# How much mass does a human need to live a satisfying life?

Humans evolved in ancestral environments as semimigratory omnivores, which has embedded several things into our instinctive sense of wellness, aka "all is alright with the world", among them a preference of living close to water, rolling hills with lots of green and sparse tree cover, an average tendency to exhibit weaknesses for certain foodgroups (such as meat and fructose), and experience cognitive biases and limitations generally steering them towards living in groups of 50-200 people. Let's assume for now these traits are preserved, i.e. we are dealing w/ Homo Sapiens Sapiens as we know and love it.

Now, I want humans in space. Now, I want A LOT of humans in space. I want humans to infect the solar system and nearby galactic regions like a plague, like an unstoppable ever expanding Zerg creep. Now, complicating things, I'm also a firm believer that human lives ought to be happy as far as possible. Thus, each human in space will need a certain amount of mass to breathe, swim, play, socially interact, have sex and procreate, and raise disgustingly happy children in.

Let's say we choose to build space habitats large enough to house a billion people each. Let's ignore to the extent possible things like the specific shape, rotational speed to maintain a livable gravity-like experience and so forth. Let us not focus of the need for ways to make up for mass loss to space for various reasons. How much mass, and how much mass per capita should I expect each of these habitats to take?

Ah, but the question is now a bit broad. Let there be constraints. So, constraints. Humans need (for each of the following, you may assume that a technical infrastructure to support them is needed, with --and I am probably guessing wrong-- a 90%:10% mass ratio between infrastructure and the thing being supported):

• air,
• day-night cycle,
• space and mass for lakes and rivers,
• space and mass for trees so a layer of soil of at least 10 feet is probably required,
• space and mass for rolling hills to wander around in,
• space and mass for cute furry/scaly/feathery things to pet and/or eat (aka a biosphere),
• space and mass for habitats to live in (caves, homes whatever)
• lots of electricity, enough connectivity to maintain a rich verbal and cultural heritage (these folks live in the future, and are not primitive, although we would probably find them strange)
• mechanical servants and tools to do some of the heavy work occasionally

Humans also like to live in communities, let's say the aforementioned villages of 50-200 people, and assume about 3 miles (5km) or so between villages.

Let's keep in mind (for the purpose of realism) that some small fraction might prefer solitude (so higher mass requirements) and that a significant fraction (5-99%) might actually prefer to live in denser communities of millions or billions of people, which means our village estimate should be a rather conservative (in the sense of being generous & expansive) estimate of the amount of mass needed.

And yes, I know that being as a flesh-being in space is expensive. I know it's easier to colonize Antarctica and the Gobi desert, or the Mariana trench for that matter. You may safely assume that's all occurred, or that Earth is a pristine nature preserve or whatever. Remember, ever spreading plague of happy humans.

And yes, I know the AIs and Uploads will probably take over, and that most intelligence in the universe in the future will likely be of the computronium rather then squishy fleshy brains variety, which would result in totally different mass requirements, I've read Age of Em too -- if it makes you happier, assume this is a pet-project of some trivial nostalgic fraction of the Uberminds. Let's just focus on the question at hand for now.

• "caves, homes whatever" - what tech level? Because caves are a lot heavier than homes and homes are a lot heavier than ISS... Should we assume modern-day surface structures with only minimal changes to allow it to be in space? Also, how slow is the biomass recycling loop? Modern day one is way too slow and too bulky for space, I'm afraid, long, indirect way from human poo to hydroponics nutrients, and composting is hazardous. May 31, 2017 at 7:29
• @Mołot You may assume high-tech homes made out of nanomachined hydrophobic aerogel if you wish. These are not primitives, just flesh humans under the "do not go stir-crazy in space" requirement, so lots of stuff to make them feel safe and instinctually at peace. May 31, 2017 at 7:31
• Oh ok. I'll try to answer but it'll take time and I bet someone will beat me to this. May 31, 2017 at 7:33
• Indeed; related. Note that I didn't claim it was a duplicate, nor did I vote to close as such. If a question is inspired by another, it rarely hurts to link to it in the question itself, if for no other reason then for clarity. I just recognized a part of the premise because I voted to close that question as off topic on Space Exploration yesterday; then it was migrated here.
– user
May 31, 2017 at 7:47
• I want humans to infect the solar system and nearby galactic regions like a plague. [...] and raise disgustingly happy children. +1 for the verbage alone. May 31, 2017 at 17:13

Speaking from experience, having 100'000 m2 per human is more than enough to be lost in any type of village or city or woods you would like to have.

Also, 250 m2 per human is good enough for a relatively big city, and you can have different things in the city - trees, grass, water to swim, small forests/parks.

21'853 m2 per human is enough to host 7 billion people with different habits and appetites for territories, including water bodies, land used to grow food, cities, villages, roads, nuclear testing sites, deserts, etc - this is the average density of Earth's population at the moment.

One of the significant uses of land is land to grow different crops and livestock, forests to use as the material source.
According to this source, 11% of the land area is used for crop production.

Different types of activities and plants require a different thickness of the ground. In my understanding forest requires the deepest of them all and forest takes about 1/3 of the available land according to this source. Grass requires the least thickness of them all (0.1m seems to be well enough).

So if you assume about 1m thickness of soil on average it is not that big of a stretch. However, it depends on the technologies you have. So we speak about low-grade technology, which is called "no special technology at all".

Requirements for soil, especially for food production, could be significantly reduced to essentially 0 soil with different types of aquaponics. The same goes for livestock - no soil requirements.

Requirements for forests could be reduced if we do not use them too much for the production of different stuff and do not rely on them as CO2->O2 conversion as such conversion is doable with the same grass, algae, etc. - no soil required.

So soil requirements could be cut in half just by changing the main contributors for CO2 conversion and ways to grow food or as a minimum the proportion of soil requirements could be reduced significantly.

All that said, assuming 1m thick soil isn't a big stretch and the average density is about 2.5-3.5 ton per cubic meter. So if we really want to make everyone happy then about 60'000 tons per human is enough. That's for the matter wich has no structural input for the construction of the habitat itself.

The mass of the habitat construction depends on the technologies you are using to build that habitat, and the sizes of it, as well a the approach you take to combine different parts of it. As an example: have a set of smaller habitats, which form a system with different climate zones, look and feel and which hosts 10-20 million inhabitants. These are connected in a bigger structure consisting of hundreds of them and hosting billions of people - this approach is cheaper in terms of the amount of construction material than a single big structure for reasons of safety as an example or reasons of protecting the external hull.

Different materials and technologies could affect a number of materials you need for the construction itself. Using carbon nanotubes everywhere could significantly improve the situation with the required materials by reducing their amount.

So if we take twice of what we need for the interior for the supporting structures of the habitat itself then we get about 200'000 tons per human.

So this number is based on the average density of Earth's population, without significant improvements in land use, without considering optimizing landscape design, etc.

If we take city norms, which a lot of people are ok with, then it seems those requirements are lower by 2 orders of magnitude. However, they will require the use of aquaponics and large-scale livestock keeping methods.

O'Neill, G. K: The Colonization of Space contains interesting calculations for an O'Neil cylinder. And if I understand it correctly it assumes in general about 8 tons per square meter for the average material requirement for the cylinder. Here we have more than that, I assumed about 11 tons.

So at the end you need to get some numbers for the average territory you assume as per human for the "to live a satisfying life", and it seems to be below of about 22'000 m2. However, who knows.

Using materials with high strength (those CNT's) and technologies (like smart matter stuff, made of those CNT's as an example) could significantly lower the requirements by mass, significantly increase the surface produced with them and allow other optimizations like 0-soil for real plants (all of them - trees, grass, etc), almost massless mountains similar to the Everest, or even fake things like oceans, deserts, forests and other procedurally generated graphics, but made in 3d space of the smart matter.