= ASTRONAUTICAL EVOLUTION =

Interstellar Travel and Straw Men

A gloomy prognosis

A bizarre article appeared on the Scientific American website on 13 January under the byline of well-known science fiction author Kim Stanley Robinson, entitled What Will It Take for Humans to Colonize the Milky Way? Bizarre, because it shows a failure of imagination from someone whose imagination is his main professional skill, contains factual errors, and discusses only the Earth-to-Earthlike-exoplanet model of interstellar travel despite the fact that the literature, going back to the 1984-1985 worldship papers in the Journal of the British Interplanetary Society, shows this to be a straw man.

Robinson is notorious for his novel Aurora, published last year and portraying a dysfunctional multi-generation interstellar worldship. Listen to the author talking about his novel on YouTube.

While the question in the title of Robinson’s Scientific American article is a perfectly fair one, Robinson is initially more interested in pouring cold water on the whole idea of manned interstellar spaceflight: “things we have learned about the universe and ourselves combine to suggest that moving out into the galaxy may not be humanity’s destiny after all”.

The distances between the stars are, obviously, mind-bogglingly vast. But not as vast as Robinson claims. Tau Ceti is the target star in his novel, with a distance of around 12 light years from the Sun. Robinson interprets this for Scientific American readers as “100 billion times farther from Earth than our moon”. A little simple arithmetic shows that the true value is 300 million times the lunar distance, a factor of over 300 less than he states.

Wisely, Robinson avoids giving any further numbers, and uses only qualitative arguments. Instead of a spaceship, we would need an interstellar ark (true). The ark would need to be small enough to accelerate to high speed, but large enough to carry a fully recycling ecological system: “The design imperatives for bigness and smallness may cross each other, leaving any viable craft in a non-existent middle” (speculative and not backed up by any detailed calculation).

He states (and again in the YouTube video) that the bigger the ark is, the proportionally more fuel it would have to carry: “this is a vicious circle that can’t be squared”. Maybe it would be a vicious circle if it were true, but it is not. A large ship would need the same proportion of fuel (the same mass ratio of initial to final mass) as a small ship for the same performance engine and the same trajectory.

He allows that humans are “highly adaptable”, but finds that the social system on board his ark would have to resemble a totalitarian state. On top of which, “Add to these social constraints permanent enclosure, exile from the planetary surface we evolved on, and the probability of health problems, and the possibility for psychological difficulties and mental illnesses seems quite high. Over several generations, it’s hard to imagine any such society staying stable.”

Did a science fiction author just utter the words “it’s hard to imagine”? Well, if his imagination is not up to the job, there are plenty of other authors of speculative fiction who can achieve that intellectual feat. Particularly when they consider that, to our ancestors living before the neolithic revolution, our present-day jungle of social, financial and legal constraints, confinement for most of our lives to the enclosed spaces of houses, shops, factories, offices and transport vehicles, exile from the African savannah we evolved in and exposure to the diseases caused by living in close quarters with millions of other people would have seemed terrifying. They would have found it hard to imagine any city-dwelling society staying stable.

After discussing the biological and social problems on board, Robinson talks about the difficulties of occupying an Earthlike planet at the destination as if no other option existed. His conclusion: “All the problems together create not an outright impossibility, but a project of extreme difficulty, with very poor chances of success.”

His answer to the question posed in the title consists of three preconditions for interstellar voyaging: (1) a sustainable civilisation on Earth itself, (2) a practice ark orbiting the Sun to provide a proof of concept, and (3) extensive robotic exploration of nearby stars to see if any of their planets are suitable candidates for human habitation.

What it would really take

Robinson’s assessment is based on the unstated assumption that humans can only realistically inhabit Earth and closely Earthlike planets (such as Mars, the subject of his famous Mars trilogy). The voyage must therefore begin from Earth, and must end at an Earth-analogue world orbiting another star (or at an Earth-analogue satellite of a giant exoplanet as in Aurora, and of course in James Cameron’s Avatar and Buzz Aldrin’s Tiber). The target world is only usable if it allows shirtsleeve surface habitation, or can be terraformed into such a state.

If this assumption is correct, then no manned interstellar voyaging will be possible at all. The project would be simply too great for a one-planet civilisation to attempt (see my technical papers on the subject in JBIS here and here).

Robinson is apparently not acquainted with the astronautical classics such as Gerard O’Neill’s The High Frontier, which is famous for making the point that the surface of a planet is not the best place to locate a technological civilisation, or John S. Lewis’s Mining the Sky, which details the resources available in the asteroid belt.

What Robinson is offering us in this Scientific American post is basically the Wright brothers, circa 1904, stating that transatlantic air travel is a project of extreme difficulty with very poor chances of success, and proving this by referring to the capabilities of the Wright Flyer. But they do allow that it might one day be possible, if a second Wright Flyer was to be built before the final, transatlantic one, in order to fly it around the east coast of America to test it and prove the concept.

Obviously, what actually happened was that, by the time people were ready to attempt the Atlantic crossing by aeroplane, thousands of aircraft were in daily use, and many generations removed from the original Wright Flyer.

Similarly, it should be equally obvious that, by the time people are ready to attempt the first interstellar crossing by worldship, thousands of space colonies with worldship-type living environments, and thousands of interplanetary vehicles using high-power propulsion, will be widespread throughout the Solar System. The problems which Robinson raises will have been addressed over centuries of gradual extraterrestrial development involving billions of people. And in order to grow the Solar System economy to the point that interstellar voyages will be affordable, an extraterrestrial population very much larger than Earth’s will be needed; larger even than can realistically be accommodated on all the terrestrial surfaces of the Solar System with lunar gravity or better (Earth, Moon, Mars, Venus, Ganymede, Callisto, Titan).

An interstellar worldship is basically an extreme space colony: to the majority of its occupants, it will be a world for living in, not a vehicle at all. Such a thing can only be built after a long process of gradual evolution of human capabilities for living in space at locations progressively more remote from Earth. There will be colonies in the asteroid belt, the Jupiter trojans, the rings of Saturn, the Centaurs, even the Kuiper belt.

But this also means that the travellers are released from any dependence on finding an Earth-analogue destination planet: an asteroid belt or collection of small moons will be preferable, as these can supply the raw materials to build more of the type of accommodation which they are used to and which they consider most natural.

The voyage can then be to almost any nearby main-sequence star, not to the minority of stars with an Earth-analogue planet. The journey will therefore be shorter: Alpha Centauri, for example, is only 37 per cent of Tau Ceti’s distance from the Sun, and very probably an even smaller fraction of the distance to the nearest star with an acceptably Earth-analogue exoplanet or exosatellite.

Robinson’s propulsion circle can be squared without resorting to fantasy physics: thanks to the Daedalus Report from 1978 (now republished by the British Interplanetary Society) we know that nuclear fusion is perfectly capable of giving cruising speeds of a few per cent of the speed of light, and hence crossing times of a few centuries for the nearest stars and several centuries to more distant ones, such as Tau Ceti (say 600 years cruising at 2 per cent of light speed).

Any exoplanet truly resembling Earth, thus with its own biosphere, will therefore be more valuable as an object of non-invasive scientific study than as a target for colonisation, removing one of the common popular objections to interstellar travel.

A ship with a dry mass of one million tonnes (less than the mass of Robinson’s ship; see the analysis by Stephen Baxter and others on Centauri Dreams) for around a thousand passengers, boosted to 2 per cent of light speed, and decelerated at the destination, using an efficient nuclear fusion rocket engine (with exhaust velocity 7500 km/s, less than that calculated for Daedalus) would have an energy budget of about 100 ZJ (one zettajoule = 10²¹ J), thus equivalent to current global industrial energy consumption (~ 0.5 ZJ per year) extended over a period of almost 200 years.

While this is way out of reach for any plausible Earth-based society (and even more so if the estimated dry mass of Robinson’s ship of 18 million tonnes is used instead, and if the speed is increased to his 0.1c), a wealthy Solar System civilisation, the large majority of whose members live in space colony conditions very similar to those of the worldship itself, could possess an economy half a dozen orders of magnitude greater than today’s before coming anywhere close to the Solar System’s carrying capacity, in addition to enjoying easy familiarity with the necessary technologies and social structures applicable to interstellar spaceflight.

Robinson concludes that the preparation for interstellar spaceflight will itself be a multi-century project, and will rely crucially on a prosperous and sustainable civilisation on Earth. On these points I am able to heartily agree with him! But ironically, despite his intention to emphasise the difficulty of the preparation, I think he has underestimated the magnitude of the preparatory development that will actually be needed, as well as its chances of successful extension to an interstellar civilisation.

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