Rockets are expensive. Not only are they limited by the weight of their fuel, but also by their cargo capacity and reusability. While commercial entities like SpaceX and Virgin Galactic have been unveiling new systems that promise alternative ways of reaching low orbit or reusing rockets, there are still a lot of limitations that prevent space travel from becoming ubiquitous in the short term.
This is not necessarily a bad thing. Space flight does not have to be easier for good scifi, in fact, I would argue that it should be hard. It helps to impose limits on the characters and promote conflict. A thriving space industry could still be expensive and thus impose limits on who goes to space and why.
That said, once your setting is to the point where colonies throughout the solar system are becoming viable I think that it’s time to start exploring other ways of getting to orbit. I think rockets will always have a place, but forms of mass transit will make the entire endeavor a lot easier.
Orbital elevators are a staple of science fiction. How it works is that a giant tether is built connecting the surface of a planet to a station in orbit. The tether is then held taught, allowing elevators to move up and down its length.
One of the primary challenges with an elevator is making a material strong enough to build the tether in enough quantities to make it work. A lot of authors choose to use some kind of carbon allotrope and this part might require you to invent your own very special flavor of carbon fiber or synthetic diamond. Remember that the tether will need to be much, much thicker than you think it will need to be.
My favorite part about this concept is that it allows a world to have regular trips to orbit and back in an environment that might resemble a modern airport or train station. Elevator pods could have large cargo areas and multiple passenger areas divided into economy, business, and first-class. You could have observation windows and restaurants. All the trappings of comfort or the lack thereof.
An elevator is probably best in a setting where space travel has become common enough for such a project to be profitable. A single planet will likely only have one or two placed in neutral or autonomous regions or controlled by a specific faction. Of course, the resources needed to build one might limit which worlds have a space elevator and which do not. If your setting involves multiple star systems it is likely that only the most developed of them will have one.
It goes without saying that such a large piece of infrastructure will make a very tempting target. If destroyed an elevator could cause immense damage to any settlements built around its base and cripple and the economies of multiple factions in a given system.
For worlds that are not yet capable of a project as massive as a space elevator but still need regular surface-to-orbit transit, skyhooks may be the perfect solution.
You can think of a skyhook like a satellite that spins as they travel along the edge of the atmosphere. Its hook can latch onto craft flying in the upper atmosphere and accelerate them into orbit, and can also grab craft in orbit and bring them down into the atmosphere.
These are a good in-between stage between rockets and elevators for travel and could probably be set up a lot faster than a full-sized space elevator could. Which would make them perfect for worlds with some orbital traffic but not enough for a full elevator. Or they could be an option for planetary factions that do not want to rely on a space elevator that someone else owns. Or in instances where orbital infrastructure needs to be set up quickly. I’ll talk about a possible scenario for that in the next section.
An Invasion Scenario
Surface combat in the far future is likely to be small-scale and asymmetric. There isn’t much use in landing millions of ground troops when ships in orbit can turn a continent into radioactive glass. But we seem to crave depictions of ground-based combat anyway.
Let’s say a planet is host to an environment that is hospitable to humans or contains some vital piece of infrastructure that would be destroyed in a bombardment and that this necessitates the use of ground forces on a large scale. The first wave of troops could be brought to the surface with a combination of capsules and landers that glide down through the atmosphere much as the space shuttle did. Some of these crafts might be designed to return to orbit with a variety of energy-intensive designs. Since we all know that military objectives beat concerns like cost and efficiency any day.
If the planet already has extensive orbital infrastructure, which it probably does if its world attacking, these initial forces would work to establish beachheads and try to capture any space elevators that might be present. The attack on a space elevator could very well commence on both ends since it would be hard to use if the people at the other end of the tether were waiting to shoot you as soon as the door opens.
But perhaps the space elevator was destroyed or the planet really doesn’t have the infrastructure. Once landing sites are secured, ships in orbit could deploy prefabbed skyhooks to provide the infrastructure of occupation. From that point on if the locals continued to resist the war would probably resemble something like the conflicts in Vietnam or Afghanistan. Massed tanks and infantry make for awesome illustrations but are nothing a few “rods from God” couldn’t fix. In the long term, the construction of a new space elevator could be seen as the ultimate mark of ownership of the planet. A massive, sprawling symbol that the invaders are there to stay.
Further Reading (And Watching)
I realize that this post is less technical than previous Science for SciFi entries. I chose to do this because I am not a physicist nor am I an aerospace engineer. Instead, I wanted to highlight a few interesting concepts in science fiction and point you all towards some resources that can be an inspiration for your next story of a planetary invasion. If you liked this content consider supporting it by signing up for my newsletter or exploring my page of recommended products on Amazon.
For a start, Atomic Rockets is a great site for anyone who wants to dig into the physics of science fiction and learn how science has been incorporated into many great science fiction classics. For a fun and straightforward explanation of skyhooks, you can look to Kurtzgesagt on Youtube. The same channel also has a great explanation of space elevators.