Let's say the world is about to deteriorate to an uninhabitable state; and will become fully uninhabitable in 2100AD.

Humanity realises that we have to leave this planet and go to Titan1 and the best time to do it is in 2080AD; we all have to get there at about the same time to increase our chances of survival.

Given our current technology, resources, and rate of advancement; would it be more efficient to build single person spacecraft for everybody or build many large spaceships for large numbers of people at a time?

1. Let's say in the meantime we've found that Titan is even more habitable for us than Earth has become.

# This is my first question here and I hope this makes sense.

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    $\begingroup$ bulk transfer is always cheaper, due to something like the square cube law. You need a hull, engines, etc around those transported, and transporting that extra weight requires more resources. The more people transported the more efficient your hull to carrying capacity gets and thus lower expense on moving those extra parts around. If they using traditional rocketry this is a huge expense for escaping our atmosphere; through frankly their need a space elevator to make things remotely practical. I decent number of still huge bulk transports. $\endgroup$
    – dsollen
    Mar 18, 2016 at 17:48
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    $\begingroup$ obligatory xkcd: what-if.xkcd.com/7 $\endgroup$
    – rav_kr
    Mar 18, 2016 at 20:41
  • $\begingroup$ When does humanity realize earth will be uninhabitable by 2100? Probably a few decades prior to 2080? Maybe it could be feasable to not try to send everyone at all, but instead discourage reproduction after some point in time to not have kids be born in a hopeless place and instead have people breed on the way to titan. $\endgroup$ Mar 20, 2016 at 17:01

6 Answers 6


One vs many ships

First off, I would separate transport into three parts: surface-to-orbit (= earth to space), orbit-to-orbit (space travel from earth to titan), and orbit-to-surface (= landing on titan). For all three parts, I would suggest different vessels because the requirements are so radically different.


Since we haven't improved any on the "Strap capsule with transport load to a rocket" design for surface-to-orbit travel in the past 60 years of space travel, I doubt we will be able to come up with a radically different design within the next 20 years. I am saying 20 years because leaving you 40 years for going from a scientific design idea to a workable prototype to a final efficient form (that passes all kinds of safety regulations when it's intended for mass transport) and then mass-producing it in a large enough numbers to evacuate earth, is very short.

Surface-to-orbit will, by necessity, be a smaller vehicle where you can transport only a couple hundred to a couple thousand people at the very most -- it needs to be somewhat aerodynamic, after all, and capable of escaping the gravity well. Also, there comes a point where the fuel-per-person ration increases for every additional passenger and makes large rocket-style shuttles inefficient. (Reason: the fuel for the additional person also weighs something, so you've got to stow some more fuel to transport the additional-person-fuel until it is needed).

Orbit to orbit

The orbit-to-orbit vessls can be huge super-ships that can transport a percentage of humanity (several hundred thousand to several million). You can also include all kinds of amenities and necessities like gravity for the inhabitants (a rotating ring) that make a surface landing completely impossible.

From a scientific stand-point, there might be a size limit due to the overheating problem. Yes, space is cold, but there is no better insulator than vacuum. And pretty much everything on a space ship produces heat -- from the humans on board to the engines to all computers and appliances. You do bleed off heat 'normally' through infra red radiation, and you can increase cooling by using all kinds of decompression cooling (i.e. let miniscule amounts of water or other liquids 'evaporate' into space). However, both ways are limited. Radiation cooling is proportional to surface, but heat is proportional to volume (i.e. heat production growth quicker than cooling capabilities when you scale up). And for evaporation cooling you need to carry something with you that you can evaporate -- at a certain point that becomes inefficient.

There also might be a couple other factors that don't scale well beyond a certain point; fuel once again bein amongst them, same for material strength (you wouldn't want your engine to take off with only half the ship because the I-beams holding everything together snapped).

A more compelling argument against one super-ship would be politics. Since I doubt that by 2080 we will have a unified world government, you will probably never be able to convince all countries to work together on one single space craft. Especially not when anything beyond simple scientific interest is on the line. Also consider that you might not want to have warring factions on the same space ship -- too dangerous that war erupts and then destroys/damages the ship. And you will have to deal with politics and command-lines on the ship, too -- keeping hundreds of thousands of people living together peacefully in a sardine can is very, very hard work.

And last but not least -- the most prominent argument against one supership has already been mentioned by the other posters: laying all eggs into one basket is never advisable.

Orbit to surface

Once again, this will have to be smaller vessels because the orbit-to-orbit cruiser can't land on a surface. And the landers have to be transported by the orbit-to-orbit cruiser.


General feasibility of evacuating earth

I doubt you will ever manage to evacuate the entire earth population. Aside from technical limitations (can we produce enough spaceships to provide a spot for every human? can we stem the transport volume required to get everyone off earth within a reasonable amount of time? can we produce enough fuel to get the spaceships off the ground? can we make the spaceships autark enough that whole cities can survive in there for years?), you have human nature as a limitating factor.

  • whoever produces the spaceships will not want to let everyone on board (criminals? people who can't pay the transportation fee? radical elements? ideology / religion?)
  • there will be non-believers (the world will end in 20 years? tell that to your grandmother!)
  • there will be people who refuse to leave earth
  • there will be a technical failure somtime down the line that kills an entire shuttle of people -- less people will be willing to leave after that
  • there will be people who can't survive the stress of take-off, let alone such a long space voyage
  • there will be countries / populations / religions that deliberately go against what the majority says (just because)
  • once it becomes clear that there are definite deadlines, panic will explode and either kill a lot of humans or damage important surface-to-orbit transport infrastructure

I'd say getting a billion (out of 10 to 15 billion in 2080) willing and capable of leaving earth is a very, very generous estimate. Even that would require transporting 300,000 people off planet per day for 10 years despite riots and technical failures.

And in space -- it took the Cassini Huygens seven years to reach Titan (https://en.wikipedia.org/wiki/Cassini%E2%80%93Huygens). If you want to have a chance at ferrying 1 billion people across to Titan, you should probably start right now.

Actually, I'd suggest Mars as an evacuation target -- temperature is reasonable when compared to Titan, you get more sunlight as cheap energy source, it is closer to earth, its gravity is closer to earth, and no atmosphere is about as helpful as a poisonous super-cold nitrogen atmosphere when trying to settle somewhere. On Mars you have at least a chance at warming the planet with the green-house effect; on Titan you can forget that.


Individual vs. Mass transport ships:

Single-person-ships will probably not be feasible due to the human factor -- families don't want to be separated, and since the journey will be measured in years you'd have to deal with the psychological torture of isolation. Even if you put them on ice for the journey, I doubt that people would send off their small children and babies like that.

Family- or group-sized crafts (10-100 people): While it might be technically feasible to mass-produce peresonal surface-to-orbit transportation, who would buy one since it's a one-way journey? I'd expect something like a regular shuttle service, and that will be optimized to get as many people into orbit as possible.

Small-sized orbit-to-orbit transportation is even more unlikely since it takes years to get there. There is a lot of knowledge and on-the-fly troubleshooting necessary for such long voyages since we don't have the routine and experience yet. Also, I would expect the ration of loaded-to-empty to scale very well up until you hit the upper boundary of technical feasibility. Just imagine cargo ships today -- bigger means more efficient. And if you are talking about the insane amount of resources for an exodus, efficiency becomes the new religion.

The only option left are large to huge orbit-to-orbit crafts. How people get on and off however is up for discussion.

  • $\begingroup$ Actually deep space void has a temperature of 2.7º Kelvin. In space you may transfer heat by radiation. As this is a solar system travel you can heat by pointing radiators to the sun and cool by pointing radiators to the opposite direction. So heat inside a space ship is not an issue. $\endgroup$
    – roetnig
    Sep 14, 2016 at 9:24

Common sense suggests that sending people in large groups would be a more viable option.

1- Sending people into space, and then to a specific, moving destination in space involves a lot of calculations, headache, testing and frustration. It would be practically impossible to give so much attention to even 1/1000th of total population on Earth. If you build huge spaceships, you only have to invest all that headache and frustration once and the payout would be big.

2- Besides the complexity and technicality, the process is also extremely expensive. Sending every human being alone would result in a budget which all the governments of the world wouldn't be able to pay for. Building massive spaceships would also be a highly costly mission, but if all governments of the world unite, they would be able to pay for it.

3- Also, don't forget that there is a huuuuuuuge distance between Titan and Earth and the journey would probably take months, if not years. Now consider the fuel costs. And the food requirements for every person. It might surprise you, but most of the weight on a spaceship before launch comes from its fuel. Even unmanned space mission carry vast amounts of fuel. Sending a lot of people together will immensely cut down on fuel and food costs too.

4- 99.999% of people don't know anything about driving a spaceship. Or how to react when/if they see a Baptestina sized asteroid straight in front of them. So then again, you would need to have a highly learned technical crew to take people safely to their destinations.

5- Lastly, it's interesting to consider how many launch pads you would require. Considering that spaceships require special launch pads, and there aren't many such launch pads around in the world, let's assume that 10,000 people are launched into space everyday, utilizing all launch pads around the world. Considering that the current population of the world is 7 billion, it would take 700,000 days to send all humans into space. That isn't a lot of time. Just about ... well ... nearly 2000 years.

  • $\begingroup$ Why does it cost less food to send them in groups? And why is it cheaper to build one large ship instead of many small ones? $\endgroup$
    – Burki
    Mar 18, 2016 at 8:01
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    $\begingroup$ #3 -- keep in mind that the delta-v ability is a function of the mass fraction and the engine exhaust velocity, as per the rocket equation. Sure, the mass fraction doesn't paint the whole picture in this case because it concerns itself with only the fraction of the total mass that is fuel, not little things like life support systems, communications and so on, but it's still a good place to start. $\endgroup$
    – user
    Mar 18, 2016 at 8:49
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    $\begingroup$ Because => Some items of food can be recycled @Burki Also, consider that 3 massive spaceships, with each the capacity to transport some thousands of passengers would take far less material and labour than the material and labour required to build 1 billion small spaceships, each with the capacity to transport 1 passenger. Also, calculate in the fuel costs. That's why governments always support public travel (more passengers in one vehicle) over personal transport vehicles. $\endgroup$ Mar 18, 2016 at 10:30
  • $\begingroup$ @YoustayIgo not sure about the food, but for the ship you are pointing at the square cube law, would that be correct (enough)? $\endgroup$
    – Burki
    Mar 18, 2016 at 10:57
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    $\begingroup$ @Burki Yes, but you also have to include in the labour costs (cubic I guess) and multiply one navigation system (for the larger spaceship) with billions (for each single). Resembles square cube law a bit (in some weird way), but since it isn't pure geometry so I think square-cube law is a nice analogy of this situation. $\endgroup$ Mar 18, 2016 at 11:31

First, I want to question your premise. Don't send them all at once. That decreases our survival chances.

  • Reuse the spacecraft for the Earth to Saturn leg. That reduces the mass you have to lift from the gravity well of Earth.
  • In fact, make it a limited triangle trade. People, equipment and landing craft from Earth orbit to Titan orbit, mostly empty ships from Titan orbit to bases near some ice asteroids/comets/whatever, fueled ships from the asteroids to Earth orbit.
  • Have the first wave of settlers phone home and tell what works (and what doesn't). Perhaps they find that wheat doesn't grow well but rye does.

That being said, build reasonably big. If you send millions of people, there should be thousands in each ship. Youstay Igo mentioned how few can pilot a spaceship. Let me add that each ship should have a medical team, life support techs, landing craft pilots, quartermasters, ...


Your question suggest that this is a binary "all or one" question. In fact though there is a big sliding scale between 1 and everyone.

What would most likely happen is that a lot of reasonably sized ship would be created able to carry several thousand people, and those ships would then ferry back and forth carrying people each time.

Getting everyone there at the same time is actually a bad idea, you want to start with a few thousand and have them start building the infrastructure for everyone. Then as you get the infrastructure ready more people start arriving and you grow at a steady and controlled rate, expanding to accommodate more people which then lets you expand faster to accommodate even more people. etc.

Titan isn't very hospitable so it's not like you can just unload everyone on the surface and be done :)


Tell people that they are being sent in ships that can each hold roughly 7000 people.

So, entire world's population of 7 billion starts to work making 1 million. ships. Using secrecy like Manhattan project so no-one knows the full design.

Then, while they are wasting their time, 1 real ship is created, and sent 10 years ahead of schedule. If the launch can't be kept secret, say they are going ahead of time to set up the base and prepare for the arrival of everyone else. 7000 people arrive on Titan, more than enough to continue the human race.

6,999,993,000 people are very disappointed to find out that the rocket design doesn't work, they haven't stockpiled anywhere near enough supplies, don't have enough fuel, and will be dead within 20 years.

The project is not feasible, regardless of ship size.


Just enjoy your after life. If you look at the big picture the Sun with engulf the solar system and leave Earth and everything else vaporized.

On a sadder note: Humans may only exist based on the will to repair the damage inflicted to Earth otherwise the extinction of the human race is possible or probable.

Rinse and repeat with any habitable planet orbiting a Sun like star.

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    $\begingroup$ Welcome to Worldbuilding, ccris, answers to questions on this site are expected to deal with solving the problem posed in it. They aren't about taking a philosophical view of the topic. You can improve your answer by proposing how humanity can escape from Earth. Have fun! $\endgroup$
    – a4android
    Jul 15, 2018 at 5:34

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