Interstellar Flight: The Generation Ship

I talked about hibernation previously - now let's talk about simply slogging it to the stars the hard way, no sleeping on the job. Many SF writers talk about generation ships as a way to get human colonists to a distant star. But there's a couple of problems with generation ships which don't make them so attractive.

First off is the sheer size of these things. In order to get a minimum viable population without the hazards of inbreeding, you'll need something like 180 individuals. Small groups have made it to colonise islands, perhaps 50 or less breeding members. A single female even could simply impregnate with frozen herself and raise her daughter to fly the ship (not that her daughter would likely be grateful for the thankless life she's born into).

The trouble is, we need a lot of people to cover all the scientific and engineering disciplines. Small armies of technicians, engineers and scientists service the space shuttle, for example. The 7 or so people on board really just push the buttons, as competent as they are. Fixing complex problems like those that would arise on a spacecraft require lots of smart brains. And there's no guarantee that subsequent generations would be as smart or motivated as their astronaut parents. The automated systems on the ship had better be pretty reliable or else extremely easy to fix. Possibly even to the point of having an AI or expert system and be almost self-repairing (can you say HAL 9000?) One might envision a generation ship with primitive humans, having lost the skills of their ancestors, worshipping the benevolent computer-deity literally controlling their world. This, plus the possibility of disease or accidents wiping out large chunks of crew, points to the need to have as large a crew as possible. And more humans equals more mass.

Mass requirements for keeping a human alive in space, and fed with soyburger and supplied with toilet paper, range from 100 tonnes to 1000s of tonnes. Biotechnology can really help here; improving crop yields and increasing efficiency of recycling systems. Certain tools, chemicals and medicines could also be grown in bioreactors. The entire ship (or at least the habitat) could be constructed of organic materials. This pretty good from a radiation shielding standpoint, the abundant hydrogen atoms in organics and plastics are great for stopping cosmic radiation and preventing the lethal backscatter of secondary radiation that occurs when a speeding iron ion smashes into structural aluminium.

In addition to the mass requirement, there's also volume. NASA studies estimate that 100 cubic metres is enough for a single human for an indefinite period. I rather think the ship's crew may grow up a bit nutty... anyway, with the life support requirements, rather more volume than that is likely. Inflatables seem the current best technology, but just how safe will they be after decades of hard radiation exposure? Perhaps they will need some extra reinforcement for a more permanent solution, but at the moment they look like a good bet.

SF is replete with stories of hollowed-out asteroids as generation ships, but the truth is that they're terrible spaceship hulls. For a start, they're weak - asteroids being composed of rubble, and would need the rock to be fused. The rock would still be fundamentally very weak for its mass. And that mass would weigh in the millions of tonnes for a generation ship a kilometre or so across. Furthermore, the rock is not such a good radiation shield. Plain old plastic, water or wood is better for cosmic radiation. And radiation shielding wants minimum volume to be used most effectively - generation ships are anything but minimum volume.

Hollowed-out comets or ice asteroids seem to be a somewhat better option - although frigging cold, they would provide water, oxygen and reaction mass. A layer of insulation could allow for an inner shell lined with water, and aquaculture. Everybody living in boats and stilt houses - how very appropriate for the island-in-space theme! Of course, the best option is still to purpose-build an actual hull for the job. And that's going to be heavy.

Assuming the minimum case of 180 people (at any one time), the ship needs to weigh at least 18 000 tonnes if 100 tonnes of life support infrastructure are necessary to keep things going - that's a WWII battlecruiser. Being more conservative, that could mean 500 000 tonnes of ship for 500 people and 1000 tonnes of life support. To match the current mass of the ISS at 400 tonnes, we would need something like 8 tonnes of life support for 50 people (and no idea of how they're going to go down to the surface). That's a pretty miserable cramped existence, eating algae glop for hundreds of years, living naked and escaping to VR all the time. But it might be possible.

The point is, at what point is it simpler to put more fuel up to go faster? With a solar sail, you've no choice - you're limited to 0.001c with a scorching approach. But fusion-powered craft might just prefer to burn more fuel and get there faster rather than build a big expensive habitat and risk the crew dancing around fires playing bongo drums when they should be getting in their landers to go the surface. Reducing the mass from 18 000 tonnes to just 180 yields a 100x jump in mass ratio, which can be cashed in for a 2.4x increase in speed over just having a 10x mass ratio (rough approximation). Instead of getting to Alpha Centauri in 200 years, a trip of 83 years becomes possible. The crew might be old duffers by the time they get there, but they can limit their numbers because they don't have to keep a society going on board. Or they could prolong their lives with life extension drugs.

Life extension also poses its own problems. While it's useful for long voyages where you want to actually live to see the target star and use your expertise instead of teaching it to your children and hoping they teach it to the next generation, it's a problem on a generation ship. Even with people sticking to two children per couple (or one child to one parent in polyamorous Heinlein-esque communal love-fests), room's going to run out real fast. Great-great-grandpa and grandma may have to be euthanised.... or their children only allowed to breed when their parents die. Which is a problem is females can only safely breed up to about 40. There better be some serious mojo in those pills if that's going to be the case.

Speaking of kids... can you imagine what a 2 year old would do in a delicate, tightly enclosed environment? Or a sulky teenager? Best just feed the crew contraceptives and boosterspice* til they get to their new Eden...



*anti-aging drug in Larry Niven's known space novels

Interstellar Exploration - Two Motivations

Chinese junks, similar to what Zheng He's fleet may have looked like. Credit: Wikimedia

Exploration is rarely about the pure, unfettered pursuit of knowledge. Scientists want to prove a theory. Politicians want to see high-tech industry stimulated and their national prestige elevated. Private citizens want to go "because it's there." History has shown that there are two broad kinds of exploration - with their own separate outcomes. The first is the show of force. The second is sustained interest.

1405 saw the launch of Zheng He's expedition of exploration from China. A massive flotilla of 317 ships and 27 000 crewmembers, with 44 huge treasure ships measuring 120-150 metres in length. Or, accounting for typical historical exaggeration, probably half that as 100 metres is the limits of what is possible with wooden ships (note that no remains of these craft have ever been found).

Christopher Columbus had 3 ships, 23 metres long, and 270 men (before the usual diseases started wiping them out). The only thing he had over Zhang He was the fact that he didn't sing soprano (Zhang He was a eunuch).

The Chinese weren't interested in colonisation or trade. They wanted to show off, impress people with their bling (hence the treasure ships) or else kick them around a bit. They wanted to remind people that China, just like every other Empire in the world, was the centre of the world. It was the Apollo project of the era, in more ways than one. Because after a flurry of these expeditions, the whole thing was called off due to escalating military conflicts and finally a big damn wall to keep the Mongols out.

Christopher Columbus of course, had different motivations. He wanted money by opening up an alternate trade route to Asia. And he didn't make such a big investment - three ships, perhaps a big thing in Medieval Europe. Although everybody thinks of him as a big success because he found this place called America, he failed to find the trade route. He wasn't even out to prove the world was round, everybody knew that.

Now, how did the New World wind up colonised? Why didn't the Chinese do it? Europe was a dirty little backwater, although the first glimmerings of scientific enquiry were beginning. Simple: China wasn't interested. China could have easily afforded to colonise and conquer these new lands. The Europeans were broke, and constantly fighting each other. But there was land to exploit, and savages to convert! There was also somewhere to run away to if you didn't fancy being oppressed!

So, we have two scenarios with which to place interstellar colonisation in. The first is the massive show of force, done mainly to impress and sustained by government interest. The second is a trickle which eventually becomes a flood, sustained by continued private interest (Hey Sven! Could I interest you in a place called Greenland!). This can tell us something about what kind of colonies we can expect, and who's going to found them.

Interstellar Flight: Hibernation

So let's say we've discovered a nice habitable planet around those nice, nearby sun-like and well-behaved stars, Alpha Centauri A or B. How are we going to get there? As it stands, there are three possible ways of getting there. Obviously it's going to take a long time.


The simplest in engineering terms is hibernation (aka "suspended animation"). This has the very important engineering advantage of not needing all the mass of a big habitat or a closed ecology. The ship can go faster or be smaller (and cheaper) as a result. Throw the humans in the hibernation chambers and let them sleep the journey away. Plus they won't go stir crazy or forget how to do their jobs. However, there are significant challenges: we don't really have the first clue as how to induce hibernation in humans.

Short-term hibernation is possible through induced (or accidental) hypothermia. Unfortunately it's dangerous - Swedish radiologist Anna Bågenholm was dunked under ice with a core body temp of 13.7°C. A drop below 28°C is often fatal. However - this hypothermia-induced hibernation can be extended for long periods. Mitsutaka Uchikoshi, a Japanese skiier, went missing and was recovered 24 days later with his organs shut down and his body temperature at 22°C. What was particularly amazing was how he survived for so long without any fluid intake.


Hypothermia can also be induced for medical reasons (with much better prospects of surival as it's controlled): English and Japanese doctors pioneered the technique of using deliberate hypothermia for heart surgery with packs of ice, and the Russians perfected the technique (the normal solution these days is to use a heart-lung machine). Basically the brain chilled to about 16°C (60°F) and the body to 24°C (75°F). This gives the doctors a 30 to 60 minute window where the heart stops and they can tinker with it, then whip the body back up to full temperature with no lasting brain damage. A recent discovery with mice showed that hydrogen sulfide combined with hypothermia could be used to induce hibernation in mice - but only mice. It probably won't be as simple with humans, but the US military is certainly interested in it for trauma applications. Extreme hypoxia might also be a trigger.

Unfortunately, humans just aren't built for hibernation; this technique must be done correctly otherwise there is severe risk of cardiac arrest. More so is the problem of simply lying inert for years on end. Wouldn't the body degenerate? What crucial processes might be impaired? It might be necessary to wake the sleepers up every year or so to let them recover, before climbing back in the freezer again - with significant impacts on life support requirements.

For really long term flights, it might be necessary to go all the way and use cryogenic preservation - ie freezing the astronaut solid and then thawing him/her out. Unfortunately, this is kind of problematic - small embryos and organs can be successfully quick-frozen, but damage from expanding ice crystals (especially during re-warming) causes all kinds of havoc. A way around this is vitrification - basically pumping the body with antifreeze, turning the body into a block of glass at -135°C. This unfortunately creates further problems in the form of the toxic antifreeze now flushing the body.

A vitrified rabbit kidney being thawed - notice the lack of frost and ice.

Although reviving the patients is tricky for both cryogenic preservation and hibernation, perhaps AI will have advanced enough for it to handle the process without having to have someone on hand. Continual waking and thawing may not be a good idea - so perhaps some brave souls will volunteer to spend a significant chunk of their lives in extreme boredom watching over their crewmates. It may also be possible to grow whole new bodies around the vitrified brains, with the ship becoming more like the mass (and cost) of an unmanned probe. Once it arrives in the system, it mines sufficient material to build a habitat and create nutrients to grow the crews' bodies. However, this is getting to the real bleeding edge of the possible - FTL may be possible before then.

The most likely scenario - hibernation through some combination of induced hypothermia, hypoxia and drugs, seems feasible enough that we could conceive of an interstellar mission if the propulsion technology is likewise developed (which seems to be the far more difficult problem). Since hibernation has many more applications than just long-duration spaceflight, it'll probably be developed sooner rather than later.

The hibernation "pods" would probably be some small climate-controlled chambers with a comfy bed, IV drip and some sensors, hardly weighing anything at all. More like the cheap-looking hibernation pods in the original Planet of the Apes than big bulky cryo-coffins. They'd be located right in the heart of the ship, surrounded by as much radiation shielding as possible, and with a bit of rotation to keep fluids behaving properly. The quarters for the awake crew would probably be right next to it, to take advantage of the proximity to the sleep chambers' radiation shielding. Upon arrival, the sleep chambers could be used as bunks for the crew.

Reliability of the hibernation mechanisms would be a big issue - springs, one of the most reliable components, rarely last beyond 60 years. New, highly fatigue tolerant materials need to be designed, along with fault tolerant systems - more so than with a ship where everybody's awake and able to fix things. This in itself is a significant challenge, but the aerospace industry tries to reduce maintenance and extend aircraft life to lower operating costs, so this is another area where industry may make the development anyway.

Appreciating What We Have Already

Sometimes we forget we're living in really amazing times. I saw this over on Wayne Hales' blog and shamelessly stole it. It looks like the cover of an SF book or a still from an SF movie, but it's not. It's astronaut Tracy Caldwell-Dyson looking out of the ISS cupola. Here's the full-sized original from wikimedia.

Wayne Hale talked about how images like this would inspire the public. I was personally wondering how she managed to look like she was lying down when she was in zero g, and whether her elbow was going to smudge the optically perfect glass.

For me, the constant SF eye candy can never dull the awesome reality of knowing that someone up there is seeing something like this right now. We have a 300 tonne space station, 3 different unmanned cargo vehicles, a space plane headed for retirement and the prospect of 3 or 4 new craft to replace it. Add to that the prospect of a private space station before the end of the decade, the Russians and Chinese possibly building their own and it gives you hope for the future. Yeah, space isn't happening as quickly as we would like but boy, what we have already boggles the mind.

If you've got the time (and bandwidth) why not watch a tour of the ISS from Youtube and get a taste of what it's like out there.