Saturday, January 29, 2011

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

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