Hooking the Sky
Adam Kall, Director of Science
4.5 minute read
The current way humanity gets into space is highly expensive, and even worse, not great at bringing things back. Discussions about a space industry have an often unspoken clause, that if we do achieve orbital manufacturing or asteroid mining, it would be expensive and difficult to get those goods back to Earth for use by the average person or industry. The main problem is that it takes a lot of speed to get into Earth orbit, plus a lot more to go farther, and you need to slow down a lot before returning to Earth if you don’t want to end up as barbeque. However, there is an exciting exploit of the universe that may help us change this whole dynamic, and really start to connect the people and industries of Earth with the rest of the solar system, known as the skyhook.
Orbital mechanics dictates that an object in a circular orbit has to maintain a minimum speed in order to stay in orbit. This equation lets us know that the ISS, orbiting at 250 miles above the surface, needs to go 17,152 mph. The ISS is a relatively small object, but what if it had a big radio tower that extended a mile higher? Well, the end of that tower would want to orbit at 17,150 mph, but because it is rigidly linked with the ISS it keeps going that 2 mph faster than other objects at 251 miles above the surface. Now the calculation for the ISS speed was really about the center of mass, so to keep things balanced we can also extend a radio tower down a mile towards the surface. This lower tip is still moving at 17,152 mph, but other objects around it move at 17,154 mph. So an object going 2 mph too slow to stay at an orbit of 249 miles could attach to this lower tower, climb the two miles up the tower, and be released going 2 mph faster than it needs to be going. This isn’t much, but that’s because we’re talking about a small distance of 2 miles. If we replace the radio towers with light-weight ultra-strong tethers, we can now extend the cable 200 miles in each direction. Now we have a 400 mile long cable, reaching from 50 miles altitude to 450 miles altitude. The difference in speed is only about 400 mph at the top and bottom, but we can add the secret sauce now and start rotating the structure.
By rotating the structure in the same clockwise direction as the orbit, we achieve a unique characteristic where the low point is sent moving backwards relative to the center of mass, and the high point is sent forward faster. Paired with the orbit it is in, the lower point can descend into the upper atmosphere at a speed much lower than orbital velocity, before swinging up and accelerating to speeds much faster than necessary to stay in Earth orbit. A possible operation would look like an advanced hypersonic cargo plane getting up to speeds like Mach 5, something which is being worked on, so it can hook up with the cable before getting swung out to a Mars transfer orbit. This would happen on a regular cycle because of the rotational period of the skyhook, almost like a train schedule.
The only problem is how this speed increase isn’t free, and each time a spacecraft is flung to Mars the skyhook system slows down. This could be boosted back up by engines, but something amazing happens when we consider the spacecraft that come back to the Earth. If they are caught by the same skyhook system and slowed down to Mach 5, they’ll actually give more energy to the skyhook. This can be designed so each mission tries to maintain a balance of mass on both the up and down trips, such that the return trip recharges the energy for the outgoing trip. For the return trips there are an abundance of natural resources in the solar system, like rare-Earth metals, that industries on Earth could benefit from. For the outgoing trips we’d need to start with the initial mining equipment, and supplies to support the men and women working throughout the solar system, but eventually we’d reach a point where more mass is coming down than going up, and suddenly there is an active and strong demand to send things into space.
This is where some wild ideas become viable. We could construct massive orbital vessels in space since the cost of sending aluminum paneling up is subsidized by the gold and platinum coming down. Putting massive solar panel installations into orbit could help solve the energy crisis, or we could start freezing greenhouse gasses and shipping them wholesale into the depths of the solar system. This still isn’t the end state, as most industries consist of taking raw material and removing waste to create a more refined product. If there is an imbalance of too much material going down to Earth, the solution could be to set up the industry on Mars instead. Now Earth can export food and people to Mars, which ships back finished, high-quality products that weigh less than the original raw material. Due to its thinner atmosphere, lower gravity, and conveniently close moons, Mars would actually be an ideal hub for the mass movement of material around the solar system.
It’s not just an intriguing idea, the benefits for us here on Earth are clear and tangible. One advantage would be that all of our electronics and electrical systems would be made out of gold. This isn’t a hedonistic display of wealth, it is because gold is actually an amazing electrical conductor and would make electronics more efficient. We are limited by its scarcity on Earth, but up in space there are individual asteroids with more gold than has been mined in human history. Similar examples exist for other rare Earth elements, with names like Neodymium, Yttrium, Dysprosium, and Gadolinium. Names which are so rare on Earth most people don’t know them, but they are abundant in space and would greatly benefit the products in which they can be used. And let’s not forget the fact that certain miracle pharmaceuticals can only be made in the microgravity of space, and since the skyhook could make this environment accessible, they would become affordable treatments. The craziest part of this whole plan is how possible materials for the skyhook tether already exist and are used on spacecraft today. For most of the space race we’ve thought of ourselves as trapped in our gravity well, but the universe actually provides a method by which we can explore the planets, we just have to be courageous enough to build it.
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