"It will be difficult enough to avoid disaster in the next hundred years, let alone the next thousand or million." "Our only chance of long-term survival is not to remain inward-looking on planet Earth, but to spread out into space." - Stephen Hawking

Many scientists believe that it's too late to save our planet, and that all we can do is prolong its death. They suggest we leave Earth - but there are a number of engineering challenges involved with leaving Earth en masse that have not been addressed.

How do we get a significant portion of the population off Earth?

  • People should be intact and alive
  • People should not be genetically or cybernetically modified
  • "Significant portion" means at least 1 in 1000 people still alive by the time your project has been constructed. You can choose specifics about the people chosen but they should be diverse, from a range of countries. Assume they can be transported to anywhere on Earth.
  • "off Earth" means low-Earth orbit. Another question will address where they go from there.
  • Money and time are not constraints, but the cheaper and quicker is better
  • Assume modern-day technology exists at the start of your project
  • $\begingroup$ With companies like spaceX and Virgin Galactic, if both ever become reliable? $\endgroup$ – Alexander von Wernherr Dec 9 '16 at 13:06
  • $\begingroup$ Does "Assume modern-day technology exists at the start of your project" mean "You start working with the technological and scientific resources of our real world as of today, but you get to pick the path forward"? $\endgroup$ – a CVn Dec 9 '16 at 13:08
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    $\begingroup$ "should not be genetically or cybernetically modified". Why not? "at least 1 in 1000 people". Losing 999 out of 1000 people is disastrous. You have not averted the catastrophe then. $\endgroup$ – MichaelK Dec 9 '16 at 13:10
  • $\begingroup$ The population is about 7.3 Billion right now. So extrapolate further and we'll estimate/round and assume 10 Billion by the time this plan comes together. You're wanting 10 million people to survive, give or take? $\endgroup$ – user10945 Dec 9 '16 at 13:23
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    $\begingroup$ @Zxyrra I mean your constraints are kind of... eh... strange. I mean, your scenario is this: "We are about to lose all of the world's population. We must save all we can! But we must not use any body-changing enhancements!". Eh, what?! The apocalypse is on its way! Do you honestly think mankind will start imposing stupid limitations like that then?! Oh... wait... en.wikipedia.org/wiki/Anti-nuclear_movement ....never mind. $\endgroup$ – MichaelK Dec 9 '16 at 14:07

You're considering the survival of the individual. This is incorrect thinking when the problem is about the survival of the species. Individuals are irrelevant in this model beyond what they contribute to the group effort.

You don't have to get vast numbers of people off the planet, you have to build safe independent colonies where new people will be born, where the population can expand in its own right. New worlds to call home.

By this model, people sitting in a low Earth orbit are not off-planet. They're still very much tied to the planet and it's a high resource drain to keep them there and keep them healthy. If anything actually happens to the planet, the people in LEO must move on, or come down, but it's highly unlikely that any of the land based factions will put the effort and resources into maintaining them there.

Low Earth orbit must maintain no more than a small transient population who use it as a stepping off point for that longer journey. Off world means out of orbit and beyond, people in LEO might as well stay on the ground until it's time to leave.

The population of LEO should be:

  1. Ship construction workers
  2. Ship maintenance crews, dockers and loaders
  3. Ship engineering and development teams
  4. The passengers and crew of the next ship to depart
  5. The crew of the ship after next

But I want to go too!

Everyone wants to go! but anyone whose skills aren't on the required list will have to pay the costs of their share of the ship, supplies and for all the support crew required for an extra person on the run. That's a lot of money for a pleasure cruise, I suggest making sure your skills are on the list and up to scratch for some highly competitive places. Either that or you have a whole lot of disposable income.

Don't worry though, costs will come down once the technology matures (and competition will drop once the best people have gone).

  • $\begingroup$ I agree that small groups of people are sufficient but for the sake of pretending the population can make it to some degree I ask you to humor me and treat it as if more people want to go $\endgroup$ – Zxyrra Dec 9 '16 at 14:05
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    $\begingroup$ @Zxyrra, I'll let them go once the technology matures :) $\endgroup$ – Separatrix Dec 9 '16 at 14:49

How do we get a significant portion of the population off Earth?

Sorry that is simply not happening. Unless some magical solution allows us to lift huge amount of weight into orbit. And no such solution exists right now.

If survival of human species is the main priority, the best way is to send out small groups of people. Probably on order of thousands of individuals. This is truth for colonizing both our solar system and outer space. And even that is going to be prohibitively expensive.

  • $\begingroup$ I agree no such solution exists right now, so I asked you to allow tech to develop both in the question and it's comments $\endgroup$ – Zxyrra Dec 9 '16 at 14:04
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    $\begingroup$ Well, you call always lower population of Earth with good old fashion war. "significant portion" from several hundred millions (or even thousands) is a lot lower than "significant portion" from 7 billions. $\endgroup$ – Mr Scapegrace Dec 9 '16 at 14:55
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    $\begingroup$ What we know of physics strongly argues that this is an unsolvable problem. Unless the EM drive really does introduce some fifth force or gravity turns out to be negatable, the vast vast majority of individuals will never ever leave earth without significant cybernetic or biological modification. There's just too much stuff required to keep our bodies alive to make the collective lift cost ever feasible, in my somewhat educated analysis. Ruling out cybernetic mods seems foolish to me for that reason. $\endgroup$ – SRM Dec 9 '16 at 19:57

The only remotely practical approach is building lots and lots of space stations. Once s few are up in space you probably should start mining asteroids for water and metal and then build more stations and ships in space from such material.

The biggest cost in space transportation is getting things from Earth to space, since we have both high gravity and a thick atmosphere. Therefore you want to haul as little stuff up from Earth as possible. Best case scenario would be transporting only people and getting everything else that you can from asteroids or moons (Mars could also work as a source with low gravity and thin atmosphere).

Space stations may require more raw material than planet colonies, but all rocky bodies in the solar system are very poor candidates for colonization. Humans need gravity to survive healthily long term and we can simulate something very similar with rotating space stations. All planets and moons have less than 20% of Earth's gravity and it's not physically possible to increase that. Venus has 90% Earth's Gravity but an air pressure than is 90 times higher and a surface temperature over 400°C, which is the most deadly place in the solar system aside from inside gas giants or the sun. Floating sky cities could hover at a height were air pressure and temperature is very comfortable and you would still get the full gravity. All you need is to protect yourself from the air that has no oxygen and is slightly acidic, but there's nothing immediately toxic that would quickly kill you if you have a small leak in your airship of suit. The skies of Venus are probably the nicest place we know outside of Earth, but space stations would probably be much easier to do. Space stations can also be moved as the sun gets bigger, but that's a process that takes place over millions of years and it's silly to make shor term plans for that.


Start with developing the cheapest and safest space access vehicle you can manage. My personal favourite is Skylon, which promises a reusable, SSTO spaceplane. It's pretty much all present-day tech, just needs refining and assembly. Costs may not be as low as Reaction Engines are promising, but they should be significantly lower than any other currently extant launch system.

Once we have cheaper access to space, we can work on space industry. Automated probes to catch asteroids and bring them to Earth/moon for processing.

If you'll allow us a small leap in developing carbon nanotubes, we can now use carbon-rich asteroids to produce cables strong enough to make a space elevator practical, allowing even cheaper access to space.

Meanwhile, the asteroids that we've been bringing in and mining out can be refitted as space habitats. Scoop out the insides, fuse the exterior rock together if it's not already fused. Install an airlock and engines at each end, and set it spinning. You've now got an O'Neill Cylinder. Line the inner surface with organic material, and you've got a livable environment for humans, plants and animals. Rinse and repeat.

  • $\begingroup$ Checked the link, but I do not think Skylon qualifies as a "escape to the space vehicle". Satellite height is not that high, they are still very deep inside Earth gravity well (that's why they orbit Earth and don't escape into the deep space). You need way more energy to escape Earth gravity well (which means more fuel, which means more weight, which means more powerful engines, which means more fuel and more powerful engines, etc.) $\endgroup$ – SJuan76 Dec 9 '16 at 15:39
  • $\begingroup$ Once you're above the atmosphere, you have a lot more options for escaping Earth orbit. Ion engines, old-fashioned chemical rockets, perhaps the 'impossible' EM drives, solar sails, nuclear pulse propulsion, etc... The tricky part is always getting into orbit - that's what Skylon is for. $\endgroup$ – Werrf Dec 9 '16 at 15:52
  • $\begingroup$ I'd like to see a math based estimate of how much fuel it takes to launch 100,000,000 people into space. If money is no object, as implied by the OP, I think a good answer should assess if our industrial base could support so many launches. $\endgroup$ – kingledion Dec 9 '16 at 19:20

There was a chapter on this in the xkcd book, "What If". Basically, if you want to get everyone off of the planet and don't care what happens to the earth, you could try building some sort of super strong and heat resistant container, and strap everyone inside. Then you set off some nuclear bombs underneath, and voila everyone's in space. Hopefully, you would have some sort of space station ready to relieve these people once they got out there.

Another relatively cheap idea, that would not result in the destruction of Earth, would be an elevator to space. Apparently there are companies actually researching this right now. Again, you'd would need a massive space station to accept all these people once they got out there. It'd be easier to build the station in space and send up a few supplies at a time, rather than building it on earth and launching the massive thing into orbit.

Anyway, I think the ladder idea is the most feasible if you really want to get everyone to outer space. Ships and rocket fuel are extremely expensive and in short supply.


In order to move massive quantities of people and equipment from Earth to LEO (and assuming there is some plan to move them outward from there to permanent colonies), then we have to get away from the "spacecraft" paradigm to something resembling a railroad.

Proposed megaengineering solutions include "Skyhooks", space elevators, and other launch devices like Startram or the Lofstrom Loop. There is considerable literature about these and other hypothetical megastructures, so no description is given here, but the essential commonality between these is they are incredibly massive compared to their payloads, and work by exchanging momentum between the cargo and the static or dynamic elements of the structure.

While most of these devices are on the edge of current engineering technology, the primary reason they are not in current use is they are not only cutting edge technology, but also massive structures in their own right, so would be extremely expensive. Without a massive flow of paying cargo, there is no incentive to build them as the cost would not be amortized over the current amount of space traffic from Earth. OTOH, there is little space traffic from Earth since it is so expensive to launch into orbit; the classic chicken and egg problem.

As mentioned by Separatrix, this is only step one. Of all the various ideas floated in my answer, only the orbital elevator has the potential to take us to part two. An elevator which extends beyond geosynchronous orbit has the potential to act not only as a momentum exchange device to lift cargo from Earth, but the long extension past Geo has the potential to transfer Earth's rotational energy to a cargo pod and (depending on the actual length of the tether) inject it on a minimum energy orbit to Saturn.

You could argue that Saturn is not ideal, but the real point is you already get enough free energy by sliding up the track past Geo that you can pretty much get a free ride to a large part of the Solar System. With that much deltaV to start, a modest rocket engine can get you most places with little extra effort.


It's possible but it will be the single largest undertaking in earth history.

An orbital ring is within our capabilities, It requires only existing technology. You will need it to move the massive amount of material needed. Surviving off earth means you need artificial gravity created via centrifugal force. humans can't breed in microgravity. You need huge scale agriculture in space to produce necessary food. You need large scale manufacturing in space, asteroid mining will help but you still need to move a lot of organic material. That means you need a huge amount of mass lifted into orbit. there only ways to do that using current technology are orbital rings or skyhooks, or more likely both.

You need to also start sending probes in every direction in hopes of finding habitable worlds by the time you have moved a significant portion of the populace into space. We might be able to make mars livable but it will take hundreds of years, but that's OK so will the exodus. It will have to start with dropping a lot of comets on the planet.

You care basically building generation ships and trying to be self sufficient in space while hoping to find a suitable world.

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    $\begingroup$ I do not assess an orbital ring to be 'within our capabilities.' Not remotely. $\endgroup$ – kingledion Dec 9 '16 at 14:46
  • $\begingroup$ Why do you think they are outside our capabilities? Orbital rings don't require the same absurd tensile strengths as space elevators. $\endgroup$ – John Dec 9 '16 at 14:58
  • $\begingroup$ The absurdely huge amount of material and manpower needed in orbit and our lack of experience building anyting that is even 0.0001% of the size, just to begin with. $\endgroup$ – SJuan76 Dec 9 '16 at 15:43
  • $\begingroup$ those are not real limitation, they make it more difficult but none of them move it outside the possible. We lack the motivation not the capabilities. It is like the moon landing was, that is to say within existing technology but very difficult. Its is difficult and will require a huge investment of time, material ,and labor, but none of those things move it outside our capabilities. $\endgroup$ – John Dec 10 '16 at 6:00

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