I know that I want to transport 500,000 people to a fictional planet. Earth has had roughly 800 years to plan this evacuation. Reasonably speaking, if people were transported to a docking station, and then transported orbit to orbit, what would be the absolute most (and realistic) amount of people I could cram on a spaceship for a 300 day journey.

Given that this story would take place almost 1000 years in the future, is my imagination the limit or are there specific rules that have to be follow regardless of scientific and tech advancement for the amount of people able to be transported and survive?


closed as too broad by sphennings, L.Dutch, Mołot, Aify, Draco18s Jul 5 '17 at 18:47

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    $\begingroup$ If it only takes 300 days, I wouldn't wait 800 years until I could construct a ship to carry 500k people but I would do the trip multiple times. Even if I need to wait 800 years, do multiple trips and don't care about the size. Also, where do you go in 300 days? The only thing that comes to mind is Mars - so you need to establish a colony on Mars. Even with a small group and just one trip you can easily have 500k people 800 years later living there. I don't think there are any physical limits for "just" 500k people, I just think it's a bad plan. $\endgroup$ – Raditz_35 Jul 3 '17 at 15:20
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    $\begingroup$ Too few characters. One more thing about 300 days. This is terribly slow in order to reach Mars. I think my comment stands for any other body in our solar system. You will not reach another star system in that time unless you have FTL. In that case I wouldn't worry about any specific rules anymore because you already eliminated the biggest one of interstellar travel $\endgroup$ – Raditz_35 Jul 3 '17 at 15:26
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    $\begingroup$ Millennium is a long time and tech changes a lot in such time frame. Your question is way too broad unless you tell us what's the technology in your world. $\endgroup$ – M i ech Jul 3 '17 at 15:27
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    $\begingroup$ It's your story. You can write it however you want. 1000 years is far enough in the future that it is impossible to make any meaningful statements about humanity's capabilities to do anything. As this question is currently written it's far too broad. If you described the technologies available to your future humans this question would become somewhat answerable. Perhaps it would be best to have your plot dictate whether the evacuation is successful or not. $\endgroup$ – sphennings Jul 3 '17 at 15:27
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    $\begingroup$ Depends on how big your spaceship is. If you attach a solar sail to the Earth and use the entire thing as a spaceship, you could transport hundreds of billions. If place a solar sail near the sun and use it as a Shkadov thruster, probably hundreds of trillions. $\endgroup$ – ckersch Jul 3 '17 at 15:35

I think your imagination is the limit; obviously "spaceship Earth" can carry ten billion.

That said, for practical purposes, more ships is a better bet; it gives you an enormous amount of resilience. No single catastrophic failure (or meteor strike) kills all 500K people, no single mutiny takes over all the ships, no disease travels through the entire crew, and you have backups: Many food ships, many fuel ships, etc.

Follow the square root rule: $\sqrt{500\,000} \approx 707$; so put around 700 people on each ship, for about 715 ships. Then the 25% rule: Presume you need 25% more space to carry fuel, food, and other specialized equipment (like hospital equipment, engineering, manufacturing, non-fuel energy production, administration, leisure). So bump it up to about 900 ships. Add about three shuttles for each. It is possible to link your ships (so shuttles are not usually necessary); a large flat disc of ships can fly through space "face first"; there is no air resistance. That even makes it easier for them to separate and fly far apart, if necessary for individual maneuverability through an asteroid field or something.

One giant ship is a logistic mistake, I doubt that would ever be the actual solution that engineers would agree upon; it endangers too much life.

  • $\begingroup$ Tell us more about the air resistance in space. $\endgroup$ – Tony Ennis Jul 3 '17 at 22:56
  • $\begingroup$ @TonyEnnis he is obviously talking about solar wind. #sarcasm? $\endgroup$ – Mindwin Jul 4 '17 at 6:40
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    $\begingroup$ @TonyEnnis What are you talking about? The only mention of air resistance in this answer is the statement that "there is no air resistance" in space. Of course, there are other similar issues that one needs to contend with, such as gravity causing the instantaneous velocity in your transfer orbit to change over time (a consequence of Kepler's second law of planetary motion), slowing you down if you move outwards in a solar system, but those are known knowns which we know already how to deal with. Compare the Pioneer anomaly. $\endgroup$ – a CVn Jul 4 '17 at 8:14
  • $\begingroup$ @TonyEnnis You mean, that there is none? If you are inclined to dismissive snarking, please double check you understand what was said before you pull the trigger. The point of that line was to say the flight configuration in space is not constrained by the aerodynamic (or hydrodynamic, in water) concerns that are second nature in our thinking; thus there is no need for a streamlined shape of the fleet, even when ships are connected. I thought a hint would suffice, but thank you for the opportunity to expand here for readers with less reading comprehension. $\endgroup$ – Amadeus Jul 4 '17 at 10:29
  • $\begingroup$ @Mindwin What you find obvious is false; I intended exactly what I said. I would have included as an equal in my response to TonyEnnis, but I can only call out one per comment. Enjoy that response as if it were addressed to you. $\endgroup$ – Amadeus Jul 4 '17 at 10:33

The question is extremely open ended. If you use a "worldship", you could potentially have millions of people on artificial worldlets which can keep everyone aboard alive for generations. OTOH you could cram them aboard something like a commercial low price airliner and have a few hundred people aboard with hundreds of ships making the trip at once.

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An example of the Woldship idea

Perhaps the real limiting factor is how many people you can get into space to board your spaceships at a given time. Current state of the art, using chemical rockets, can only take a few people into LEO at any given time, so if this is the only way to get into space, then you will be extremely limited (building the space infrastructure will also be difficult)

The best way to break the tyranny of the rocket equation is to use some sort of momentum exchange system to bring people into space. This also means building the spaceships and infrastructure will also become easier as well.

Sending people and equipment into space via a space elevator is probably the best solution, although the building of a space elevator involves solving technology questions like creating super strength materials in the quantities needed. Other solution such as giant mass drivers or devices like the Lofstrom loop also require some extreme engineering, as well as manipulating huge amounts of energy.

So the answer to the question is essentially set in how easy it is to bring material and people from the Earth to space to create the spaceships and man them. Systems that limit the amount of equipment and people will necessarily tend towards small spaceships and small numbers of people, while momentum exchange systems which can transport large amounts of men and equipment will allow for large spaceships and large numbers of people to be accommodated.

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    $\begingroup$ "some extreme engineering" ... "manipulating huge amounts of energy" I wonder what people in the 1200s would have said of our current technology level? While people had perhaps dreamed of things like going to the moon for aeons, it was only a hundred years ago that we had got to the point that we could actually imagine a craft that might theoretically be able to get to the moon which we could construct. (It took another half century to actually design and build one that could make the trip.) $\endgroup$ – a CVn Jul 3 '17 at 20:10
  • $\begingroup$ Since we are talking about the limits of chemical bonds (in the case of a skyhook) or manipulating the kinetic energy equivalent to a small atomic bomb in the case of a launch loop, it seems we still have quite a way to go $\endgroup$ – Thucydides Jul 4 '17 at 4:04

Available for the task energy will be one of the first limiting factors.

And demands for the energy in the case will depend on the technology you are using for the space ship - for to create it, for to launch it, for to slow down it at the destination.

300 days is a rather small time range to travel to any planet outside our system if we do not talk about some planet nine or 10th or 11th which happens to be in our system unnoticed.

However, if you stay out of FTL and as an example assume that those 300 days are subjective time for those people, then you have means to regulate the plot as you wish.

the closer the speed of the ship is to the speed of the light, slower is the subjective time for passengers, and more energy is required to launch the ship.

Time dilation is: $\Delta t' = \cfrac{\Delta t}{\sqrt{1 - \cfrac{v^2}{c^2}}}$

relativistic kinetic energy: $E_\text{k} = m \gamma c^2 - m c^2 = \cfrac{m c^2}{\sqrt{1 - \cfrac{v^2}{c^2}}} - m c^2$

So basically if their destination is Alpha Centauri, they need about 4 times slower time, thus they require $(4-1)\cdot m c^2$ of energy.

If we assume 10'000 kg mass of craft per passenger (which is 7 times less mass than the mass of ISS per crewmember) then for that 4 times dilation time travel, they require energy which our sun produces in 1.2 seconds time.

A trip to the center of our galaxy will require about 26000 times slower time for the passengers, and it is about 8.5 hours of work for the sun.

There are limits, but they are pretty high, and in your situation, it is more likely to be limits of the technology available, or destination point if it so happens you would like to send them billions of billions of light years away.


Your question is really open ended. What kind of technology would there be in 1000 from now, in what I assume is year 3020 to 3820? what would the population of the earth be at that time?

Using the calculator here, you only need to transport 36 people with a growth rate of 0.012 per year in order to have a population of 502,010 within 800 years. The birth rate matches USA 2017 statistics, but the birth rate for a colony ship would be considerably higher.

I am sure that is not what you are asking, but you should keep it in mind.

Using that same calculator to answer the question of how many people there might be given a very conservative 0.012 birth rate, in 1000 years there will be 90,920,905,889,691 people (conveniently ignoring space and resource constraints on Earth).

Obviously "saving the human race" with only 36 people is possible but 100x that number would be significantly better. With 3600 per ship, and only 200 years of time the population would be 39,120... so only 13 ships would be required to reach 508,560 (base population of 46800). Using the same formula, you could calculate generational ships with initial crews of about 4000 and ending with 45,000 people at the end of the 200 year journey.

If you are saying that it only takes 300 days for the journey, it might take up to 700 days for a round trip (with 50 days repair and resupply at each end) meaning that you can have a single ship holding 4000 people (plus crew for out+return trip) and making 125 trips within 240 years to transport a full 500,000 refugees...

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    $\begingroup$ I think this assumes too much about the story plot. Suppose that over the course of the 800 years, there is a concerted breeding program starting with many thousands of people, trying to quickly evolve or enhance specific physical traits in forty generations, and only taking the 'best' 500,000 of the result as a seed population. So you could have a trillion offspring, but each must still "prove viable" by age 18, or its descendants are excluded from the pool of final candidates. Let them live out their lives, just drop them from the roster. Selective breeding of humans for life elsewhere. $\endgroup$ – Amadeus Jul 4 '17 at 10:10

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