Sci-fi scenario with flights to a colony planet. Tech level is otherwise close to today (e.g. no androids or desktop nanofactories). Planet is quite similar to Earth, so e.g. agriculture works, you can grow wheat and raise cattle.

What would be the likely/optimal ratio of passengers to cargo transported to said planet?

Let's say it costs as much to transport one passenger as five tons of cargo, and available transport resources could per year bring 10,000 colonists and 50,000 tons of cargo - that would be splitting the available resources equally.

At one extreme, clearly there would be no point hauling 100,000 tons of cargo and nobody to make use of it.

At the other extreme, you don't want to put 20,000 colonists on another planet with nothing more than carry-on luggage; even though it's a nice habitable planet, they will still need tools and supplies.

Does the above even split of one colonist per five tons of cargo make sense, or can we reasonably say each person would need more cargo, or less?

The closest analogy I can think of is the colonization of the Americas. (The main difference being that in the sci-fi scenario, there are no natives.) What was the overall ratio of people to cargo transported from Europe to America during that time period?

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    $\begingroup$ By 'cargo', do you mean just what the colonists need/use when they get to the planet? In other words, not the food, air, water, etc. needed on the journey? In other, other words, using your example, the goods transported to the Americas, but not the food eaten along the way. $\endgroup$
    – Giter
    Mar 6, 2018 at 16:18
  • $\begingroup$ @Giter Right, yes. $\endgroup$
    – rwallace
    Mar 6, 2018 at 16:32
  • $\begingroup$ With the colonization of America, cargo went in both directions. $\endgroup$ Mar 6, 2018 at 16:45
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    $\begingroup$ If your tech level is close to today, then you will have much bigger problems: fuel. Imagine a ludicrous amount of fuel in relation to the weight of the actual cargo, and you will need a whole lot more than that. I suggest reading about the Tsiolkovsky equation. In order to leave Earth's gravity well, you need fuel. But to lift that fuel, you need even more fuel. And to lift that even more fuel, you need again more fuel. Take a look at the rocket which sent us to the Moon, and how much of it was just fuel. Now multiply that by 50000. Wait, you can't just multiply, as it grows exponentially. $\endgroup$
    – vsz
    Mar 6, 2018 at 21:45
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    $\begingroup$ Having large amounts of cargo delivered on one ship before committing to the arrival of passengers seems like a good idea. If for some reason the cargo delivery fails, would-be passengers can then delay their voyage until a successful delivery can be made. As a related real-world example, Ernest Shackleton set out just before the start of World War I to cross Antarctica; his plan was to have enough supplies to make it about 2/3 of the way across, where he would find supplies left by another crew that was supposed to leave them there. As it happened, ... $\endgroup$
    – supercat
    Mar 7, 2018 at 0:18

6 Answers 6


Just how earthlike is the planet? Assume it is a shirtsleeves environment for humans, at least in the nice spots ...

  • Assuming dehydrated/dry food, with a few supplements, it should be possible to live on less than 1 kg of food per day. Double that for sundries like toilet paper, aspirins, or new socks. 730 kg for one year. (If the world is reasonably Earthlike, the growing seasons should have about the same length.)
  • 250 kg per person should be plenty for a sturdy all-weather tent, camping cot, sleeping bag, stuff like that.

So with one ton, your colonist can live for a year.

Say the colonists want to clear fields, put seeds into the ground, and get a bountiful harvest.

  • The interior of a standard athletic track is one hektar (ha). Modern grains might required 100 kg of seed for 1 ha, and yield several tons of grain on Earth. Your alien world will have unknown pests and other calamities.
  • The figure assumes modern mechanized agriculture.

Still, an off-the-cuff estimate suggests that your colonists should be able to feed themselves with starchy staple foods after the first growing season, if Earth grains will grow. For the second year, the colonist will need much less than a ton -- perhaps some vitamin or mineral supplements, and the occasional imported chocolate bar from home.

Note that I haven't suggested cars, or tractors, or aircraft. Decide on the ratio of jeeps or tractors per colonist. A well-supplied colony could have close to one vehicle per colonist. A bare-bones colony could have less than one vehicle for every 10 or even 100 colonists. Say an average farm has 200 workers and perhaps a dozen vehicles: a few tractors, a few trucks, perhaps a combine harvester.

An off-the-cuff estimate says less than 1 ton of vehicle per colonist. How much fuel and spare parts per vehicle? Unless they will find a way to make fuel locally, bringing vehicles makes no sense at all, so it will be just a little initial stock, until the oil wells and refineries, or solar farms and electrolysis systems, or soyfields and biodiesel refineries are set up.

Other workers will require their own "tools of the trade." Machine shops, hospitals, whatever. Assume that those, too are 1 ton per worker.

That suggests a ratio of 2 tons of cargo for each colonist.

But I made some very back of the envelope estimates here. What is the goal of the colony? Become a self-sufficient technological society as fast as possible? Become a genetically healthy human society as fast as possible? Living room for the crowded masses of the homeworld?

I think anything from 1 ton per colonist to 10 tons per colonists can be justified.

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    $\begingroup$ But humans being humans, they will want more and more creature comforts from 'home'. Grain alone does not make for a happy, contented life. As Keynes famously said, people will always WANT more than they NEED. $\endgroup$ Mar 6, 2018 at 17:08
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    $\begingroup$ @JustinThyme, it could be that these people want to get out of the crowded, polluted slums of Earth, and that they're not willing to save for another decade to buy more cargo space. Or they're highly qualified specialists who can and do demand "basic necessities" like hot and cold running coffee if they're to work in the "outback" ... $\endgroup$
    – o.m.
    Mar 6, 2018 at 17:28
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    $\begingroup$ @JustinThyme the ads say "Want to own a planet? Enlist in the Colonization Office!" They don't say "Want a margarita-making-sexbot?" or "Want a 320G brain phone?" Colonization is for people willing to leave these kind of comforts (at least until the colony is up and running and trade has been stablished). $\endgroup$
    – SJuan76
    Mar 6, 2018 at 21:07
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    $\begingroup$ @o.m. Hot and cold running coffee, huh?... :) $\endgroup$
    – T.J.L.
    Mar 6, 2018 at 21:16
  • $\begingroup$ ...and of course don't waste fuel on useless stuff like men. For the cost of a 200lb man you could have two young fertile petite women and a dewar full of sperm samples. $\endgroup$ Mar 6, 2018 at 22:24


At one extreme, clearly there would be no point hauling 100,000 tons of cargo and nobody to make use of it.

But that would make a fine scifi scenario, and give some good asymmetric energy to the narrative.


Ultimately, some 170 million pounds of vehicles, equipment and "white gear" -- that's all the non-military stuff that contractors have been using, like furniture, generators, chemical toilets, air conditioners, non-classified computers and more -- are being left behind. What they cannot, or will not, donate or sell to the Afghans or to allies is being destroyed so that it doesn't rot in place or fall into Taliban hands. Some reports indicate that most of the $6 billion worth of materiel will indeed be sold for scrap or thrown in the junkyard.

Modern US military endeavors come with an excess of materiel - given the enormous cost of the endeavor overall and difficulty replacing individual colonists, having any impediment due to lack of equipment is pennywise poundfoolish. So too your colony. Those behind it overpack. Unmanned cargo ships have been coming for years and parking themselves in orbit or on the surface. They do not want their colonists to starve, or struggle for lack of something that is easily provided.

The huge excess of stuff available to the colonists could have shades of military contractors providing loads of whatever they had at top dollar. It might be unusual stuff, or stuff with no practical use on the planet in question.

  • $\begingroup$ +1 for cargo ships parked in orbit - once they're empty, they can either be left in orbit to function as satellites (quite possibly being designed to separate into multiple satellites) and/or harvested for scrap metal. $\endgroup$
    – Rob Watts
    Mar 6, 2018 at 21:32

I suspect the ratio would change as time passed. Early flights might have more cargo and less passengers, and those passengers being highly specialized and trained to setting up the colony - agriculture, manufacture, building. As things get established, passengers would increase as less materials are needed from the home planet.

That's assuming said planet is close enough that more than one ship is planned. If you're thinking of a more of a "drop the ship and cross your fingers" strategy, I think you'd still have a very carefully chosen team of base staff, with a larger number of laborers and settlers. I'd lay odds there's be no children at all, and depending on the length of the journey, rules against having any on the way. The final breakdown of staff would likely be based on the climate of the planet - what's available as opposed to what would be needed quickly, and what would need building and maintaining.

In either case, I'd bet an initial team would be no more than about 500 people - more than enough to get a colony going, but not so much to put a strain on initial resources. And I'm assuming that a great deal of that initial shipment is building material - prefab housing and the like.

Giter makes the good point about air and water - using your analogy, I suspect a great deal of those early colonists' supplies were used (up) on the journey. I'm guessing that's why you approximate such an expense in carrying a passenger?

  • $\begingroup$ Right, part of the reason it costs as much to carry a passenger as maybe five tons of cargo, is because the passenger needs extra consumables for the voyage. $\endgroup$
    – rwallace
    Mar 6, 2018 at 16:35
  • $\begingroup$ And yes, the ratio might change over time. So let's say after that initial phase, after the initial team has done the job and new colonists are arriving by the tens of thousands, what should the ratio be then? $\endgroup$
    – rwallace
    Mar 6, 2018 at 16:36
  • $\begingroup$ The answer might not be a number but a formula. How much would a particular type of passenger need in supplies to get set up until they can become a self-sustaining and contributing member of the colony? Allowing for one new prefab building per X number of passengers, x amount of supplies, etc. This is also assuming that the entire ship is reusable - There could be a design where the ship itself, or some trailer-like portion will serve as housing or other structures. The ship itself would BE part of the supplies $\endgroup$ Mar 6, 2018 at 16:44
  • $\begingroup$ Methinks it would be economics that decides. Someone has to pay for it. $\endgroup$ Mar 6, 2018 at 17:04
  • $\begingroup$ Colonization might likely be subsidized by the government, at least for the first few deliveries, to get things started. The best case scenario is a self-sustaining colony, hopefully one that can conrtibute to the economy $\endgroup$ Mar 6, 2018 at 17:45

With the colonization of the Americas, there were cargo ships and passenger ships. The cargo ships delivered nothing but cargo, once the settlers became a viable market.

I suspect that, in any colonization venture into space, even with generation ships, there would be separate cargo ships from the passenger ships. Cargo ships are much more economical, requiring no life support, for instance. They would be launched at the same time, on the same trajectories. at the same destination.

They would be scheduled to arrive roughly at the same time as the passenger ships.

Thus, the passenger ships would be required to take only what was necessary for the trip, and a bit extra to bridge the gap until the cargo ship came.


However, there is a different model to that of the colonization of America. There is also Australia. It was established as a not-for-profit penal colony, with no expectation of return (literally and economically). Such a venture would have a different ratio of cargo vs passengers. Creature comforts of the 'settlers' would not necessarily be a consideration, nor would 'development of the profitability of the colony'. Bare necessities only.

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    $\begingroup$ On slave ships, the passengers WERE the cargo. $\endgroup$ Mar 6, 2018 at 16:32
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    $\begingroup$ There might indeed, but then the question becomes how many passenger ships versus how many cargo ships to build and operate. $\endgroup$
    – rwallace
    Mar 6, 2018 at 16:34
  • $\begingroup$ @rwallace I would suggest, then, that this would be an economic market force answer. The ratio would vacillate from year to year, as to which would be more profitable - cargo or passengers. Supply and demand. If there were sufficient demand, returning a sufficient profit, I am sure there would be merchants willing to ship at great volumes. The catch would be in what is available to ship back. $\endgroup$ Mar 6, 2018 at 16:43
  • $\begingroup$ There would also possibly be profit to be had if the colony starts manufacturing goods and services of value. The trip to the colony might even become a loss leader against the more profitable return trip. $\endgroup$ Mar 6, 2018 at 16:46
  • $\begingroup$ Right, but the characters investing in the transport company, have to make initial guesses about what is likely to be profitable. Return trip is for the moment going to be basically natural resources, ore or suchlike, some value but not profitable enough by itself to pay for the operation. $\endgroup$
    – rwallace
    Mar 6, 2018 at 16:48

I don't think you could ever bring enough cargo to support the colonists. At sub-light speeds, the only viable way to feed your passengers is a self-contained ecosystem.

What cargo you do carry would be a small amount of emergency food, hard to make engine parts and backup seeds & embryos in case you have an ecological disaster mid-flight.


Cargo shipping is always going to be a cost/reward balance - at some point, for almost any possible cargo, it's going to become more viable to produce the cargo at the destination than it is to ship it across. When it comes to interstellar travel, it's likely that the costs for shipping will be very high indeed. In all likelihood, you'll mostly be shipping machinery and similar hi-tech products to enable end-user production and possibly a few ultra-high-end goods, if you have any uber-rich colonists (how much might a billionaire pay for a truffle?).

The exact numbers though are going to depend on how the travel works in your universe, why the colonies have been established and how far away are they. A good start might be how isolated the colonists are - if it's a one-way trip then that suggests difficult travel. Will they be holidaying back on Earth? Then it's probably pretty simple to get between places and trade will be much more likely.


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