All Astronautical Evolution posts in 2019:

The Destiny of Civilisations – Fire, Iron and Gold (November)

The Destiny of Civilisations – A Problem for SETI (November)

The Holy Grail of Space (October)

Return to the Moon, 50 Years On (August)

The Case for Interstellar Flight (June)

SpaceX Dragon 2 Success (April)

Killing the Doomsday Fallacy (Feb.)

All Astronautical Evolution posts in 2018:

How Far Can We Take the Copernican Principle? (Dec.)

Dawkins and the McGraths: a Biologist versus two Theologians (Nov.)

The Atheism Question (Oct.)

The Religion Question (Sept.)

I, Starship (June)

Back to 2017:

Scenario Block Diagram Analysis of the Galactic Evolution of Life (Nov.)

Comments by Alex Tolley (Oct.)

Elon Musk’s “Great Martian” (Oct.)

Elon Musk’s Mars Plans: Highlights from His Second Iteration (Sept.)

What is a Supercivilisation? (Aug.)

Quantifying the Assumptions Behind the METI Debate (July)

Five Principles of a Sustainable Manned Mars Programme (June)

Pale Red Dot: Mars comes to Oxford (May)

Back to 2016:

Elon Musk and Mars: Looking for a Snowball Effect (Oct.)

New in 2020:

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2022: What’s to do on Mars?…

2021: New space company Planetopolis…

2020: Cruising in Space…

2019: The Doomsday Fallacy, SpaceX successes…

2018: I, Starship, atheism versus religion, the Copernican principle…

2017: Mars, Supercivilisations, METI…

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…

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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 147, 8 June 2019 – 50th Apollo Anniversary Year

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The Case for Interstellar Flight

The challenge

I recently received an e-mail from a researcher at the Open Philanthropy Project, a relatively new foundation supported by one of the founders of Facebook.

“At Open Philanthropy,” – he wrote – “we’re very interested in the long-term future of humanity, and a lot of the research behind our grantmaking revolves around trying to think about what will make a difference to future generations. We’d like to have a better understanding of what possibilities technological developments might open up for future generations, and as part of that, we’re thinking about what kinds of space travel and settlement might be feasible in principle, in the very long run.
“I’ve been trying to work out whether there is a very conservative scenario for how interstellar travel could happen which would convince even a staunch sceptic of its in-principle feasibility, and have kept coming back to proposals akin to the worldship concept.”

I replied with some thoughts which I shall flesh out a bit here, aimed at producing precisely such a conservative scenario which would convince even a skeptic that interstellar travel is feasible in principle – as well as being a worthwhile and morally praiseworthy enterprise.

(1) Growth

For the past couple of centuries, civilisation has been consciously structured for growth. This applies simultaneously in a number of distinct areas: economic, technological, population, infrastructural, cultural – in a mutually reinforcing pattern.

There are now calls for a move towards a zero-growth society. At the current stage of our development this would be not only unrealistic, but potentially catastrophic for the poorer sections of the world’s population, who have not yet achieved a tolerable standard of living (as Gerard K. O’Neill, author of The High Frontier, was at pains to point out). By “unrealistic”, I mean that the likely political will to terminate growth is weaker than the entrenched forces driving growth forward.

Clearly, material growth on Earth must eventually come to an end. This recognition is driving progress in energy efficiency and recycling of material wastes. These trends both work with the grain of the legitimate desire for a better life, and help develop our civilisation towards long-term sustainability not only on Earth but ultimately beyond our mother planet.

(2) Our cosmic situation

The universe offers vast opportunities for further growth. Almost all the energy and material resources of use to industrial civilisation are extraterrestrial.

Yes, material growth must come to an end on Earth quite soon, but Earth civilisation as the metropolis of a dynamic space-based society will be very much more secure against collapse than an Earth civilisation which closes off its horizons and aims for intellectual as well as material and energy stasis.

K/T impact event

If our long-term future as an industrial species is of interest to us, then we are of necessity required to develop our civilisation to the point where it is robust against a range of potential disasters:

The only guarantee of robustness against the full range of existential threats is to diversify our civilisation into our astronomical environment, starting with the Solar System. If Earth falls, Mars may yet survive, and vice versa.

In doing this, we would be replicating the pre-industrial situation where empires in Europe, India, China and the Americas could rise and fall without impacting greatly, or even at all, on one another. For a civilisation with globe-spanning technologies, only a multi-global (i.e. interplanetary) distribution of centres of power can recover such resilience.

(3) Space colonisation

I see a natural progression of human colonisation of our cosmic environment. The starting-point will probably be Mars, assuming that the low gravity does not pose insuperable health problems.

Stanford Torus by Don Davis

The next step will be space settlements manufactured from asteroidal materials. These will be more difficult to construct than planetary surface bases, and will therefore come later in time. However, once the link with planetary surfaces has been broken and a space colony lifestyle established, expansion throughout the Solar System becomes possible. After these colonies have developed towards technological and social maturity over many generations in an expanding population, their design can be taken as the nucleus for interstellar worldships.

The progression from current terrestrial city-dwelling to extraterrestrial settlements is a huge step, comparable with transformations in the past such as the move from nomadic to village life, and from village to city life. It will take many centuries for this new lifestyle to mature.

The high cost of space travel relative to terrestrial travel, plus the high cost of constructing livable habitation at all possible extraterrestrial destinations (Mars, Venus, the asteroids, the large satellites of the outer giant planets), will have a bottleneck effect: growth of an extraterrestrial population will be from small founder groups who are able to afford those costs.

Growth in space cannot therefore be seen as a safety valve for current problems on Earth, except in the important respects of intellectual growth, inspiration, and cultural and technological cross-fertilisation. Terrestrial problems caused by population growth and industrial pollution must be addressed on Earth itself, but their successful solution will enable long-term sustainability not only on Earth but on Mars and other locations.

(4) Propulsion

For many, propulsion has been the killer question – potentially the fatal flaw in dreams of interstellar travel. The distances between the stars are simply too great to cross in any reasonable journey time, unless one relies on using an engine which is so powerful that it is pure fantasy.

Let’s create an outline vehicle design for carrying humans to another star. Suppose that a worldship has a mass of one million tonnes – twelve times the mass of, say, the RMS Queen Mary – including accommodation for about one thousand passengers, their shuttles and tools for use after arrival, and the ship’s propellant tanks and engine.

A comfortable cruising speed for our worldship might be two per cent of the speed of light. At this speed, it would need about 220 years to reach the nearest neighbouring star (the triple system of Alpha Centauri), 530 years to reach the nearby sunlike star Epsilon Eridani, or 600 years to reach the next nearest sunlike star Tau Ceti. Such multi-generational journey times will look a good deal more achievable once the space colonies described above have been running smoothly for a few centuries in remote regions of the Solar System.

We can estimate the energy cost, assuming a reasonably efficient rocket engine which makes one relatively short burn to accelerate the ship away from the Solar System, and a matching deceleration burn on arrival: the total energy consumption works out in the region of 150 ZJ (150 × 1021 joules; 1 ZJ = 1 zettajoule). This is about 300 times annual worldwide industrial energy production at the present time (taken as 16 TW). In other words, the energy burnt in its engine by this one ship on a one-way voyage would be enough to power all of present-day global civilisation for 300 years. It might well seem impossible.

Daedalus by Adrian Mann/BIS

The question is, then, whether a technology can exist which can pack such immense energy into a ship-sized package. This was in fact answered as long ago as 1978 by the British Interplanetary Society’s starship study, whose result was the Daedalus design. The study demonstrated in principle (and in considerable technical detail) that nuclear fusion is a sufficiently energetic power source for interstellar crossings to nearby stars with large vehicles on a timescale of a few centuries. No breakthrough physics is required, just ingenious engineering. (The actual mission profile used for Daedalus was a flight to Barnard’s Star, 6 light years away, in 50 years at a cruising speed of 12.2% of light, but without deceleration at the target system.)

The Icarus project is in the process of revisiting Daedalus, and already some useful insights have emerged to ease the problems of building and flying a vehicle of such power.

The propulsive energy density of nuclear fusion fuel for the kind of pulse engine envisaged for Daedalus and Icarus is around 60 TJ/kg (60 × 1012 joules per kilogram; see the Freeland paper in the references below). The amount of fusion fuel required for the outline starship above is therefore 2.5 million tonnes, or just 2½ times the mass of the ship without fuel.

(For anyone technically inclined: the mass ratio is 3.5 and the exhaust velocity is then 9,580 km/s, slightly less than that calculated for Daedalus, so reasonably plausible.)

For a rocket vehicle, this is nicely in proportion; chemically powered rockets launched from Earth today require considerably more propellant per unit hardware mass. The resources required might prove to be problematic, however: while deuterium is abundant wherever hydrogen is found, other proposed fusion fuels such as lithium or boron are moderately rare.

The project would be enormous by present-day standards – about as inconceivable as building the Queen Mary might have seemed to our ancestors around the time of the Norman Conquest. Here, too, a development timescale on the order of one thousand years would be about right.

We come back to the question of economic growth. A starship that consumed centuries-worth of global industrial energy in a single voyage would have a cost millions of times greater than any present-day terrestrial organisation or government could possibly afford. But growth is not restricted to Earth. A worldship like this could only arise after centuries of growth of a Solar-System-wide economy, one which developed space engines to the point where they could be adapted for starship propulsion, which developed a space-colony style of living to the point where people would be confident that their living-quarters would maintain them in comfort and security for centuries to come, and which grew to the point that the cost of a starship fell to a small fraction of the total economic activity around the Solar System.

Equally important will be the creation of industries which can mine raw materials from asteroids and small moons – coping with the difficult unearthly conditions of a low-gravity dusty vacuum – and which can refine and process them through to high-quality products suitable for construction of high-tech space vehicles and space colonies.

This will not only be essential in order for human civilisation to spread around the Solar System. The fact is that most interstellar destinations do not have closely Earth-analogue planets. The first worldships will arrive in systems whose raw materials are found only in asteroids, desert worlds and gas giants. The lifestyle at the destination will therefore require the construction of new space colonies in any case, and occasional Mars-like surface bases.

The old cliché of our departing Earth in order to arrive at an Earth clone, or Earth 2.0, orbiting another star is totally unrealistic. The nearest such planet must be too distant for early starships from the Solar System to reach. And when such a world does eventually come within range, it will by definition already have its own biology, and will therefore be far more valuable as an object of non-invasive scientific study than as a location for colonisation by humans. For the immigrants must have already long been adapted to life in fully artificial structures, over many generations, if they are even to have travelled there in the first place.

(5) A galactic civilisation

Once a civilisation has begun to expand on an interstellar scale, its ultimate dominance of the entire galaxy would be secure – except of course in the case that another species from a different star industrialises at about the same time that we do, plus or minus a few million years, when we would share the galaxy with the descendant civilisations of that species.

After arriving at an unoccupied planetary system and consolidating its presence there over perhaps a few thousand years, a second-generation civilisation will have the material and power resources, and the economic capability, in its new home to send new colonising expeditions further afield.

In early human history, the first civilisations in Mesopotamia, Egypt, the Indus valley, the Yellow River valley and the Americas lived independently of one another: one could collapse without harming any of the others. In today’s globalised world, human civilisation has become monolithic, every part dependent on the health of every other part. But on a galactic scale the diversity of civilisation is recovered.

By forming a population of civilisations in this way, the long-term sustainability of any one civilisation is no longer necessary for our human heritage to survive. Just as a species survives even though individual members die, so a galactic-scale population of civilisations can survive and prosper even if every individual civilisation within it is doomed to ultimate decline and fall. This is why anybody concerned to preserve the human heritage for the long term must be committed to space colonisation.

Thus if, for example, humans fall and become extinct in the Solar System, other humans, or post-humans who preserve the general human heritage, can recolonise the Solar System from their home in the Alpha Centauri system or wherever it may be.

(6) Artificial intelligence

But what about machine intelligence? Isn’t it about to drive us biological humans extinct?

People expressing such concerns include Professor Nick Bostrom, of the Future of Humanity Institute in Oxford, and Max Tegmark, of the Future of Life Institute.

As it happens, I don’t share the popular hype about a technological Singularity which will cause biological humanity to be driven to extinction. It’s clear to me that technological development is tending towards a plateau of technological maturity, where further progress is increasingly frustrated by several factors:

Since they too are physical beings, the machines we create will be as subject to these constraints as we are. Thus for example the fantasy that a machine which matches human intelligence will be able to design and build a machine which is more intelligent than itself, producing an uncontrolled intelligence explosion, is ruled out by the Law of Requisite Variety, which states that a less complex system cannot control a more complex one. Thus intelligence can only increase by evolutionary trial and error, not by design.

It seems clear to me that we are in the process of merging with our machines, while still retaining our essential humanity. I see biological and digital intelligence as being complementary parts of an integrated cybernetic civilisation, rather than as competitors.

So far as interstellar travel goes, the arrival of machine intelligence greatly simplifies the problem. Machines are very much easier to maintain than biological humans. They can withstand millennial journey-times and harsh environmental conditions much more cheaply than we can. While a plausible manned worldship needs to weigh a million tonnes or more in order to support a self-contained human society, a plausible robotic interstellar probe – but one still with serious capabilities for exploration at its destination – would be a thousand times lighter and cheaper.

By “serious capabilities” I mean a probe carrying a sufficient number of orbiters and landers to be able to make an unambiguous detection of any biology that may exist on or under the surfaces of terrestrial worlds in the target system, and at least begin to characterise their microbiology sufficiently to answer the question whether it is related to terrestrial biology, or represents an independent original synthesis of life – surely the deepest underlying question which motivates scientific exploration of other stars.

Just as with planetary exploration, the first interstellar explorers will clearly be robotic spacecraft. The long radio delay times of years to decades separating them from command by the nearest human controllers, combined with the complex decision-making they will need to make after arrival in the target planetary system, will force them to be highly self-sufficient.

In the future development of our civilisation, humans and machines will both play active and creative roles.

(7) The next steps

What should organisations such as the Open Philanthropy Project be doing now, if they wish to support progress towards a long-term human future both on Earth and away from it?

In my view, the major issues right now are:

The first of these issues is being addressed about as fast as possible by American NewSpace, particularly by Elon Musk and his company SpaceX, and by Jeff Bezos and his company Blue Origin. The bottleneck at present is the extremely high cost of transport to orbit, and the resulting extremely low traffic level. The solution, in the form of large-scale space passenger transport for industrial research and space tourism, cannot accelerate towards reasonable traffic levels on the order of, say, 10,000 space passengers per year, until these commercial spacecraft are flying regularly.

The recent success of the Dragon 2 test flight is a major step forward, though the test stand explosion following it has now delayed the schedule.

Regarding the second point, one would have thought that, with all the concern about the polluting effects of burning fossil fuels, governments around the world would have long been engaged in a race for more advanced technologies such as nuclear fusion. It is a mystery to me why this is not the case, and why the international ITER programme has been proceeding so slowly and in the absence of competition. But one sometimes hears of independent work such as the research being done by Lockheed Martin, so a breakthrough is possible at any time.

Clearly, as we expand our civilisation into space and especially as we venture further from the Sun, compact nuclear fusion will become an indispensable power source – quite apart from its rocket propulsion application.

The third issue is not being developed very strongly at present and therefore presents more of an opportunity for innovation. I would like to see a settlement created in a desert area on Earth which can act as a prototype for sustainable cities both on Mars and on Earth itself. The emphases would be on recycling human waste so as to grow all foodstuffs within the city walls, rather than relying on a global agricultural hinterland, and on local manufacturing, which again does not rely on a global supply network. The first major stab in this direction, the Biosphere 2 project, showed how much more there is to do in this respect.

Clearly, these are processes which are in progress anyway, but not yet with any focused purpose of assembling a complete system which – despite a wilderness setting and no global supply chain – is both sustainable and attractive to live in.

Competing visions of the future

The above points constitute a scenario for the future. There is no way they could be a prediction – far too much is uncertain for any definite predictions about the human future to be made!

Therefore we must obviously take into account other possible scenarios, in particular ones which interrupt the current trajectory of growth and progress before any self-sufficient bases can be established off the mother planet. Some of these are:

There is therefore still plenty of material to fuel the concerns of those with a pessimistic view of the human future.

One variant is, in my view, implausible, and this is the clichéd story of a starship carrying to a new Earth the survivors of a catastrophic collapse of Earth. As I should have made clear by now, no such ships will depart without the support of a vigorously healthy Solar System wide economy, and such a large-scale civilisation would hardly be prepared to watch Earth collapse without making any effort to support it.


There, I think, we have the ingredients for a “very conservative” scenario for interstellar travel:

  1. Human civilisation needs to continue its current trajectory of growth, but with increasing efficiency so as not to overburden planet Earth;
  2. We must expand out into the Solar System in order to create a civilisation which is robust in the long term against both natural and anthropogenic disasters;
  3. There is a natural progression of colonisation, first of planetary surfaces, then building space colonies which can spread progressively more widely around the Solar System over the next millennium;
  4. Interstellar journeys will then become technologically feasible, with journey times of a few centuries, using nuclear fusion for propulsion (physics which is already well understood) and a space colony style of living both on the interstellar flight and at the destination;
  5. Once a second-generation civilisation has established itself in a new planetary system, then it may send its own colonising expeditions further afield in turn;
  6. Regardless of the ultimate balance of power between biological humans and artificially intelligent machines, the rise of the latter makes interstellar flight more plausible;
  7. The most useful preparation for a sustainable future on both Earth and other worlds which is not yet being seriously addressed would be to start to build cities in terrestrial deserts, where technologies, recycling, life-support systems and the social and political organisations appropriate for largely or fully self-contained societies can be tested and developed.

Anyone with specific questions on the details of interstellar flight may find it helpful to get in touch with Kelvin Long, Andreas Hein or Rob Swinney at the Initiative for Interstellar Studies, of which I am also a member. Their headquarters is near Bristol in the UK. Their work has so far been mainly in the fields of education and publishing, and they’ve been involved in the planning for Yuri Milner’s Breakthrough Starshot. Anyone who wanted to set up some practical work directed towards an interstellar future for mankind might well find them to be the people to work with, and they certainly have the enthusiasm and technical knowledge necessary for this difficult subject.

Further reading

Stephen Ashworth, “Aridopolis: Project for a Sustainable Earth/Mars Desert Settlement”, Principium: The Newsletter of the Initiative for Interstellar Studies, issue 18 (August 2017), p.4-11.

R.M. Freeland III, “Project Icarus: Fission-Fusion Hybrid Fuel for Interstellar Propulsion”, Journal of the British Interplanetary Society, vol.66 no.9 (Sept. 2013), p.290-96. (Proposes lithium deuteride fusion fuel.)

Robert G. Kennedy, “The Interstellar Fusion Fuel Resource Base of Our Solar System”, Journal of the British Interplanetary Society, vol.71 no.8 (August 2018), p.298-305. (Proposes hydrogen and boron-11 fusion fuel.)

Some of the most realistic speculation about the future of machine intelligence has come from Josh Storrs Hall in his book, Beyond AI: Creating the Conscience of the Machine (Prometheus, 2007).

My own technical papers published in the Journal of the British Interplanetary Society cover the following topics:

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