From Interplanetary to Interstellar Spaceflight

Good morning, and a big thank you to Kelvin Long, Rob Swinney and the Initiative for Interstellar Studies for organising this session, and to the staff at ISU for helping me to get connected.

I should like to discuss some aspects of spaceflight which I covered in detail in my two worldship articles published in JBIS in 2012. The references and links to these papers may be found on my website. I have another article on the similar theme of economic and energy growth due to be published in the same journal this year.

I’d like to offer you a view of progress into space which is solidly grounded on what we can realistically expect to be physically and economically feasible. Forget for the present space warps, wormholes, faster than light travel and other science fiction marvels. Maybe some of these will eventually come true, maybe not. If and when they do, we can update our plans to take them into account. But until that happens, it would not be good engineering practice to get carried away by hype or by speculative fiction.

I’ll say a bit more about that at the end of my talk, but my central focus today is on the following question: how much of a stretch is it to progress from where we are now on Earth to achieve interstellar flight? Might it not be necessary first to master interplanetary space travel, around our own Solar System, before setting off on voyages to other stars? What might this mean for our roadmaps of progress?

My talk is structured around five sub-headings, asking the following questions:

1. Robotic probe or manned starship?
2. Matter-antimatter or nuclear fusion propulsion?
3. A launch from Earth alone or from a broader Solar System civilisation?
4. Humans or robots in charge?
5. Towards a technological singularity or a plateau?

I’m sorry there was no time to prepare slides for this session, but I hope you’ll still be able to follow my train of thought well enough without them.

(1) Robotic probe or manned starship?

Firstly, “interstellar flight” may mean two very different things, depending on whether an interstellar journey is undertaken by a robotic probe, or by a manned starship carrying human passengers. These vehicles are different in two important ways.

Firstly, in mass. At the Starship Century Symposium in London in 2013, Professor Ian Crawford stated that he would like to see a probe with a mass of 150 to 200 tonnes decelerated into the target planetary system. It would carry an array of orbiters, landers and atmospheric entry probes. Smaller masses have been suggested, and Yuri Milner’s Breakthrough Starshot initiative is contemplating sending probes with a mass on the order of grams, thus a million to a hundred million times lighter.

The mass of a vehicle able to support human life for long periods of time in isolation from the Solar System will necessarily be very much greater. Published proposals have ranged from 25,000 tonnes up into the hundreds of billions of tonnes. Since the kinetic energy of a vehicle is proportional to its mass, and since very large kinetic energies are demanded by the high velocities necessary to reach even the nearest stars within a reasonable period of time, such large masses entail major problems in developing sufficient energy and power for nuclear rocket or laser sail propulsion.

As a quick example to focus the problem in our minds, let us suppose that the International Space Station, together with a few spacecraft attached plus an interstellar engine and fuel tank, is sent to Alpha Centauri with its six astronauts on board. We round its mass up to 500 tonnes, and assume that the engine operates at maximum rocket efficiency. We decide it should complete the one-way journey within a human working lifetime of 45 years.

Under these conditions, the speed needs to be one tenth of the speed of light, and the total energy in the rocket propellant comes to 1.4 × 10²¹ joules, ignoring energy lost in the form of waste heat. This energy is equivalent to the entire industrial energy consumption of Earth’s global civilisation over two years. (Note that the BP Statistical Review of World Energy, published annually and accessible online, is a respected industry source of information about current energy consumption.)

More realistic assumptions would have given us a ship whose mass was at least 50 times greater, pushing the energy up to a level equivalent to present-day global consumption on Earth continued over a century or more. Furthermore, this energy has to be converted into rocket thrust over the course of a few months or years within a single physical structure which is vastly smaller than the global infrastructure on Earth, necessitating extremely high power levels per unit mass. For example, releasing 10²¹ joules of energy within a single year of acceleration or deceleration represents a power of 32 TW, concentrated into a single engine or cluster of engines. This is three to four orders of magnitude higher than the capacity of the largest power stations on Earth.

Robotic probes could possibly cheat on the propulsion requirements, and this is the second difference between probes and starships. Unlike human beings, they can be made tolerant of high accelerations. This would allow a probe to dive close to the Sun, unfurl a solar sail and use the pressure of sunlight to accelerate to a significant fraction of the speed of light. The acceleration using raw sunlight must be completed while close to the Sun, because the intensity of sunlight falls off with distance according to the inverse square law. But an auxiliary laser may then be used to give the probe an extra burst of acceleration.

You sometimes hear that the laser sail method of propulsion is more plausible for interstellar flight, because it possesses the advantage that the fuel has been left at home: there is no need to accelerate the fuel together with the vehicle. In this way what people sometimes call the “tyranny of the rocket equation” has been bypassed. But this is only partly true. The rocket equation does more than relate the mass ratio to the exhaust velocity and total delta-vee achieved, for it allows one to calculate the efficiency with which the energy released has been converted into the motion of the vehicle.

That efficiency reaches its maximum in the region where the exhaust velocity and delta-vee are of the same order of magnitude. For a rocket this corresponds to a mass ratio of 4.9. But with laser sail propulsion, although the thrust is produced in a similar manner to a rocket by direct physical pressure of an exhaust stream on the vehicle, the exhaust velocity is fixed at the speed of light. Unless the total delta-vee also approaches that speed, the energy released is used with low efficiency. If a rocket can be designed with a suitable exhaust velocity, it may well be more energy efficient than a laser sail arrangement.

For a small probe with a mass of grams to kilograms, such poor efficiency may be acceptable, because the energy cost is a small proportion of the total cost. But for a starship whose total energy budget comes to a large multiple of Earth’s present-day total annual energy consumption, economics would dictate the most efficient possible propulsion system. In practice the question will be complicated by the fact that the price per joule or per kilowatt hour of energy in a laser beam generated by an orbiting power station will generally be different from the price per unit energy in nuclear rocket fuel.

A further point would be that the rocket may be used to decelerate at the destination, whereas a laser sail cannot realistically do this unless there already exists a laser installed in the target system. As a way around this, Robert Forward proposed using a multi-stage sail which splits into two as the vehicle approaches its target. The laser beam from the Solar System is reflected off one sail in order to decelerate the vehicle, which is attached to the other sail, thus allowing it to come to rest at the target star. The same geometry would even allow the same energy beam from the Solar System to accelerate the vehicle back towards the Solar System, making a return interstellar flight conceivable.

But the practical problems of focusing high-power beams over multi-light-year distances may well make conventional rocket propulsion preferable, if suitable rocket technology has become available. Another drawback with Forward’s ingenious scheme is its inflexibility: the reflective sail and the sail with the ship itself must rendezvous with a beam which was transmitted from the Solar System a number of years before. A rocket-propelled ship, on the other hand, can change course as it approaches in order to find the optimum parking orbit.

For any kind of manned starship, therefore, I tend to assume that it will use rocket propulsion, with nuclear fusion as the energy source. You may like to ask yourselves whether I might have missed some realistically possible propulsion system which would be preferable?

Note also the concept for braking an interstellar probe or a starship on arrival by use of a magnetic sail, or magsail. This was recently analysed in considerable mathematical detail in JBIS by Robert Freeland, a member of Project Icarus. It appears that at speeds of less than 5 per cent of the speed of light, a magsail is not necessarily effective.

(2) Matter-antimatter or nuclear fusion propulsion?

While we’re on the subject, a word about matter-antimatter propulsion. This has been on the menu of possibilities ever since the German rocket pioneer Eugen Sänger described his photon rocket in 1956. The modern understanding is that the most efficient way to use antimatter would be to mix a small quantity of antihydrogen with normal hydrogen gas in the reaction chamber. Assuming that the annihilation energy could be efficiently captured by the remaining hydrogen, this would then exit a magnetic nozzle in the form of a plasma at an exhaust velocity which could be precisely controlled by varying the matter-antimatter mixture ratio.

Antimatter as an energy source is intellectually seductive. The Sun generates enough energy in total to fuel hundreds of starships per second, if only that energy could be captured and harnessed. But it must be captured close to the Sun, before it has been too greatly diluted by the inverse square law. One way would be to build a power station near the orbit of Mercury, which converts the radiant sunlight to electricity and that to a laser or microwave beam. The beam may then be transmitted out to a great distance, either to push against a light sail, or to be recaptured by an antenna, reconverted to electricity and used to power an electric or magnetoplasma rocket. But, as we have seen, the laser sail option suffers from the low efficiency with which the beam energy is converted to kinetic energy in the spacecraft. The electric rocket option, too, suffers from low efficiencies as the beam is reconverted back into electricity at the spacecraft, and then converted again into the kinetic energy of its hydrogen exhaust.

Using the power station in low solar orbit to manufacture antimatter, however, offers the prospect of improving the fraction of solar power captured by the station which ends up in the kinetic energy of the spacecraft. The antimatter fuel on board a matter-antimatter rocket would represent the most concentrated form of energy possible with known physics, unless of course one is willing to contemplate the engineering of microscopic black holes. The two major uncertainties with the antimatter option would be the efficiency with which the power station manufactures the antimatter – presumably antihydrogen – in the first place, and the efficiency with which the annihilation energy generated in the rocket’s reaction chamber is captured by the other particles of fuel.

But even if it is assumed that these processes can be made to go at acceptable rates, two other factors emerge which take the shine off any kind of antimatter-powered vehicle. They both stem from the fact that the matter-antimatter reaction is the precise inverse of nuclear fusion: while fusion is extremely hard to ignite, matter-antimatter annihilation is extremely hard to prevent from ignition.

A tank holding liquid hydrogen and oxygen such as on the Space Shuttle, or holding precisely engineered pellets of fusion fuel such as in the Daedalus design, can have a mass very much less than that of its contents. The Space Shuttle external tank carried 28 times its own weight of hydrogen and oxygen propellants. A tank designed for a starship, thus one manufactured in space and never subject to accelerations of more than a fraction of a gee, can be still lighter in proportion. But confinement of antimatter in the significant quantities required to drive a starship – tens to hundreds of tonnes – would require the surrounding tank to have a mass which was a multiple of the mass of its contents. The antimatter would presumably have to be stored in the form of pellets of frozen antihydrogen which were ionised sufficiently that they could be held and manipulated with electric and magnetic fields, but not so strongly that they exerted a significant repulsion on one another. Each pellet would require many times its own mass of electrodes in order to control it fully and prevent it from coming into contact with any matter structure until it had been injected into the reaction chamber. A system of sensors using laser or microwave beams would also be needed to monitor the exact position of each antihydrogen pellet at all times, and to adjust its state of ionisation if necessary.

The only way around this, I think, would be if a state of matter could be found in which matter and antimatter particles were combined into a single subatomic entity, but did not react so long as the temperature was held low enough. I’ve sometimes wondered whether an antiproton could be trapped within a carbon-60 buckyball, but of course in reality the electron shells surrounding the carbon nuclei would not be sufficient to repel the antiproton whenever it came close, and it would quickly be attracted to a carbon nucleus and would annihilate with a proton there, particularly as one electron would be missing from the molecule in order to balance the electrical charge.

Another flaky idea I had was to manufacture antineutrons, and attempt to create a stable nucleus consisting of a proton in combination with an antineutron. My assumption was that the antineutron would not react unless it encountered a neutron, but those of you whose understanding of particle physics and quantum mechanics is better than mine will, I’m sure, very quickly be able to tell me that such a nucleus would not be stable. So it seems we are left with a complex, heavy and power-dependent matrix of electrodes and microwave sensors, whose mass is many times greater than that of the antihydrogen fuel it contains.

The second factor which tells against the use of antimatter is the possibility of malfunction. Any engineered system can break down on occasion. With a fusion rocket, if there is any technical problem then the reaction will simply fizzle out, or not take place at all. With a matter-antimatter rocket, any malfunction in the electromagnetic systems monitoring the antimatter and holding it precisely in place would be liable to lead to an explosion, the annihilation of the entire antimatter supply and thus the immediate destruction of the entire vehicle.

Thanks to the Daedalus study which the British Interplanetary Society published in 1978, we know that nuclear fusion is theoretically capable of accelerating large vehicles to speeds of several per cent of the speed of light, allowing interstellar crossings to nearby stars to take place with journey times in the range of one to five centuries. While the engineering challenge is immense, the physics of nuclear fusion is well understood, and the resources for manufacturing fusion fuel are plentiful.

This is particularly the case since Robert Freeland’s paper appeared in 2013 as part of the ongoing reassessment of Daedalus undertaken by the Icarus Project. This paper showed a way to avoid the use of scarce helium-3, substituting for the deuterium-helium-3 reaction a combined fission-fusion reaction using lithium deuteride fuel. Both lithium and deuterium are commonly available in our own and other planetary systems.

So it seems that the industrial use of macroscopic quantities of antimatter must remain speculative for the time being. The nuclear fusion rocket seems still to be the most practical propulsion system for interstellar flight.

(3) A launch from Earth alone or from a broader Solar System civilisation?

Returning to the main theme of this presentation, let us move on to consider the industrial base necessary for interstellar flight. Given the enormous energy and power required, the next major distinction which needs to be made is between flights which may possibly be launched from Earth alone, and those which will inevitably depend upon a large-scale industrial infrastructure extending to the Moon, Mars, perhaps also Venus, and to the Asteroid Belt and beyond.

In my first worldship paper, I concluded that Earth alone could despatch an interstellar probe with a mass of a few tens of tonnes at a cruising speed of around 5 per cent of the speed of light, taking ten years to construct and fuel the probe. While smaller than Ian Crawford’s ideal probe, there would still perhaps be room on board for multiple orbiters and perhaps even a lander, if a planet had been detected which offered the possibility of surface biology.

But I concluded that an interstellar spacecraft much larger than about 50 tonnes, or flying much faster than 15,000 km/s, could not be launched from Earth alone within the constraints of known physics. You may like to take this as a challenge, if you can see a way to invalidate my assumptions.

A manned starship, taking many decades and more probably many centuries to make a crossing, is the ultimate interstellar challenge within the bounds of engineering we can conceive of at present. Unless people could be put into long-term hibernation, it would need to be a worldship with passenger accommodation at least on the scale of the largest luxury ocean liners. I concluded that in order to launch such a vehicle an interplanetary economy would certainly be needed. Let’s consider why I came to this conclusion.

Firstly, there’s a simple economic argument. No single starship or worldship can use more energy than the whole of human civilisation for all its other purposes. To do so would be unrealistic, because industrial energy is fundamental to everything we do. Whether considered in its own right, or using the energy cost of such a mission as a surrogate for its monetary cost, an energy economy is required which is very much greater than any which can be maintained on Earth alone.

One is therefore driven towards a view in which starships are constructed by a consortium of interests both on planetary surfaces, such as on the Earth, Mars, Callisto and so on, and in free-flying space colonies, such as O’Neill cylinders, in orbit round Earth and Mars, in the Asteroid Belt, the Jupiter satellite system and indeed eventually further away, in the outer Solar System.

Secondly, we’ve already mentioned the extremely high power demanded for an interstellar propulsion system, tens of terawatts or more, many orders of magnitude greater than anything we can build today. Such high power demands high efficiency, if the engine is not to melt itself with its own waste heat. Such an interstellar engine cannot appear overnight. It must be the product of many generations of engines, just as modern jet or rocket engines have a long and incremental design history. The extreme reliability required for an engine on an interstellar voyage of one or more centuries in duration indicates that the technology must be very well understood, having matured through long use within the Solar System.

But there is a problem. In order to get around the Solar System within reasonable periods of time, an interstellar-capable engine is not needed. Remember that even the largest distances between Solar System planets are four orders of magnitude smaller than the distances between neighbouring stars. A trajectory from Earth to Neptune would be around 30 astronomical units long, while the distances to Alpha Centauri and to Barnard’s Star are in the range of 270,000 to 370,000 AU. Even if one relaxes transfer times to these nearby stars to the order of centuries as opposed to years for transport between Earth and Neptune, the discrepancy in terms of the necessary velocity is still two orders of magnitude. It would therefore be possible to get around the Solar System, and particularly out as far as say Jupiter, with an engine, such as Chang Diaz’s VASIMR magnetoplasma engine for example, which will not scale up to the performance required for interstellar propulsion.

Maybe we will be lucky, however, and find that an engine, perhaps based on nuclear fusion like the Daedalus engine, can be developed to be competitive for long-range transport within the Solar System. Once the technology had matured, the performance could then be turned up for interstellar use. Maybe.

Note that the development of interstellar-capable engines would be increasingly likely the greater the distance that human exploration and settlements had spread outwards from the Sun. And in fact the gradual diffusion of colonies into the Kuiper Belt beyond Neptune would greatly improve human preparedness for interstellar flight in many ways. If a fifth giant planet were to be discovered orbiting the Sun several hundred AU out, as Professor Mike Brown believes, this again would serve as a stimulus for bringing the technologies for higher power propulsion systems to maturity while still remaining within the Solar System.

My third point on the necessity for a Solar System civilisation before interstellar passenger-carrying worldships can be built concerns human life support. A worldship is an extreme space colony. One ship, or a small fleet of them, must be more self-sufficient than any human community has ever been before. Again, it is not realistic to imagine these ships being flown before extensive experience has been gained closer to home.

To my knowledge, the closest simulation so far of a human colony in a space environment has been the controversial Biosphere 2 project, in which eight people spent two years in near-total isolation from Earth’s environment, thus biosphere 1. The biospherians attempted to grow all their food and recycle all their waste within the structure. I recommend to you Jane Poynter’s book describing the experiences of the biospherians. “After thirteen months in Biosphere 2,” she wrote, “we were starving, suffocating, and going quite mad” (The Human Experiment, p.245). Yet without local food production there can be no permanent human settlement beyond Earth, either in our own planetary system or beyond it. The Biosphere 2 project stands as a useful reminder of just how much there remains to be done.

Furthermore, the biospherians did not even attempt to manufacture replacement clothes or other hardware items. Manufacturing on Earth has tended towards specialisation, with each factory serving a global market. In space the opposite is required: small-scale factories which can supply their local population with a wide variety of different products. These issues were debated at some length by William Hodges, Eric Jones and Ben Finney in the book Interstellar Migration and the Human Experience, one of the astronautical classics.

If we suppose that the multiple complex problems of life beyond Earth are capable of being incrementally solved, we can consider space and planetary colonies gradually spreading throughout the Solar System. As they spread, one can see them incrementally increasing their self-reliance, reliability and livability. Habitable stations in space and on planetary surfaces can spread from the Earth-Moon system to Mars, to the Asteroid Belt, and so on. By the time they reach the moons of the outer planets, the Centaurs and the Kuiper Belt, life on such a colony will be highly self-sufficient, and closely akin to life on an interstellar worldship.

After arrival at a destination star, too, the travellers will rely on space mining and construction techniques which need to have already reached a high pitch of perfection. Forget the science fiction cliché of astronauts stepping onto an Earthlike planet and breathing its air in as much comfort as they would enjoy on Earth itself. Such planets may exist orbiting other stars, but it must be certain that there are very few of them, spaced too widely for a voyage there directly from Earth.

The first order of business of the new arrivals to a previously unvisited planetary system will therefore be to set up industrial infrastructure for mining and manufacturing on its asteroids and small moons. But probably not on any terrestrial planets in the system, because the more such a world resembles our Earth, the more scientifically interesting it becomes from the point of view of discovering possible indigenous life. The star travellers would therefore avoid colonising it until it had first been thoroughly explored.

(4) Humans or robots in charge?

A possible alternative architecture exists in the proposal to transport, not normal human beings, but human passengers who have been put into a state of induced hibernation or suspended animation, or even fertilised human embryos. See for example a recent paper by Adam Crowl and collaborators on what they call embryo space colonisation. The idea is that they will be tended by robotic systems which can be miniaturised as human beings cannot. The robots are required to maintain their human cargo on the journey, and after arrival use local resources to construct the massive and bulky accommodation which people need. Only when this is done are the human passengers taken out of cold storage: the sleepers are awakened, or the embryos incubated, and thus the new planetary system receives its first human population.

Because such a ship could be much less massive than a worldship, and its in-flight endurance greater, this would lower the mass of the expedition. Perhaps the costs could be brought down to the point that Earth alone could afford them?

This question is certainly debatable. I believe that when one looks into how such an expedition would be developed, it becomes a lot less attractive. Consider what needs to happen on arrival. In order for the robots to be effective at building a new microcosm of human civilisation from the ground up, this capability needs to have been first demonstrated in the Solar System. The requirement for mature technologies of mining and manufacturing from primordial resources is not bypassed. The requirement for mature life-support systems and socio-political arrangements is likewise still present.

What the sleeper ship architecture has done is to reduce the ship mass, and hence propulsion costs, but at the cost of requiring a demonstration for a comparable period of time within the Solar System of human hibernation, or of human embryo storage followed by robotic gestation and raising of babies outside the normal biochemical environment of a living human mother. This may or may not turn out to be possible.

Either way, the need for extensive preparation within the Solar System remains, on a timescale of at least a number of human generations.

My most recent considerations of this subject suggest that the step from a civilisation on a single planet to one spread widely in the Solar System will not be easy, and that the overall human population and energy economy will plateau out for a period of a few centuries before growth in space takes off. This paper is due to appear in JBIS during 2016, and when it does I shall add the link to my list of publications.

We should, I think, not be in too much of a hurry to see the first people depart on interstellar voyages. Rather than a hundred years providing enough time to prepare, my arguments lead to the conclusion that it will be more like a thousand years from now before the first manned worldships begin to depart from the Solar System. If we are to become a civilisation which colonises planetary systems, then we must inevitably start with our own.

This will, however, be a thousand years of extremely hard work, with many difficult and interlocking problems in technology, life sciences and society needing to be solved before humans can live securely and sustainably beyond their planet of origin.

(5) Towards a technological singularity or a plateau?

Arthur C. Clarke famously wrote: “Any sufficiently advanced technology is indistinguishable from magic.” From the point of view of an uneducated person, or one living in a pre-industrial society, this is perfectly true.

However, Sir Arthur did not say: “Any sufficiently advanced magic can be turned into technology.” The fact that we can imagine some fabulous capability does not prove that it is possible. Somewhere there may be limits to what technology can do. Where might those limits be?

The subject of interstellar studies raises this question in its most extreme form. Consider some of the expectations which science fiction has primed us with:

• Starships will be able to complete interstellar journeys in a matter of days or hours.
• Their engineering will use future breakthroughs in understanding the fundamental physics of spacetime.
• Or, in the case of alien species, this has already been done, millions to billions of years ago.
• Alternatively, human interstellar travellers will be able to make long journeys in suspended animation, and wake up at their destination in perfect health.
• Alternatively, human consciousness will be uploaded to computers, and the resulting digital persons will live in a rich and complex virtual reality.
• Alternatively, humans like ourselves will be accompanied by completely artificial, super-intelligent robots.

Developments such as these have been portrayed so often in science fiction that the tendency is to take them for granted. Yet at the same time the indications from the real world are that progress is slowing down. Consider three areas of particular relevance to interstellar engineering:

• Controlled nuclear fusion.
• Astronaut missions to Mars.
• General artificial intelligence.

In all three areas, progress has not been as rapid as was originally expected, leading to the popular joke that they are 20 years in the future, and in 20 years time will still be 20 years in the future, just as they have been for many decades in the past. Clearly, the joke is an exaggeration. Progress is being made. In nuclear fusion, the triple product or Lawson criterion has been going up over the decades. The technologies for Mars flights are steadily improving, for example with SpaceX’s demonstration of supersonic retro-propulsion. And computers are still racing ahead in terms of hardware, software and machine-human interface.

If, therefore, one takes the view that, say, faster than light space travel will one day be possible, that can only be speculation at present. It cannot depend upon real-world evidence, because that evidence does not at present exist. Such evidence as does exist suggests that the rate of technological innovation is slowing down. An important part of science places limits on what is physically possible:

• The laws of thermodynamics limit the energy efficiency of machines to less than 100 per cent.
• Gödel’s theorem proves the impossibility of a logical system which is both self-consistent and complete.
• Einstein’s special relativity limits the speed of material bodies to less than the speed of light.
• Heisenberg’s uncertainty principle limits the amount of information which can be known about quantum systems.

In addition to purely scientific constraints on our knowledge and capabilities, new technological developments frequently introduce unwanted side-effects, such as pollution, and a restructuring of the jobs market as some jobs are made obsolete and new ones are created which require special training. The logical consequence of an acceleration of technological change towards a singularity would therefore be an equal acceleration of problems produced by the application of those changes to society, putting a brake on the process of change.

Consider for example the appearance of engines of sufficient power to drive starships. They would make immensely destructive weapons. Or the appearance of artificially intelligent machines whose intellectual capacity was superior to human intelligence. Such beings would create insoluble conflicts between those people who wanted to hand over to the machines important decision-making functions in politics, religion and economics, and those who did not. The super-intelligences would quickly discover that, as the title of a famous Russian science fiction novel goes: it’s hard to be a god.

In any case, as I said earlier, the indications are that at the present stage in history we are heading, not towards a technological singularity, but rather towards a technological plateau. The process of industrialisation based on science and technology is producing a single step forward from a medieval type of society to an interplanetary one, and on a timescale of about a millennium. Interstellar travel will still be possible eventually, but it will depend upon the maturing of the technologies for creating sustainable industrial civilisations on Earth and throughout the Solar System.

The idea has become popular in some quarters that manned interstellar spaceflight will become possible about a century from now; in other words, without depending upon the prior large-scale human settlement of the Solar System or the painstaking working through of applications of our existing knowledge of physics. The prospect of such near-term interstellar flight would depend upon breakthroughs in fundamental physics, and in technological applications of the new knowledge, and in managing the social consequences of those applications. Nobody can say that these are impossible. But while they have not yet happened it is necessary also to plan for the alternative scenario of the technological plateau.

That scenario is one of consolidating and perfecting technologies based on the physics we know today, and of integrating them into society. Of course we must keep an open mind towards the possibilities that science fiction writers dream of. But as engineers we must focus the main part of our efforts under present-day conditions onto securing the place of human civilisation in the Solar System, on Earth and off it. Again, if we are going to convince the uncommitted that interstellar travel is a realistic prospect for our descendants, then it must be shown to be possible with the tools we have today, or could realistically have in the near future, without waiting for a breakthrough which may or may not ever come. Not magic, but just sound engineering and realistic economics.

This is why I thought it important to focus in this talk today on our incremental growth into the Solar System, and why that growth leads naturally to interstellar travel within the next millennium or so. I hope you found it interesting.

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Stephen Ashworth has been a fellow of the British Interplanetary Society for the past 30 years, and has contributed both popular articles to Spaceflight magazine and technical papers to its Journal. His blog is entitled Astronautical Evolution. His science fiction novel The Moonstormers was published in a range of electronic formats in 2012. He lives in Oxford, UK, and works in scholarly publishing at the Voltaire Foundation, part of the University of Oxford.