Thursday, December 17, 2015

Surviving a trip to the stars

kw: book reviews, science fiction, space fiction, starships

Here's a conundrum. You want several generations of people to live aboard a starship or space station; to remain healthy, they need gravitation, or the semblance of it via rotation. In a small vessel, Coriolis effects when people move about are downright disturbing, so you want the rotating diameter to be large, the larger the better. How large can you go?

In various SciFi stories I've read of can-shaped craft about 2 km in diameter, rotating on the long axis, and also of various kinds of ring-shaped craft. In Aurora by Kim Stanley Robinson, the starship has two rings with a common hub (the "spine"), so it is a nice double ring, and the diameter is 15.3 km. It rotates to provide 0.83g, to prepare the inhabitants for the gravity they will find on their target world in the Tau Ceti system. However, in the second half of the novel (spoiler alert), one of the rings and half the spine is detached and returns to the Solar System, and on the return journey the rotation is increased to provide 1.1g. This is a decision by the ship's AI systems, because of the expectation of great debility among the returnees, so they won't be wholly disabled by Earth's 1g field. A kind of suspended animation is used because food has run out, and they'll arrive weakened and starving.

What are the stresses holding a large, rotating ring together? I won't go into the whole analysis. Suffice it to say that the fiber stress increases linearly with diameter for a specific g force. This is because a ring twice the diameter will have twice the mass, but is held together with exactly the same cross-sectional area. I remember calculating years ago that a steel hull could be no larger than about 2 km in diameter, and just barely hold itself together. If you want to have dirt and people and buildings inside, you need to make it smaller so the steel can support the extra mass.

However, we have better materials. Kevlar is almost twice as strong as steel, and weighs a quarter as much. So a Kevlar hull with a diameter as great as 21 km could just barely hold together. Make it only half as large, and it can then support internal stuff equal to the mass of the hull. Then we have carbon fibers, which are more than 1.5 times as strong as Kevlar, though they are a little denser. A carbon-fiber hull could be as large as 27 km. I am told that carbon nanotubes are a great deal stronger than this, but nobody knows how to make them kilometers long, and the glue in a matrix holding a bunch of them in an overlapping configuration isn't strong enough to permit their full strength to be employed.

Maybe they will have solved that dilemma by the 25th or 26th Century, when the ship to Tau Ceti is sent out. If not, carbon fibers are a pretty good material, as long as they aren't degraded by space grit during a decades-long flight at 0.1c (some 67 million mph or 108 million kph). The way author Robinson gets around the abrasion conundrum is by having some sort of magnetic shielding field warding off anything smaller than a few millimeters, and radar and avoidance taking care of the rest.

Of much more interest to the author, and even readers as nerdy as I am, are the biological, chemical and social situations in an ecosystem that remains closed for, eventually, 170 years, and is later re-closed for a further 180+ years. A super-engineer named Devi worries constantly about the future of the colonists—and their animals—who are smaller and on average less capable than their ancestors of 6 generations previously. But no reason is given for why her daughter Freya becomes the tallest person aboard, at 2.02 m (6'-7.5"). Devi is also faced with the gradual unbalance of various nutrients and other elements in the ecosystem. Phosphorus, for example, is gradually getting bound into insoluble minerals from which it is increasingly costly to re-extract. Crop yields are falling. Late in the voyage, certain bacteria are found in hidden globs of water in low-and no-gravity spaces along the spine, where they are degrading various materials, threatening the physical integrity of the ship's systems, and maybe even of its hull.

Then we come to the people. No matter how committed, emotionally balanced, and perhaps politically uniform the original generation might have been, genes rearrange themselves with every generation, and there is no predicting what mix of traits might dominate after several generations.

The term "island biogeography" is used a few times. There is an important aspect of a small, isolated population that is worth exploring a little. Suppose you've managed to gather an extremely diverse collection of a 2-3 thousand people, such that the maximum variety of favorable alleles of the human gene set present in the founder population. They pair up and have two children per couple (allowing for accidents, the "replacement rate" is historically 2.1). Each child has a random assemblage of exactly half the alleles of each parent. The two children of each couple will carry forward close to 75% of the total genetic variation found in their parents. If the parents somehow had totally unmatched allele sets, then some 25% of these will be lost to future generations. The statistics get remarkably harder to determine with more realistic mixes of alleles. But some numerical experiments I've done, kind of a Monte Carlo simulation, indicate that after 4 generations the remaining gene set is about 67% of the original. Also, a few genes, quite at random, have increased their representation among that generation by a factor of 3 or 4, while a larger number are about twice as prevalent, and the rest are about as common as ever. Let's hope the genes growing towards dominance are going to make future generations more healthy. Since there is an artificial cap on conceptions and births, natural selection only appears in the form of miscarriages or stillbirths, though I suppose some children will be deemed too flawed to be allowed to reproduce, and a few "lucky" couples get to produce an extra child to compensate.

That "cap" produces the greatest source of suppressed resentment: you have to have a strict totalitarian system, rigidly enforced in certain areas, or the mission will fail. You can't run such a small, closed ecosystem as a democracy. Midway through the book political polarization leads to violence and warfare. Robinson is ready for one aspect of this: an earlier generation that experienced in-ship conflict wisely programmed all manufacturing systems that, if someone tries to produce a firearm or projectile weapon of any design whatever, it will explode and dismember or otherwise damage whoever tries to use it. So warfare is carried out by throwing things and making stabbing weapons and clubs.

One message of this book is that faraway planets could be more dangerous than we imagine. The toxic, high-energy chemicals in the soils of Mars, chemicals that cannot be produced in quantity on a living planet, are one example used to illustrate that "terraforming" an alien planet may be many hundreds of times more difficult than we'd like to think. Authors galore have imagined terraforming, carried out in a quasi-human time frame of 50-200 years, but Robinson's characters find themselves discussing the need to keep their ship working for several thousand years while their remote descendants turn the planet into something that won't kill on contact. That's why a third of them return to Earth.

Inorganic-chemical troubles are just the beginning. A kind of super-prion is the stated cause for the original planetary body to be abandoned. The place had been found to have lots of oxygen in its atmosphere, but this was considered "abiotic". A mistake, it seems. An oxidizing atmosphere is so out of balance it cannot arise inorganically. Better to go to a planet or moon with an atmosphere in near-equilibrium, so you can be sure no life has taken hold. Any life of any kind on an alien planet is likely to be much too toxic for us to survive there.

The discussion of the closing years of the return, when the ship must calculate all kinds of gravity-looping billiards through the solar system (the "catcher" laser system gets a late start and can't slow down the ship all the way), goes on for too long for my taste. It is dramatic, and I suppose Robinson had some kind of simulation software to set up the planetary passes…or maybe he just put it all together of whole cloth. It is a piece of great writing, quite gripping, but wore me out.

There is a curious oversight in a couple of places. In the midst of a discussion of river deltas and braided streams in one place, and the v-shaped formations seen in beach wash, a character muses that perhaps this is the origin of the term "delta-v". Anyone with a glancing acquaintance with physics knows that the "v" here refers to velocity, and delta-v means change in velocity. When you want to maneuver from asteroid to asteroid, for example, attaining the right delta-v while using minimum fuel (encapsulated in the term "specific impulse" for straight-line acceleration, but lots more complicated otherwise) is the most critical consideration. Such considerations led to the Apollo astronauts receiving training in jeeps they drove in circles in the New Mexico sands, learning the tricks of docking satellites without running out of fuel. In another place, the use of GPS navigation on the Tau Ceti planet's moon is mentioned, with no indication that a set of GPS satellites was deployed. I found that a bit jarring. As long as I'm in error-checking mode, there's one more: on p 443 the neap tide is stated as the larger. Not so, the larger tide is the spring tide, which occurs when sun and moon are either at conjunction (new moon) or opposition (full moon). Neap tides occur at the first and third quarters, and are much smaller.

Enough negative blather. It's a great tale. Robinson's writing is a tremendous pleasure to read. The book is full of interesting ideas and discusses a great many things we need to consider when planning to colonize any bit of "outer space". A very fun read!

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