All Astronautical Evolution posts in 2011:

The battle for the future (Dec.)

The personal satellite / Olaf Stapledon / Europe needs a strategy (Nov.)

100 Year Starship Symposium / BIS newsletter Odyssey (Oct.)

Development Roadmap for the Worldship (Sept.)

Tumlinson’s challenge for NewSpace / SRI calls for global Apollo day / Poem celebrates Apollo (Aug.)

Dear Mr Dordain… / Mat Irvine: From Imagination to Reality (July)

Society between cosmic growth and utopian dreams / UK Space Conference (June)

Available in any colour so long as it’s black / UK Space Conference and Sir Arthur Clarke awards / David Baker slaying the Space Age myths / Getting involved in the BIS / Cartoon: can Spirit come home now? (May)

Mankind in space: the next fifty years / What future for intelligent life in space? (April)

Does evolution progress? The multi-shell model (March)

Social software for a spacefaring civilisation / Best of the blogs / Space cartoon of the month (Feb.)

2010: Manned spaceflight at the crossroads / 2011: Smoke signals from two dragons (Jan.)

New in 2015:

Short story The Marchioness

AE posts:

2017: Mars…

2016: Stragegic goal for manned spaceflight…

2015: The Pluto Controversy, Mars, SETI…

2014: Skylon, the Great Space Debate, exponential growth, the Fermi “paradox”…

2013: Manned spaceflight, sustainability, the Singularity, Voyager 1, philosophy, ET…

2012: Bulgakov vs. Clarke, starships, the Doomsday Argument…

2011: Manned spaceflight, evolution, worldships, battle for the future…

2010: Views on progress, the Great Sociology Dust-Up…

Chronological index

Subject index

General essays:

Index to essaysincluding:

Talk presented to students at the International Space University, May 2016

Basic concepts of Astronautical Evolution

Options for Growth and Sustainability

Mars on the Interstellar Roadmap (2015)

The Great Sociology Debate (2011)

Building Selenopolis (2008)


Issue 73, 4 September 2011 – 42nd Apollo Anniversary Year

All content is by Stephen Ashworth, Oxford, UK,
unless attributed to a different signed author.

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A Development Roadmap for the Worldship

Stephen Ashworth

Presented at the BIS Worldships Symposium, 17 August 2011

The focus of my thinking on worldships is an attempt to achieve a broad overview of the whole problem of manned interstellar flight. This is not easy because there are enormous uncertainties about how the future will work out. It would be nice to set out a simple roadmap of half a dozen milestones that we need to pass in order to get from here to the construction of the first worldship, but such a roadmap, if it is honest, quickly branches out into a number of divergent possible scenarios.

The author's Wayland starship concept

As a highly relevant example of the sort of uncertainties we face, take nuclear energy. The original nuclear submarine, the USS Nautilus, first put to sea under nuclear fission power in January 1955, only 12 years after the first artificial chain reaction had been demonstrated in a Chicago squash court (2 December 1942). Research into controlled nuclear fusion, however, began in 1950 with Andrei Sakharov’s design for a tokamak and 1951 with Lyman Spitzer’s stellarator, but now, six decades later ... who could have predicted then that controlled fusion would be so hard to master?

I want to talk a little about three questions which lie squarely on the worldship roadmap, and which may help to clarify the situation:

  1. Will the human species become extinct before we even have a chance to build a worldship?
  2. Do we really need to find an Earth-like exoplanet orbiting another star?
  3. How might developing capabilities in life support in space and in propulsion lead to worldships?

* * *

So to begin with, a potential roadblock needs to be cleared out of the way. Might continuing development of technologies, including those needed for spaceflight, cause the human race to become extinct before we ever reach the stars?

Computer processing power and memory size are currently experiencing a phase of exponential growth. So the view has arisen that when computers surpass the human brain in some characteristic such as number of floating-point operations per second, they will become more intelligent than us, and their robotic incarnations will be capable of continued exponential self-improvement, will compete with biological humans for resources, and will drive us to a marginal existence, if not outright extinction.

The Cybermen are coming!

Bill Joy was one of the founders of Sun Microsystems. In 2000, Wired magazine published an article by Joy expressing his fears that some permutation of genetics, robotics and nanotech would cause a technological apocalypse sometime around 2030, and eventually destroy us. Together with people like Ray Kurzweil and Hans Moravec, he foresaw the possibility that the human species might be replaced by computer-controlled robots. He pointed to the presumed capacity of these incipient technologies for independent and uncontrollable self-replication, and to the additional likelihood that a psychotic individual might be able to access them for destructive purposes.

More recently, SETI astronomer Seth Shostak has applied this idea to his own field. He speculated that biological intelligence is only a brief intermediate stage in the gestation of real intelligence: machinehood. Thus we are not likely to find any alien civilisations comparable with our own elsewhere in the Galaxy. They have all evolved in the space of a few brief centuries into robot societies.

Clearly this notion strikes at the heart of our presumption that interstellar travel might carry biological human beings to the stars. But presenting a single speculative scenario as if it were fact is a job for science fiction writers, not for scientists. What are the alternatives?

First, we need to set aside the poorly defined term “intelligence”: the issue is not whether computers will attain consciousness or artificial intelligence, but about whether they will start to think of themselves as separate from humans and take decisions in such a way that their interests come into conflict with ours. Thus they may say: we have decided that the obnoxious human rabble shall be confined to the ghettos of planet Earth – and have the power to enforce that decision!

In order for this to happen, the machines have to be aware of themselves as distinct from humans – a separate “robot species”, as Bill Joy puts it. What he does not say is that, just as computers are becoming exponentially more powerful, the man-machine interface is also becoming more intimate. Many of us remember a time when communication with a computer was carried out using a stack of punched cards; then it was keyboard and mouse; now we have a touch pad; and the cutting-edge is sensing a user’s eye movements and even their brainwaves. Meanwhile, computers spend all their time processing, sifting and reorganising human-generated content, from Facebook, Twitter, YouTube, the BIS website, and a myriad of other sources.

In other words, the trend is not towards digital intelligence emerging from the products of biological toolmaking as an irresistable competitor. Rather what we are seeing is the increasing integration of our biological brains with digital computers, in the context of an increasingly integrated global economy of people, ecosystems, machines, and flows of information, energy, goods and money.

The end-of-humanity scenario which Joy, Shostak and others propose is a possible future for mankind – or rather, a possible absence of future. But it is not the most plausible scenario, because computers are not likely ever to consider themselves separate from humanity. In just the same way, our own brains never imagined they were separate from our bodies. The two grew together in symbiosis.

* * *

So let us presume that by the time worldships are possible, human beings with squishy bodies and brains like ourselves will still be around to travel on them. Will they be looking to find an Earth-analogue planet at their destination?

In science fiction – such as James Cameron’s disappointing movie Avatar – it is possible to imagine a fast dash from Earth to a twin of Earth orbiting a nearby star. In reality this is not credible.

How close is the nearest Earth-double likely to be – complete with an oxygenated atmosphere, a 24-hour day and surface water? One may assume that we will not find it in the Alpha Centauri system, given the fact that a double star is so different from our own singleton Sun. In their 1984 worldship paper, Bond and Martin quote an estimate of 140 light-years as the average distance between such planets, and that was before anything was known about the unexpected variety in planetary systems, notably the existence of “hot Jupiters”.

At the time it was assumed, for example by Drake and Sagan, that the arrangement of planets in our Solar System was typical, but now one modern researcher has gone so far as to say: “there is no such thing as a typical planetary system”.

We can therefore safely expect that the nearest ready-to-occupy exo-Earth is probably at least 30 Alpha Centauri distances away. Reaching it within a human lifetime requires a cruising speed of an improbable 95% of the speed of light. Barring an amazing propulsion breakthrough, there will be no fast dashes to Earth-analogue planets. It will be a very protracted worldship journey if it happens at all.

Meanwhile, some 30,000 stars of all types are to be found within that 140 light-year distance. The question arises as to whether there might be a reason for people to travel to some of the nearer ones.

What is it really all about? What is a worldship?

A worldship is basically a mobile space colony. In order to reach another planetary system at any realistic speed, it is necessary first to colonise interstellar space, because that is what the worldship is. Therefore it is necessary before that to colonise interplanetary space within the Solar System. If a sequence of space habitats is constructed, starting from modest beginnings, but going on to develop increasing self sufficiency and sustainability of occupation, and located at increasing distances from Earth, then the worldship is simply the end-point of that evolution, with an engine and fuel tank attached.

Space colony (Donald Davis)

Consider the prospects for extraterrestrial colonisation within the Solar System. Potential planetary targets may be considered to be those whose surface conditions are not too extreme, and whose surface gravity is at least lunar strength. This gives us five bodies to work with: the Moon itself, Mars obviously, and also Callisto, Ganymede and Titan. These worlds have a collective surface area of 426 million sq km (2.86 times the land area of Earth).

Supposing, however, that the Main Asteroid Belt is used for the construction of O’Neill-style space colonies, a habitable surface area of over 400 billion sq km becomes possible, thus 1000 times greater. How is this possible, when the mass of the Asteroid Belt is a mere one-20th that of the Moon? It is because of the different ways in which gravity is supplied. An artificial tensile structure in rotation uses mass to produce surface area with gravity about a million times more efficiently than a natural compressive structure which just piles up the mass into a ball. In addition, clearly, the inhabitants of a space colony have control over the level of gravity which they experience – a function which is not available on the Moon or Mars.

Thus we arrive at a key insight: for species which require land surface with gravity, an atmosphere and surface water, at least 99.9% of the opportunities for extraterrestrial growth lie in artificial space habitats, not on planetary surfaces.

From this follows a second key point: for species which have developed ways of converting asteroidal material into habitat, rather than being reliant on planetary surfaces, branches of civilisation can be set up at almost any main sequence star, because all such stars almost certainly possess orbiting asteroids, comets and small moons, while very few are likely to possess close Earth analogues.

And from this a third point: such species will find it perfectly natural to move from one star to another in a worldship, because the living environment on the worldship will closely resemble what they are accustomed to in their system of origin and what they will construct after arrival at their destination system.

These points may seem obvious, trivial even. Yet only two years ago the BIS marketed a DVD entitled “How to Colonise the Stars”, written and produced by Christian Darkin (Anachronistic Productions, 2009). Not a single mention is made of extrasolar comets or asteroids – the raw material for colonising the stars – or of their resource potential, or of colonies in space, or of any kind of in-space construction whatsoever. Throughout the 50-minute programme, narrator and interviewees alike – including many BIS luminaries – adopted the tacit assumption that interstellar colonisation was only conceivable on a planet that was sufficiently Earth-like to be occupied with only modest terraforming efforts, but not so Earth-like that it had advanced life of its own. A condition that in reality would be almost impossible to fulfil, and which would in any case bypass almost all the material and energy resources that the Galaxy has to offer.

Referring back to the original worldship issue of JBIS from 1984, we find that Bond and Martin too are focused on the idea of finding a terrestrial planet at their destination, which may be terraformed to suit human needs. D. L. Holmes, however, adopted a more enlightened view: he expected asteroids and small moons to be more attractive resources to human immigrants, though without giving any mention of their huge growth potential in comparison with planets.

Space colony Island One (Don Dixon)

Interstellar voyagers will not find a terrestrial garden of Eden conveniently located in the surface water zone of every Sun-like star they wish to visit. Even if they do have a propulsion system capable of crossing the interstellar vastness within a single lifetime, their extreme isolation in the wilderness – both in transit and for some time after arrival – will force them to make their vehicle their entire world, whether the number of human occupants is in the millions or fewer than a thousand. Such a vehicle is itself the prototype of the habitats which can most conveniently and most reliably be constructed in the widest variety of extrasolar planetary systems.

* * *

In the construction of such a ship, the design problems posed by the living quarters are no less severe by present-day standards than those of the propulsion system.

Getting a closed cycle ecological life support system up and running is only the start of the problem. Robert Zubrin has claimed that food production under artificial light on any meaningful scale is not feasible: how do we raise the efficiency of agriculture and prove him wrong? The problems of radiation shielding and artificial gravity are obvious enough, but establishing the long-term viability of a relatively small isolated human population has many subtle aspects. Politics, economics and psychology are all involved. Supposing that individuals display criminal or psychotic behaviour: how does one deal with a riot on board?

For long-term worldship missions a full understanding of human reproductive biology is essential. We have to be able to guarantee genetic viability, population stability, male and female fertility and healthy growth of the embryo in what will inevitably be an imperfect simulation of our ancestral terrestrial environment.

Again, our current civilisation is based upon division of skills and labour on a global scale among seven billion people. How will the occupants of even the largest worldship, tiny in comparison, preserve all relevant skills and manufacturing capabilities for spare parts and consumer products? One may think it more exciting to write papers on nuclear fusion and antimatter, but how will everyday items be manufactured and recycled? What about the clothes the travellers wear? Over the course of a long journey they’ll wear out. Unless there’s a clothing factory on board, and its associated supply chain, we’ll arrive at Alpha Centauri looking like this…

Visitors from the planet of the nudists...

The answer can only be a long process of gradual adaptation of human biology and sociology to artificial space habitats, amounting to multiple lifetimes of prior experience within the Solar System before the first worldship is ready to depart.

So two themes emerge to dominate our roadmap to the worldship:

  1. Gradually increasing sustainability of space habitats independent of Earth.
  2. Gradually increasing propulsion performance and consequent reduction in interstellar journey time.

Suppose, for example, that we take present-day capabilities as follows: sustainable life support in space for the three years of a return trip to Mars, which some people think is achievable today, and rocket exhaust velocity the 30 km/s of the ion engine of the Dawn probe now orbiting Vesta. A modest average annual increase of 1.4% would lead to a doubling of these values every 50 years or so. If the human population in the Solar System increases at the same rate, implying rapid interplanetary colonisation after about 2050, and if per capita wealth increases likewise, then the cost of high-energy interstellar flights is covered by general economic growth. I’ll add a nominal 25 years to the flight time to take account of acceleration, deceleration, and exploration at the destination through to the point where the worldship is able to reproduce itself.

Under these assumptions, the curve of falling journey time to Alpha Centauri meets and crosses the curve of rising guaranteed life support endurance around the year 2357, when the travel time is about 350 years.


Such a simplistic model is based on the idea of smooth technological progress, which is an over-idealisation of a complex multi-faceted reality. It represents only one scenario out of many possible ones.

Supposing an unexpected propulsion breakthrough is achieved which allows virtually unlimited velocities at moderate cost? Then the date of the first interstellar flight is brought forward. This is however a highly optimistic scenario.


On the other hand, perhaps the slow progress with controlled nuclear fusion so far is hinting at a different outcome, one in which expected propulsion breakthroughs fail to materialise, putting a ceiling on performance. If at the same time guaranteeing the sustainability of life beyond Earth is unexpectedly hard to achieve, the curves might never even cross at all.


Without high-energy propulsion, but with reasonable progress in developing sustainable human life away from Earth, the roadmap to the worldship takes on this appearance: a scenario in which only millennial-scale interstellar travel is possible for the foreseeable future.


Regarding propulsion, the key insight which impresses me is that, despite Bob Parkinson’s sterling work applying the Daedalus engine to Solar System applications, an engine suitable for interstellar propulsion is not actually necessary for Solar System use. Top of the range performance for a magnetoplasma engine driven by solar power, beamed solar power or a fission reactor is expected to be 500 km/s. This is entirely adequate for trips between Earth and Mars comparable with ocean crossings in modern liners on Earth, though Mars is sometimes the distance of New York from Europe, sometimes somewhere near Australia. A trip between Earth and Saturn would take two months; from the inner Solar System out to Neptune and the inner edge of the Edgeworth-Kuiper Belt: six months. Would a market for faster crossings develop? Impossible to say.

When considering advanced propulsion systems, therefore, whether powered by nuclear fusion, antimatter or vacuum energy, whether releasing that energy as rocket thrust or as the artificial curvature of spacetime, it is entirely possible that they will never be developed through to the reliability required for an interstellar journey. Unless they possess some operational advantage which feeds into cost savings relative to existing chemical, solar or fission powered engines for Solar System transport, the market to develop them will not exist.

Of course it’s always possible that by that time the Solar System is blessed with an enlightened government willing to fund them regardless of cost. Or perhaps a large-scale long-range tourist industry may appear, or alternatively a need for rapid transport of military forces, either of which might demand the high exhaust velocities which only more advanced engine technologies can deliver. The industry depends upon unpredictable social trends.

Might a robotic precursor precede manned worldship voyages, or would it even be necessary? If propulsion technologies race ahead of life support endurance, then an interstellar probe like Icarus might well be used to gather details of an exoplanetary system in advance of a worldship launch. But suppose that worldship technologies are ready when the journey time is still a number of centuries, as in the first case we looked at above. One would have to decide whether to put off the manned flight for a long time in order to wait for a probe to arrive and report back, or whether to launch a worldship when only telescopic information about the destination was at hand. Unless propulsion was enjoying a burst of innovation, the robotic probe might well be seen as too slow to deliver worthwhile results.

Space colony (NASA)

* * *

So the roadmap to the stars inevitably contains multiple possible future scenarios, reflecting real uncertainties in how technologies and societies will shape up in centuries to come.

Granted continued progress, however, five provisional conclusions may be drawn:

  1. Most of the growth potential for civilisation in the Solar System is in free-orbiting space colonies, which are in their nature prototypes for the living quarters of a worldship. Their isolation in space, scattered progressively further around the Solar System, would also help to insure civilisation as a whole against the types of new technology disaster which Bill Joy warned us about.
  2. If magnetoplasma engines powered by nuclear fission are the best we can bring to operational maturity, then journeys of one to two millennia will eventually be undertaken in space colony derived worldships.
  3. If on the other hand more powerful engines become available, the time taken for a realistic interstellar crossing plus the time required to develop resources at the destination still demands a worldship environment for the travellers. A tiny astronaut crew in a fast, utilitarian transport vehicle remains a possibility, but a remote one.
  4. After arrival, human starfarers will naturally use the tools they have brought with them to replicate their worldship environment in a new crop of space colonies orbiting their new host star.
  5. When we eventually do find a close Earth-analogue planet, its value to us will be as an object of non-invasive scientific study and recreational visits, not of resource extraction or colonisation.

Wayland: extrasolar landfall

[Note: this talk is based on a paper which is currently in preparation for the Journal of the British Interplanetary Society.]

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