldn’t be anchored to Earth as the relative motions of our planet and the moon won’t allow it.
The paper explains that the simplest version of Spaceline would be a cable barely thicker than the lead in a pencil and might weigh around 40,000 kilograms (88,000 pounds). It could be made from Kevlar or other existing materials, rather than hard-to-make carbon-based materials that are the key to the conventional space elevator.
Space travelers would fly to the end of the dangling cable, which will be kept taut by the gravity of Earth, and transfer to solar-powered robotic vehicles that would climb the cable to the moon. The voyage might take days or weeks and return trips would simply reverse the process.
According to Penoyre and Sandford, the Spaceline might be more economical, especially for bringing raw materials back to Earth from moon based mines. A lunar space elevator system might pay for itself within 53 trips by transporting lunar materials to a space station between the moon and the Earth, according to a paper published by the American Institute of Aeronautics and Astronautics.
Building the first space elevator on the moon, will be easier because of the weaker gravity, which will allow us to make a flimsier one. If we build one on Earth we have to do it right the first time.
Tests of space elevators are already underway. The Japan Aerospace Exploration Agency (JAXA) launched a miniature version of a space elevator in September 2018 to see how it reacted to the space environment.
The Space Tethered Autonomous Robotic Satellite — Miniature Elevator (STARS-Me), was designed by researchers at Shizuoka University and it involved two CubeSat satellites, each of which could communicate with the ground, with a 14-meter (46-foot) tether between them. The goal was to crawl a Bluetooth-enabled climber up the tether from one CubeSat to the other.
The launch was successful, according to a February report, but the researchers had difficulty communicating with one of the CubeSats. They have yet to verify whether the climber successfully traveled up and down the tether.
Even with all the challenges ahead in the undertaking of humanity’s biggest project, the payoff of having a working space elevator would be immense. It might be the first step to truly conquering the stars. Maybe we never build a space elevator, but in trying to do so we will learn an awful lot. When it comes to the exploration of the universe, there can’t be too many dreams of a glorious future.
Some further reading: