The all-new and long-awaited feature film for A Certain Magical Index is out tomorrow on Blu-ray/DVD Combo, and we’re taking a look at the science behind the anime in this movie, as part 3 of our “A Certain Scientific Analysis” series! In the movie, Toma teams up with Index, Misaka, and a ton of other fan-favorite characters to stop a mysterious plot revolving around Endymion, a space elevator that transports people into the heavens. But… as Index asks, what’s a space elevator anyway? Learn about them under the cut!

A space elevator is a theoretical means of space travel that consists of a long (long, looooong) superstrong and lightweight cable that extends from earth into space. This construction allows a “climber” transporter to move up and down the cable to transport the payload—machinery, people, etc. In the movie, Endymion is constructed after an accident on the spaceplane Orion, which crashes to the surface (but miraculous leaves the 88 passengers alive). The space elevator Endymion is created as a safer alternative to space travel—and it turns out space elevators are also much cheaper to operate than fuel-powered space planes! This is because rocket propulsion requires so much propellant that the vast majority of a rocket’s weight has to be the fuel. Thus, (once we get over how insane the idea of a space elevator is on paper) they’ll really represent the future of space travel.

So we know that a space elevator is, as Index describes, a Tower of Babel-like cable that extends into the heavens, but how does this work? Surely if you just built a long cable out from the surface of the planet, it’d fall over, so you’d need tons of building materials to buttress the elevator, right? This is actually where it gets interesting. A space elevator is not built like some of the tallest buildings in the world—it’s not just an extremely tall elevator built from the ground into space. It’s easier to think of it as a giant tether connecting the earth to an object tens of thousands of miles above ground.

Think of a ball attached to a rope, and what happens if you swing it in a circle. The ball moves in a circular path, and the speed and rotation keep the rope taut. That rope is your space elevator. A counterweight (like the ball) is placed far, far above the Earth’s equator and connects to the Earth via the long cable. In the Index movie, this counterweight is the platform far into space where the climax of the movie occurs. As the Earth rotates, the centrifugal force from the rotation and the downward gravity keep the cable taut and allow climbers to travel up and down it to transport a payload. Just imagine a tiny vehicle moving up and down your rope, and you get the idea.

This part’s a little more in depth, but bear with us for a second. The entire system’s center of mass must be above the altitude of what’s called geostationary orbit. As you may have learned in science class, an object like a satellite orbits the Earth by traveling in a circular path, through a combination of gravity and the object’s tangential velocity—these objects don’t need additional fuel to continue traveling around the Earth. Since the Earth rotates about its axis, an object at geostationary orbit above the equator follows the earth’s rotation with a permanent position in the sky relative to the ground. An object that isn’t at geostationary orbit would appear in a different place in the sky as the Earth rotates. With the space elevator, when the center of mass is in geostationary orbit, this allows the space elevator to rotate with the Earth, which gives us a system that’s stationary relative to a location on Earth.

Just for a sense of scale, the altitude of geostationary orbit is approximately 22,236 miles above the Earth’s equator. The Earth itself has a diameter of 7,918 miles, meaning that the elevator has to be at least 3 times longer than the Earth is wide! Of course, standard building materials like steel aren’t strong and lightweight enough, but innovative new materials like carbon nanotubes could work.

So know we generally know how a space elevator works. How does the Endymion fare in a battle of fact versus fiction?

In the movie, Uiharu notes (with delightfully nerdy, fangirlish glee) that only Academy City could have really built this elevator this fast, and also that the Endymion is exceptional because it’s located in Academy City, Japan, and not the equator. Geostationary orbit is only above the equator, and since an object at geostationary orbit does not need additional force to maintain its velocity as it follows the rotation of the Earth, a space elevator built elsewhere does require additional energy to maintain its movement. The platform and the end of the space elevator thus need to actually be constantly fueled to move through space to keep the elevator stable relative to Academy City. However, in a city where kids are trained to develop esper powers, security robots monitor everything, and a middle school girl can generate electricity within her body, we’re sure it’s within Academy City’s ability to fuel this.

The other interesting point about space elevators is that generally the elevator cable should be thickest in the middle, where there’s the most tension. As you can see from pictures of Endymion, the base is the widest and the elevator narrows as it recedes into space—as one might expect from a real elevator. That said, this isn’t unreasonable, and it just means the bottom end is extra reinforced. Plus, the blue spiral base is at the very least pretty.

So that’s Space Elevators 101. Tell us in the comments if you know any other cool space elevator trivia, or if there’s anything we missed! If you’re looking for other cool explanations of the science in the Index/Railgun series, be sure to check out the other two installments in our A Certain Scientific Analysis, one on the Railgun protagonist Misaka and one on the uniquely deadly Raildex antagonist Accelerator.

Be sure to order A Certain Magical Index – The Movie – The Miracle of Endymion today, and check out the trailer below!: