(This page is about the proposed orbital megastructure. For those looking for the no-landing air extraction device, see Fulton System)

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Basic Information

A skyhook or space-tether is a proposed sustainable reusable orbital acceleration device that could significantly reduce the cost and energy to send a spacecraft off the planet. It's likely this could reduce the costs of a trip to Mars (or elsewhere in our solar system) to 20% of what a traditional rocket system would require or less, and may reduce the travel time to mars to 33% the length of a rocket-trip. This megastructure is basically a big rotating weight in upper orbit, with a huge (hundreds of miles long) tether on it that frequently dips down into the upper atmosphere. Instead of building a spaceship with enough thrust to reach escape velocity, you only need a craft capable of getting up into the air and latching on to the tether as it swings down into the atmosphere. Once hooked, the tether would work like a trebuchet and hurl the craft into outer space.

This works best if you have two (or more) tethers, one on Earth, and one at your destination to catch you and lower your craft into the atmosphere of that planet. Phobos, the martian moon is the perfect foundation for an extra-large tether that could be used not just for Mars-Earth return transits, but also as a nexus for travel to the asteroid belt or Venus. One of the coolest features of this sort of project is that the chain of two skyhooks "feed" each other: launching a ship costs energy from the system and slows the hooks spin, but catching a returning ship and bringing it down for a landing actually injects more rotational energy into the system speeding it back up. So as long as transit from A to B is roughly as frequent as B to A, the system sustains itself. Unfortunately, the first mission or two to any given destination won't be able to rely on a reciprocal skyhook for their return trip: they'll have the task of installing such a skyhook on their to-do list first.

The technology for all this is basically already figured out. We don't have to invent new materials or uncover hypothetical scientific principles to do it, like we might for a space elevator. Instead, we just need some engineering applied to getting a practical prototype into orbit. It's the sort of thing that a major government or megacorporation could do twenty minutes into the future.


1. Kurzgesagt Video on Skyhook - This cheery animation covers all the basics in just a few minutes. Well worth a watch.

Game and Story Use

  • Effectively this makes Mars/Phobos a major waypoint on the journey to other planets. Mars is likely to have a fair amount of political power and importance in such a system. There may be some sort of big refueling station or orbital station at a near-Mars Lagrange Point, or just on the surface of mars (since dropping down to and launching up from the surface is now so much more affordable).
    • It also makes it a big target for space pirates or feuding governments during wartime. He who controls Phobos controls the solar system!
    • Even at times of peace, there's likely to be a big no-fly zone around the tether's airspace, if for no reason other than preventing an accident or space disaster.
  • Starcraft technology in a setting that relies on skyhooks will likely be distinctive and specialized. Ships will likely be small, as they have no need for huge engines or fuel supplies, and smaller ships will have less disruptive impact on the skyhooks orbit when being catapulted. They'll be designed to survive the shock of sudden acceleration from an external force, which may make them resistant in the case of a collison or crash landing. They're likely to have a manipulative arm or drone system for catching the tether. They'll be nimble within an atmosphere, like a jet plane or space shuttle, so they can catch up with the tether as it speeds past them (potentially at Mach 10). But they might have somewhat limited ability to adjust their own course once in outer space (as they don't need big thrusters to get up to cruising speed and may not have much fuel for applying course-corrections) and won't be capable of making escape velocity on their own.
    • The craft are likely to be completely dependent on those tethers. Which is great for setting up dramatic tension or a ticking clock if something's gone terribly wrong up ahead at the tether you're racing towards. It also means you can strand the PCs somewhere for a while, as they wait for a rescue mission or repair crew to get the skyhook back online. If the local skyhook is broken, you can't get off world at all and incoming craft will have a hard time slowing down enough for re-entry.
  • After the End, a spinning skyhook may be a visible sign of the heights mankind had once attained. It undulates and whips through the upper atmosphere, a flickering spinning streak in the sky. This might be the source of all sorts of mythology for those cataclysm survivors who have lost the ability to reach up to it.
  • A skyhook could potentially be used as a mass driver style weapon as well, to hurl interplanetary missiles at another world. You'd need some sort of drone or launch craft to get the projectiles up high enough to connect with the tether, but again that's cheaper and easier than launching a traditional rocket at escape velocity. It's probably not a big dumb kinetic projectile like a real mass driver, it's more likely to involve a warhead or clusterbomb of some sort, and will need a way to adjust targeting as it approaches the victim planet since the tether isn't likely to be aimed at your target by default.
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