It's an exciting time in the history of human exploration. We're only just beginning to probe outside our biosphere. Low-orbit, long-term space inhabitation like that seen on the ISS has been the norm for many years now. As of the time of writing, 7 people were in space. Our long term goals seem set on permanent outposts on Mars.

Orbital re-entry of crewed spacecraft is a complicated dance of physics. You ever do that challenge where you have to build a cage out of straws and cardboard to protect an egg from a fall? Well, it's like that on steroids. Generally, if you are coming in from another interstellar body you will be going very fast. You can't decelerate over around 90m/s/s (or 9gs) or let the interior of the ship get too hot without killing your crew. You've got an enormous amount of kinetic energy to disperse, and the best way to do that is by braking in the atmosphere at a relatively shallow angle of descent. This means that reentry maneuvers cover a lot of airspace. You will likely either orbit the earth entirely or a large portion of it before you finally reduce your velocity enough to touch down.

The same is true of orbital ejections from Earth. Generally we try to launch from near the equator as we can, and in the direction of the Earth's rotation (easterly). This is so we can steal a bit of this initial speed from the Earth itself. To get ourselves from one stellar body to another we might do a Hohmann transfer, which means there is going to be some arbitrary point in the stable orbital trajectory where we need to fire some impulse to get onto this new trajectory (where line '2' begins below). That arc over low-earth orbit can be of any length, from a few moments to multiple orbits around the globe, and stretch over any number of nation-states.

(Courtesy of WikiMedia)

There have been significant developments in the ability for nation-states to control their low-earth orbit even in the past few decades. Anti-satellite systems are a rapidly growing sector of defence, with current systems claiming the ability to target low-orbit satellites. In 2007, the Chinese military destroyed a Chinese weather satellite through their ASAT (anti-satellite) program. In 2008, the US Navy destroyed a US spy satellite using a ship-launched missile. In 2019, India joined in the fun with their own test destruction of their own satellite. Clearly, such systems are alive, well, and advancing quickly.

A final point in this equation is the scheduling of these orbital maneuvers. The best times for sending a shuttle to Mars are when it is closest, an occurrence that only happens every 3 earth-years. If we do get to the point where we have permanent settlements on Mars, these 3-year cycles of sending supply vehicles will be pretty necessary for keeping those colonies alive. Their chances grow greatly if these supply lines are able to reliably deliver resources as the colony, or colonies, grow. If a launch window is missed, it's not just a case of putting a bit more fuel in the rocket and pointing it a bit differently - entire missions must be redesigned and retooled, a dangerous situation if the colonies were relying on any of those resources.

Two things I believe are going to become true in the space race of the future:

  1. There are going to be more and more of these incredibly delicate supply lines leading to offworld semi-permanent colonies.
  2. Nation-states will get better at controlling the low-earth orbit airspace above their borders.

These two aspects of the future create a fairly stressful situation of (while not quite "Mutually Assured Destruction") something we can perhaps call "Mutually Assured Imprisonment". If multiple nation states - especially those that control large portions of land near the equator - escalate a situation of depriving other nations of access to safe entry or exit from low earth orbit, we may find ourselves quickly embroiled in a standoff that starves our colonies and deprives any nation from sending missions to space that are not universally supported. Space travel is already playing dice with the devil - introducing antagonistic forces could be its death spell.

The results of such a standoff - or even conflict, where one nation destroys a ship or satellite as a result of some political power-play - could result in a low-orbit environment that will set back the mission of space exploration for decades or even centuries. Such a conflict is likely to generate a large amount of debris, which would make putting satellites in orbit untenable. It would increase collision dangers for ships entering and leaving the Earth's surface, and make entire categories of satellite impractical. A planet with Kessler syndrome can experience a runaway domino effect which turns our currently peaceful low-orbit into a chaotic storm of fast-moving debris which will take a very, very long time to subside.

So, it seems that the price of space is friendship. A world turned in on itself may deprive us all of the universe beyond it. And yet our world is not one of friendship. It is one of domination, exploitation, and cruel power. My only hope is that we didn't nuke ourselves into a sterile rock during the Cold War as an indication of future behaviour. But it is a harsh and salted ground to plant such a beautiful seed in.


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How does the SpaceX Starship change this? If it works, it'll be 1-2 orders of magnitude cheaper for the US to launch stuff than everyone else. I suspect that this makes it also easier to launch in weird orbits and stuff that are entirely over US territory, for example. (Costs more fuel, but that's OK if things are so cheap anyway.)

Thanks for the thoughtful reply Daniel!

Cheaper spaceships are definitely cheaper to replace if some oppositional nation-state blows them up, but there's only so many times you can play that game before you end up with the Kessler syndrome issues.

If you're referring to the vertical launch and takeoff of SpaceX shuttles, that really only affects the last moments of reentry. A SpaceX shuttle still does the vast majority of its braking in the atmosphere.

Launching in weird orbits is absolutely a strategy that nation-states could use to mitigate these risks, but in a field where your margins are already white-knuckle tight, such an arbitrary restraint is at best a huge inefficiency on our ability to progress space exploration. 

  • The $/kg price point is the driving metric of how quickly we can expand our space colonization, so increasing that by an order of magnitude because we can't just get along seems suboptimal.
  • Ships cannot simply be allocated to arbitrary launch manuevers - fuel tanks need to be made bigger, designs adjusted, missions planned.
  • Hohmann transfers, as mentioned, carry with them an arbitrary amount of LEO over an arbitrary stretch of airspace. Restricting your space of possible Hohmann transfers to only "safe" ones will possibly reduce your launch windows to an unmanageable level.

Thanks for the interesting post! In times of war historically, European powers would send out privateers and raiders (and later submarines and bombers) to destroy the shipping of their enemies. Presumably this raised the cost of shipping things around, and in some cases completely strangled shipping. (E.g. very few vessels got in and out of the Third Reich from the Atlantic and Pacific and Indian oceans.) I imagine something similar would be true of space; in times of war, some nations would be unable to access their colonies but others would still be able to access them, just with higher cost due to having to launch in weird inefficient orbits.

(I'm not sure how big a deal Kessler is -- how easy is it to armor against? How easy is it to cause deliberately, e.g. by spraying sand out the side of a big tank-o-sand satellite? How long would a Starship last if it were being launched not into LEO but into a HEO to escape Kessler as quickly as possible--how intense would the Kessler have to be for it to not make it out of the danger zone with reasonably high probability?)

I imagine something similar would be true of space; in times of war, some nations would be unable to access their colonies

Maybe I'm vastly underestimating how self-sufficient these colonies will be, but my impression from current plans for permanent habitation is that they will depend on shipments from Earth for basic supplies for quite some time. Strong claim held weakly, someone please prove me wrong on that. But I imagine it's going to be a heck of a lot of extra straw on an already overloaded camel's back.

Kessler syndrome isn't a huge issue when you're just shooting off somewhere else and not spending a lot of time in LEO. But I think not having the ability to have infrastructure like satellites, fuel dumps, stations, skyhooks, or whatever the heck else you feel like putting there is going to be a problem and again make the whole operation a lot less efficient.

Good point, early space colonies could be very not-self-sufficient, more so than European colonies at any point in the past.

But I think not having the ability to have infrastructure like satellites, fuel dumps, stations, skyhooks, or whatever the heck else you feel like putting there is going to be a problem and again make the whole operation a lot less efficient.

Are you open to updating that assumption?

Well, the best way to get an idea of how actual spaceflight physics work is to play kerbal space program.  But, to summarize:

(1) kessler doesn't apply to both very high and very low orbits, leaving room for satellites (insufficient energy for the debris to reach very high orbits, very low orbits have drag that cleans them)

(2) fuel dumps do not work (and no, spacex doesn't plan to use them) as 'the rocket equation doesn't work that way'

Can't argue with a tweet but it doesn't change the rocket equation.

It does change my opinion about whether the rocket equation renders fuel dumps a bad idea though.

What's the argument that the rocket equation makes fuel dumps a bad idea?

The tyranny of the rocket equation means that we're going to really struggle to make it worth it. For the same reason that we can't just make fuel tanks bigger, it is very inefficient to send fuel out from the same gravity well as you want to refuel from - orders of magnitude.

The thing to remember when we talk about "kg of fuel per kg of cargo" is that the vast majority of that fuel is burned in the lower atmosphere. The majority of the work of shooting a rocket off to space is just getting it moving. So if you want to ship enough rocket fuel up to form a fuel dump with something like hydrogen rocket fuel, then you need to expend vastly more fuel than you end up storing.

Well yeah, but that's just an argument for looking for extraterrestrial fuel sources. Insofar as your fuel comes from earth, putting it in a depot isn't obviously worse than launching it on an as-needed basis, and arguably it's better. And insofar as your fuel doesn't come from earth, then similar considerations could weigh in favor of a depot. (The tanker from the Belt comes once a year with a buttload of fuel, puts it in an earth-orbit depot and leaves)

the vast majority of that fuel is burned in the lower atmosphere.

Then why not launch from high-altitude balloons?

This is air launch -- using a balloon is just one variant. All of the schemes I've seen seem to have fairly small payloads, I assume the trade-offs don't work well above some threshold. 

If we're talking about orbital propellant depots, the individual launches of fuel don't have to be very big, as long as the price per kilogram to LEO is favorable. Rockoons are but one method of circumventing the rocket equation. Many others are known, with some being more realistic than others in the short term.

The reason is that there is no need for crews or for a staff at the dump to have a place to "stretch their legs". Nor is there any advantage to have a vast stockpile of fuel 'just in case', you might as well launch it as needed. Most of the reasons to have a dump irl do not appear to be true in space. And the rocket equation means you pay in mass fractions regardless of scale.

Hmm, without knowing what the reasons for fuel dumps are, I can't tell how convincing those points are. I am not very convinced myself. Off the top of my head here are some reasons you might want an orbital fuel depot:

--You are limited to certain windows of opportunity for launches, or perhaps you are pad-constrained, or both. So you can't just launch a fleet of tankers alongside your regular ships; it pays to spread out the launches, and have tankers go up when regular ships don't need to, and then regular ships can go up and have fuel waiting for them already, anytime. (I guess this is an objection to your "might as well launch as needed" claim.) (I think this is the biggest reason; currently it takes several tankers to refill one starship I think, which means you either have to launch all of them at around the same time as the starship, or you have to have the starship wait around for one tanker to fly up and down and up and down. Might not be feasible given launch pad and launch window constraints.)

--Your tankers aren't optimized for holding onto fuel in vacuum or for long periods of time, or maybe they aren't optimized for transferring it quickly to other kinds of ships. So you build a depot that is.

The second part - you basically need a solar power gas recondensor. Or to have one onboard. I take your point, fuel refrigeration is a role that you might not have ships do and you could keep a module in orbit to do.

But the hassle of docking and maintenance - and possibly zero net cost savings - might make it cheaper to just launch fuel condensing modules on be vehicle for the mission itself.

Hello Gerald! For sure. To be honest the Kessler syndrome was an afterthought and I may be overestimating its impact. I think the far more relevant danger is active measures against a launching or landing craft. Things like fuel dumps (you are totally right in that it doesn't make sense to take fuel from Earth up into LEO, I was more thinking about bringing fuel from some other much-lower-energy gravity well like the moon) would probably be better placed in Lagrange points.

I mean you can replace all this with "airline travel". Already nations can shoot down airliners that travel within hundreds of kilometers of top end air defense systems like the S-400. So if a one of the central European nations decided to make anywhere they can hit a 'no fly zone', and they had nukes so no one can invade, they could de facto ban air travel in Europe.

This is true for a lot of technologies - ultimately this planet is small.