In the story I'm writing, three humans with stone-age technology and no knowledge of space accidentally awaken thousands of cryogenically preserved humans.

These humans immediately get to work on reaching space due to the presence of an AI gone insane from existential dread on the planet they are on. They will use large numbers of spaceplanes and multiple skyhooks to reach infrastructure designed for interstellar travel.

The problem is that these humans have been cryogenically preserved for thousands of years, meaning that the skyhooks would have to have maintained their orbit for all of that time without any human involvement.

Thankfully, advanced AI and nanotechnology exist in this story, and such things might help in keeping the skyhooks up. My question is how would they do it, and how might nanobot swarms and super AI help with this if they needed at all.

  • 2
    $\begingroup$ It's worth pointing out (as I try to do when this comes up) that "nanotech" is not the magic goo that it often is represented as in fiction. Making nanobots do something useful is tricky, and powering them while they do so is worse. Green goo is much more likely than grey goo. If you've come up with some sort of aether-powered nanobots, the latter problem is resolved, but at that point you may as well call it magic. $\endgroup$
    – jdunlop
    Dec 10, 2019 at 22:30
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    $\begingroup$ What is a skyhook? This word has several incompatible meaning in science and technology. Is it Daniel Dennett's general concept of an irreducibly complex technology? One of Winzen's high altitude balloons? A momentum exchange tether? $\endgroup$
    – AlexP
    Dec 10, 2019 at 22:51
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    $\begingroup$ Seconding AlexP - Without your definition of Skyhook, this question is unanswerable because there are simply too many completely different things called "Skyhook" $\endgroup$
    – Andon
    Dec 11, 2019 at 0:54
  • $\begingroup$ As an apprentice mechanic I was asked to fetch a skyhook once. $\endgroup$ Dec 11, 2019 at 4:51
  • $\begingroup$ @MichaelMano, good for moving heavy stuff, I've sent people off for one many a time. That, a long weight, and some elbow grease. $\endgroup$
    – Separatrix
    Dec 11, 2019 at 8:25

4 Answers 4


I'm assuming by 'Skyhook' you mean a counterweighted space elevator, yes? If so, then honestly, this isn't a problem. If you build it properly it's going to stay up for thousands of years all by itself.

It's going to be anchored somewhere tectonically stable, so it's no more likely to have problems at earth's surface than the pyramids have had in the thousands of years that THEY'VE been around.

The main threat to it would be impacts from stuff in orbit, and nobody is going to build something THAT expensive without either clearing that part of earth's near-space environment of anything that might hit it, or building defenses into the structure of the skyhook itself that will intercept and deflect/destroy incoming objects. Or both. Ideally both.

So, really, if the people who built the thing knew what they were doing, the only REAL threat to it would be a direct hit from an asteroid or something, and those just don't come along long enough to be a statistically serious threat to a target that size even over a couple thousand years.

You mentioned 'multiple' skyhooks and I'm going to throw it out there that it's very unlikely that there would be very many, the cost of moving stuff around the planet to get it TO a skyhook is trivial compared to the cost of building one. You might have three or four scattered around the Earth's equator, but you wouldn't have more than one anywhere near each other no matter what.

TLDR: If you can build a skyhook AT ALL, you're both willing and able to make sure it stays up for a Really Long Time.

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    $\begingroup$ A Skyhook is not a space elevator. They are not anchored to the Earth, and therefore require means to regain momentum lost to atmospheric drag, and momentum transferred to spacecraft using it as a launch assist. $\endgroup$
    – CAE Jones
    Dec 10, 2019 at 22:32
  • $\begingroup$ @CAEJones I would argue in that case that he shouldn't be using Skyhooks in his story in the first place, and should use Space Elevators instead. $\endgroup$ Dec 10, 2019 at 22:51
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    $\begingroup$ The point of skyhooks is that they're much, much easier to build, and do not require materials with the tensile strength of space elevators (last I heard, we still aren't sure that graphene can handle it). On the Moon or Mars, maybe, but on an Earth-like planet sky hooks are probably far more economical. Story-wise, though, they do present difficulties that a space elevator wouldn't, yes. $\endgroup$
    – CAE Jones
    Dec 11, 2019 at 8:53
  • $\begingroup$ -1 a skyhook is not a space elevator. $\endgroup$
    – cms
    Dec 11, 2019 at 17:45

The skyhooks are sentient.

They have been given a mission, these intelligent skyhooks. They maintain altitude, repair themselves and wait for orders. And wait. And... and ...


Fortunately the skyhooks have not gone crazy in the many decades of waiting. They can talk to each other and they do. They give each other orders and then rescind the orders. The order and the countermand must rhyme. The skyhooks understand and speak several dozen human languages, which makes rhyming more fun.

They might want their new masters to rhyme also. They might insist. The skyhooks might have gone a little crazy after all.


TL;DR: super-AI and functional nanotechnology may as well be magic. You can handwave this as you see fit.


This is going to be awkward, but not insurmountable. Tethers of any kind, including non-rotating skyhooks, rotating "rotovators" and fixed space-elevators will be subject to continuous damage from micrometeors, not-so-micro meteors, cosmic radiation, reactive oxygen species in the upper atmosphere, weather... the list goes on. You can't build one before you've got the ability to maintain it. This might not necessarily need nano technology: regular-scale stuff could be done via maintenance robots that coudl crawl up and down the tether, knitting broken strands back together or removing irrepairably damaged strands entirely and extruding new ones into the tether to keep it in one piece. Your tethers will already have to do this.

For non-fixed tethers, eg. your skyhook, you will also have to deal with the problems of orbital decay. You're dragging a big heavy cord through the upper atmosphere, and all that drag is going to slow you down and eventually you'll crash. If the tether was built with parking in mind, it might be possible to rotate it up so it was parallel with the Earth's surface, or perhaps even raise it into a higher orbit which is longer-lived. This might be a sensible way of doing major maintenance on the tether of the sort that can't be done whilst it is still "in flight".

In either case, changing orientation, changing orbit or even just simply maintaining your orbit requires power and reaction mass. You can get power from the sun, but those solar cells don't last forever, especially in the punihsing environment of space, so you need spare parts. Those spare part stockpiles probably won't last a thousand years (because what would be the point of building a stockpile that bit?) so you'll need an in-orbit industrial base, and you'll need a supply of raw materials, probably in the form of rocks flown in from the asteroid belt or lifted off the moon. Those rocks would also provide reaction mass for the manoevering rockets, without which you won't be able to stop the tether's inevitable orbital decay and firey demise.

A suitably super super-AI could probably arrange this complete industrial base for itself, if it didn't already exist, assuming it had access to some initial fabrication facilities and suitable reaction mass. It might also conclude that there are more sensible things for it to do than to wait for the frozen meatbags below to warm up, and it would be fully capable of waking them up itself and retrieving them. As it hasn't done that in your setting, you'd have to wonder why, and rather hope that it was at all interesting in maintaining a human-friendly space infrastructure that it would have no need of itself...

  • $\begingroup$ To answer your question about the mega AI, well, it sure isn't interested in human-friendly space infrastructure. In fact, it's partially insane because it tried to simulate human emotions and real consciousness within itself, which caused it to instantly crack from existential dread. Thankfully, it doesn't have a good reason to destroy the skyhook either. Or a reason for anything. The sweet release of insanity prevents it from experiencing the dread, but having a superintelligent AI that is insane does not make the planet a great place to live on for the human residents. $\endgroup$
    – John Lewis
    Dec 17, 2019 at 22:48

They can "swim"

If they're "dragging" in atmo, and they have some power source (solar would be fairly trivial), perhaps they can have some sort of propulsion system that leverages that for stationkeeping, i.e. suck in the air that's slowing them down in the first place and exhaust it in an advantageous manner. Best case, they have air screws (okay, maybe jet engines) in advantageous locations. Worst case they're built something like a Bussard ramjet. (An ion drive might work, also.)

They are self-maintaining

You have AI and nanotech. Maybe the skyhooks are built with fabrication capabilities and have a fleet of AI-operated service craft to take care of them, including AI-directed mining of either the planet surface or space-borne resources (whichever is easier). The fleet of service craft can collect more fuel / raw materials as needed.


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