Follow the reluctant adventures in the life of a Welsh astrophysicist sent around the world for some reason, wherein I photograph potatoes and destroy galaxies in the name of science. And don't forget about my website, www.rhysy.net



Saturday, 9 February 2019

Nazi Farmyard Iiiinnnnnn Spaaaaaaaaace !

NAZIS !


Everyone loves Nazis. Well, loves to punch them or shoot them or whatever. As human beings, their sole redeeming feature is their dress sense, which is what makes them perfect pantomime villains. From their dabblings in the occult to their terrifying superweapons, Nazis make ideal movie enemies. Not to mention that they were evil as hell.

So evil that they turned into zombies in Dead Snow. Much more scary than the losers with little garden torches in 'murica.
Today's post is not another rant about free speech or politics. Nope, this one will answer a much simpler question : what would happen if you put Nazis in space ?

You are no doubt thinking that the answer is simple : a movie even stupider than Dead Snow.


You're not wrong. But there's one aspect of the Nazis that raises some genuinely awkward questions for space travel. Yes, you've guessed it - I'm talking about eugenics. While genetically modifying humans on Earth causes no end of uproar, the idea that we should try and adapt ourselves for the purposes of long-duration space travel is somewhat safer (probably because there's no immediate danger of anyone actually trying it).

The idea's been around for ages, with perhaps a certain grim acceptance of its inevitability. After all, space is such a very different environment that modifying ourselves to make it easier to live on other worlds might be legitimately necessary.

That's the plot of Netflix's truly awful The Titan. At least Iron Sky is vaguely entertaining.
But trying to adapt ourselves to lower gravity or different temperatures isn't remotely feasible for now. There is, though, a much more pressing matter than needs to be addressed : how many people does a multi-generational space mission need ? As we all know, there's only so much inbreeding humans can tolerate before they get a bit... Targaryen. On a small spaceship that could be a real problem.

Only with extra toes on each hand and whatnot. It would be bad, is what I'm saying.
Do we really need multi-generational interstellar spaceships at all ? If we keep things within the bounds of current physics and known technologies, then definitely yes. It would take us millennia to reach the nearest star at the speed of our fastest spacecraft to date. There are known ways to speed this up, but none with much of a chance of getting us there within a few generations. So how many people do we need to take along in order for the crew to not inbreed themselves to death ? Can we let people breed with whoever they want, or are we going to need at least some minimal breeding controls ?

Lots of people raise lots of objections at this point. Some suggest alternatives like hibernation, but this is far from proven. Some suggest we just send robotic probes, but these people suck and I don't like them. Others suggest not taking a living crew but just frozen embryos, to be raised by robots upon arrival. Yeah, nice, but you try programming a robot to raise a child and have it not turn out to be a gibbering wreck. Even many human parents can't manage that.

Because that's not creepy in the slightest.
A more sensible approach might be to take a skeleton crew (no, not literally, skeletons are terrible at sex) with just the bare minimum of people and a large fertility bank. That way the population would always have a huge genetic pool to draw on, preventing the need for a gigantic crew to avoid inbreeding. Not a bad idea at all, actually, but not perfect either : the crew would still need strict breeding regulations and embryo (or other genetic material) storage is untested over such a long period. There's never been a population so reliant on artificial insemination. Of course, any multi-generational ship probably would take along as much frozen genetic material as it possibly could, but it still seems like a good idea to establish how small a population could maintain the species the old-fashioned way. We know with certainty that that works.

That's where HERITAGE comes in. This is a code written by my former housemate and prolific author Frédéric Marin. Now, I should warn you that by training Frédéric specialises in studying active galaxies by looking at their X-ray polarisation, which is a littttle bit different from the problems of managing a bunch of horny space Nazis. And I know, I've said many times that being a specialist doesn't mean you get to prattle on about any subject you have a passing interest in. That's true, but :
- It's possible to study multiple fields in great depth
- If you do step outside your specialist field, I never said you should be ignored completely - you're allowed to have hobbies
- If you reach sufficient competency in any field to get into a peer-reviewed journal, you ought to be taken reasonably seriously
- We'd be only too happy to take genuine constructive criticism from people who understand this stuff better than we do
- GOD DAMMIT IT'S A PAPER ABOUT SPACE NAZIS.


Let's simulate the space Nazis then !

HERITAGE is a code designed to simulate the survival of multi-generational crews. It's an agent-based approach, so it has little "objects" representing each crew member. Each one has various attributes, such as gender, age, fertility, and genetic history. That is, the code can track the family tree of each crew member and calculates the degree of inbreeding over time (assuming that the initial generation are all completely unrelated). This then affects the probability of infertility. If too many couples fail to reproduce, then eventually the crew will die off completely. The spaceship itself isn't modelled, except to impose a fixed, arbitrary population cap - a population higher than that is deemed to have failed. What we want to try and get is a nice, steady, small population.

The parameters of the would-be colonists are somewhat random, and this can have important consequences. For example a woman with a high infertility may not produce any children in one simulation, but one with the same level may still reproduce in another. And that affects the next generation in an unpredictable way. So the code runs the mission setups multiple times - 100, to be exact. This accounts for the random variations and gives far more reliable statistics : for example, sometimes missions fail and sometimes they succeed with the exact same starting conditions, so we can quantify the probability of success.

Paper I in this series was a basic introduction to the code presenting some comparisons to previous works. There have been previous claims for the population requirements, though surprisingly few which try to numerically model it. Those that do are in stark disagreement : one favours a few hundred and the other a few thousand or even more.

HERITAGE can be used to set up the initial generation in accordance with the parameters given in the earlier studies. As a first step though, Frédéric decided to just let the crew really go at it, with no constraints on breeding whatsoever. In this promising setup for a space-based orgy that would put the most depraved pornography to shame, the crew don't give a damn about such trivialities as diminishing resources or incest or anything boring like that. Pretty much all they do is make babies.

Which is quite difficult in a space suit.
As you might expect, this worst-case scenario is Bloody Stupid and doesn't work. The initial population of 150 grows rapidly, reaching 5,000 after 200 years* (and that's including a random disaster which wipes out 30% of them early on in the simulation). God knows what it would be after a few millennia but it would be a lot.

* All missions in paper I last 200 years, for consistency with the previous works.

The other scenarios fared a bit better. Both of these imposed some basic, very similar breeding restrictions, e.g. a maximum number of children per woman and/or an allowed age range for procreation*. Both give very similar results in terms of the crew numbers over time : there are still people on board, but they're not terribly healthy or numerous. The smaller crew is pretty much doomed - the amount of inbreeding is rising steadily at the end of the simulation while their numbers are declining. The larger population fares much better, especially in terms of inbreeding, but the crew numbers are still in a worrying decline.

* Which at the rather high end of 35-40 years old means the crew is filled with space milfs.

I seriously wouldn't if I were you. This settlement founded by a handful of mutineers from the Bounty in 1790 is not far off extinction.
Interestingly, none of these scenarios impose any genetic rules on who can breed with who. Rather they only manipulate the initial age distribution of the population, the age at which people are allowed to breed, and the number of children allowed per person. By keeping these ranges narrow, the age of each generation remains very similar. So there might be half the crew being all age around 50 while the other half are all age 25, say, with no-one in between. The motivation behind this was to keep the population from rising exponentially whilst maintaining the maximum genetic diversity available to each generation. That's not how I would have done it, but that's what the other people did.

What this showed was that yes, you could probably manage with a big enough crew, but keeping the breeding rules fixed for all time was a silly idea. Sometimes the crew numbers are quite close to the population cap, so less babies are required. At other times the population numbers are smaller, so more people need to get busy. Limiting the age range at which people can breed is the wrong way to go, as is insisting on a two child per couple rule. As with all things, rules have to take account of varying circumstances.


That's not very Nazi. Be more Nazi.

While paper I tested the code against other people's claims, paper II had a more sophisticated goal : compute the minimum number of people needed for a sustainable population. This time genetics was allowed to determine who could get jiggy with who. And it used a much longer duration for the simulation : 6,300 years, the length of time it would take to reach Alpha Centauri at the fastest speed currently possible. We're talking about a span of time far longer than the ancient empires of Rome and Egypt combined, which is no mean feat. In fact, with fixed breeding rules even a population of 14,000 people isn't enough to survive this long. So we definitely need to vary things !

The first tests didn't have any genetic controls, but just varied the permitted number of children. If the population reaches 90% of the ship's capacity, breeding is prohibited (save for anyone who got pregnant while the population was smaller). When the population falls again, the maximum number of children permitted per couple is 3 so the population can grow again, unlike in the earlier tests when it was only 2. Sounds sensible, right ?

It is, but it doesn't work. The breeding age was initially kept to the same narrow windows as in the previous tests. That's fine at first. But as there are random variations in fertility and breeding ages, over time the age of the different generations starts to slowly disperse : there are plenty of healthy, sexy people on board, but there are fewer and fewer within the narrow permitted breeding age range. With the standard age window of 35-40, the ship dies in a thousand years. An impressive survival time to be sure, but an unnecessary extinction. With 34-40 they make it for 3,000 years. But at 33-40 they surpass a critical threshold and reach a population which is stable indefinitely.
The big drop at ~2,500 years is a random disaster. The recovery afterwards shows that the breeding rules allow a truly stable population to survive for very long periods.
So in fact here it seems that more liberal policies on procreation are what's needed, not more fascist ones. Of course those rules still have to be enforced though, because no rules at all leads to utter disaster, but they're less restrictive than the previous cases.

The problem is that the crew aren't perfectly healthy. They show inbreeding levels, on average equivalent to banging one's first cousin (the code doesn't account for what people would naturally choose to avoid and just pairs up people randomly, so on occasion people go full Lannister). It's not fatal to the health of the crew, but it would probably nice to avoid.

Time to get just a little bit Nazi.

Or as the paper says, "For a purely genetic safety purpose, we will then restrict inbreeding within crew members in the following section." And that's why we have outreach articles.
The code can not only monitor inbreeding, but also actively prevent crew members from reproducing with others who are are too closely related. The more strict the inbreeding, the greater the number of missions which fail because there are too few available breeding partners.

The inherent randomness of the process is very important here. Missions either fail or succeed within the first few centuries. Those where the population reaches the ship's arbitrary capacity (500) inevitably succeed and their population remains stable forever after : the population is so large that inbreeding is relatively easy to avoid. Those which don't are driven to extinction. One particularly nerve-wracking example shows a population which reaches 200, perilously falls back to a mere 100 but then makes a spectacular recovery into a perpetually stable 500.

So the obvious question is : what's the minimum starting population we can get away with ? We can do that by keeping the same (population-dependent) breeding rules and systematically testing each scenario, running it multiple times to determine the success rate. Here's the result :
This is for the case of completely preventing all inbreeding. For a population of less than 32, this is absolutely impossible and the missions always fail. Above this, some missions do succeed. But to be really sure of success you need at least 98 : above this, all missions reach their destination successfully with no inbreeding.

If you were really careful, you could probably get away with less than 98 even if you allowed some inbreeding. The code accounts for inbreeding affecting fertility but not much else : it doesn't do anything really sophisticated like, say, having certain genetic strains be able to counter the negative effects of inbreeding in one's ancestors. And the paper established the minimum starting number, it doesn't actually say the minimum sustainable number - the successful missions are ones in which the population always grows to the permitted maximum. So the sustainable population level at which inbreeding can be prevented is certainly somewhere between 100 and 500, but we don't know the exact value. Still, we can say that you don't need thousands or tens of thousands : you need a small village, not a town.

And you don't need to be that much of a Nazi. You need some breeding rules, but in the end not that much worse than people tend to accept naturally anyway. Sure, you can't get it on with your hot, sexy brother, but if you want to do that you've got deep psychological problems anyway. The most prohibitive restriction is the number of children, but whether people are likely to want to raise huge hordes of screaming brats on a spaceship is quite another matter.

Sorry Hitler, but this spaceship is all about maximising diversity.

Cows in space ?


Popular documentary series Astro Farm explored the issue of farming in space in some detail. For paper III, we didn't even attempt to compete with that level of quality. Apart from some refinements to the code that use more accurate fertility data (which turns out not to affect the main result), this paper looked at how much food we'd need to keep the 500-strong population of Nazi space milfs happy and healthy.

The easiest way would be to feed the crew nothing but sweet potatoes. This crop produces the highest amount of edible energy per farmed area, so this is what you want to keep the required area as small as possible. The code was modified to include the height and weight of each crew member (which vary over time according to medical data), as well as different activity levels which determine how much energy they need. Using only sweet potatoes and aeroponics (the most efficient farming method), a farming area of 0.012 km2 is required - an area about 110 m on a side.

One can only imagine what happens when you take a bunch of pseudo-reverse-Nazis, whack 'em on a spaceship and send them hurtling into the void for 6,300 years with nothing to eat but sweet potatoes. It would probably be better to give them a more varied diet, so we did this using recommended dietary requirements. And yes, that includes cows for milk production.


That raises the required area to about 0.5 km2 - an area 700 metres on a side. Even using conventional farming this only doubles to 1 km2 because most of the area is for the animals rather than crops, and you can't grow cows aeroponically. At least, that procedure is certainly not recommended.

Also, the area allowed per animal was based on data for decent living conditions, not battery farming. Happy space cows FTW !
These numbers seem pretty small, so we did some sanity checks from real-world data. These don't have the benefit of aeroponics or perfect climate control or year-round harvests, yet the numbers are still only a few times what we get. So we're likely in the correct ballpark. Our small but intrepid crew don't need vast areas of farmland to sustain themselves.

Some quick, crude estimates find that in a ship rotating to give Earth-level equivalent gravity, we'd need a cylinder 224 m in radius and 320 m long. But if we allow multiple floors then things get smaller. In a rotating spaceship each floor will have a slightly different level of gravity, getting weaker and weaker towards the centre of the cylinder. If we allow farms on floors to gravity as low as 0.9 g (1 g = Earth gravity, 9.8m/s2), then the ship could be just 106 m long. Or at 0.5 g it would be just 25 m long. We could probably even reduce this more, since plants probably don't care too much about gravity. It's a big structure, to be fair... a  small skyscraper in space. But it's not anything remotely like as ambitious as the gargantuan O'Neil cylinders proposed back in the 1970's.

Such a ship would be nice and all, but massive overkill.

Summary

What you need for a minimum viable interstellar mission is about a hundred people who don't mind the government intervening in the bedroom and a pretty large spaceship. What we haven't yet established is the really important figure : the minimum mass. Mass dominates everything in space exploration because of the ferocious cost of launching anything out of Earth's deep gravity well. Even with the reductions coming from resusable rockets, the cost isn't like to fall to the point where we could even contemplate an interstellar mission anytime soon.

And let's not kid ourselves : there's an awful lot more work to be done to establish a realistic minimum mass. We need to account for shielding to protect the crew from cosmic rays, for a plausible engine design and the mass of propellant (or some other technique) needed to get the ship to a useful velocity. We need habitation space for the crew, unless we want them to share the stables with the cows. We need water, which is likely to be the topic of the next paper. And we need to account for how efficiently biowaste can be recycled, otherwise our farms would be no better than taking on a whole load of stored food. We made the implicit assumption here that waste can be recyled with 100% efficiency, so the mass of the starting crops is all that's needed. The opposite extreme case of zero recycling demands a food mass of many millions of tonnes, which would be extremely annoying.

More difficult, perhaps, would be how to ensure the crew don't go stark raving mad after six millennia floating in their tin can. No-one has ever designed a society able to manage that level of stability before. With such a small population, warfare is simply intolerable - the risk of complete extinction too great. How the hell we could manage that, I don't know.

But to end with a positive note, we've shown that even with near-term technologies, an interstellar mission isn't totally outlandish. It is, based on what we know so far, at least possible. That is only a first step, but an extremely important and necessary one. For a project like this, one has to accept that we can't tackle all the issues at once. Instead we have to chip away at the whole massive, complex edifice bit by bit. Our ship full of pseudo-reverse-Nazi-milf-space farmers is just the start.

Though the next step probably won't be to add dinosaurs, unless anyone out there can think of a good excuse... ?

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