Ian D. K. Kelly 22nd July 2010 Vn

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Interstellar Travel

Per Ardua
Ad Astra

Interstellar Travel

Per Ardua

Ad Astra

Ian D. K. Kelly

22nd July 2010

Copyright © 2011 Ian D. K. Kelly, All Rights Reserved

First published 2007 by
Agrintha Books Ltd, Exeter UK


This edition © 2007, 2011

Copyright © 2011 Ian D K Kelly, All rights reserved. No part of this publication may be reproduced, stored in any retrieval system, transmitted or recorded by any means whatsoever without the prior written permission of the publisher.

ISBN 978-0-9553399-4-3

First published 2007 by
Agrintha Books Ltd, Exeter UK


This edition © 2011

The Author

Ian D. K. Kelly is a computer scientist, who trained as a mathematician. He is in­terested in almost everything – including linguistics, fairy stories, philosophy, and astronomy. Ian plays the piano (and church organ), teaches music and conducts several choirs, has written books about computer translation (that’s com­puters translating between human languages), pantomimes (“Oh no he hasn’t!” “Oh yes he has!”), and novels for both children and adults. He claims that if he gets to Heaven, he’ll be a librarian, will hear the real end of J. S. Bach’s The Art of Fugue – and drink fine wine all day.

Ian D. K. Kelly


Emerald Pie, 2007, Agrintha Books, Exeter, ISBN 978-0-9553399-1-2 A children’s novel. Suitable for children from seven years old.

A Lad in Knaphill, and His Magic Lamp, 2007, Agrintha Books, Exeter, ISBN 978-0-9553399-5-0 and Cinderella and Her Bearded Sisters, 2007, Agrintha Books, Exeter, ISBN 978-0-9553399-6-7. These are pantomimes (“Oh no they’re not!” “Oh yes they are!” “Not that joke again!”). You are warmly invited to use these pantomimes, and make any alterations to them for your own purposes – but if you do use them, please send a donation to Knaphill Methodist Church, Surrey GU21 2DR, UK. Thank you.

“PROTRAN – An Introductory Description of a General Translator”, in Ebert, R., Lügger, J., Goeke, R. Practice in Software Adaption and Maintenance, 1980, North Holland, ISBN 0-444-85449-5

“PROTRAN – A generalised translation tool for natural and algorithmic languages”, in Overcoming the Language Barrier, Verlag Dokumentation, Munich 1977. Proceedings of the third European Congress on Information Systems and Networks, EEC. ISBN 3-7940-5184-X

“Thesaurus Vectors”, 1980, Newsletter No. 9, Natural Language Translation Specialist Group (BCS).

With Tucker, J.V.H.: “Jesus Smithson” in Boas, Guy: A Teacher’s Story, 1963, Macmillan, London

Progress In Machine Translation: Natural Language and Personal Computers, 1989, Sigma Press & John Wiley, ISBN 1-85058-156-8

With Goshawke, W., Wigg, J.D.: Computer Translation of Natural Language, 1987, Sigma Press & Halstead Press (John Wiley), ISBN 1-85058-056-1 and 0-470-20913-5

The Carpenter’s Carpet, 2007, Agrintha Books, Exeter, ISBN 978-0-9553399-3-6 Teaching stories from the world’s religions and traditions. Suitable for children from five years old.

How to Read This Book.

Read it through, for the first time ignoring all the footnotes, and all the sections that are shaded – these are merely technical descriptions and justifications. They are important technical descriptions, and the argument does depend upon them, but – in the fist instance – you should trust me. At the end you will know the main thrust of what I am saying.

Then read it through again, this time reading and checking the figures and the footnotes and the shaded sections and the technical niceties, as you see fit. If you find any errors, or you can show that my conclusions are incorrect or do not follow from the figures do please let me know either by letter to the publisher or by e-mail to idkk@idkk.com or both. Or you could yourself write a book presenting better arguments or counter arguments - I would be delighted to hear of either.

Above all, enjoy reading this: it is about a very important topic – more important that most people currently realise.


Where to begin? Where to end? I have had many suggestions and much help. I have re­ceived a lot of advice, and taken very little of it. The facts and good ideas are all due to other people – just the mistakes are mine.

At the very least I have to thank for (variously) their ideas, their support, their teaching, their examples, their criti­cisms (some slight, some deep), their friendship and their love [names in alphabetic order by surname]: Shaheen Aziz Ahmed [who, many years ago, tried to teach me schol­arship, art-apprecia­tion, and Urdu]; “Russell”: J. W. R. Anderson [Rux0r]; Profes­sor Archbold (UCL) [the first person ever to call me “Mr. Kelly”, and a masterly teacher of alge­bra]; Marcella Arnow [charming friend and intellectually focused work-mate: the most English American you can imagine]; Peter Bacon [charming Christian wit with, I believe, a frog named after him]; Grace Barr; James Barr [perfect gentleman, and devout Christian]; Mr. L. Berkeley [inspired teacher of history, lovingly known to generations of pupils as Buffalo Bill]; Allan Bouchard; Judith Bouchard; Pamela Bourne; David Brooks [whose 21st birthday party was a turning point in my life]; Dr. Rudranath Capildeo [politician, mathematician, polymath, who in just a few words encouraged me to look upwards, and be my real self]; Martyn Catlow [who quietly taught me the true meaning of the word courage]; Jenny Clayton; Peter Clayton, Roger Clayton; Gerald Cole [who, by example, taught me to love language]; Ian R. Dale; Irene Dale; Carol Duval [ah, Carol – can we ever understand all the poems in the world?]; Rick Duval; Alan Earnshaw [yes, it’s kinetic energy – not momentum – sigh]; Allan Freeman; Barry Griffin [master of both logic and meditation]; Walter Goshawke [Ac­cipiter Gentilis]; Cathie Hartigan [glad2b]; Richard Heathfield [C_Dreamer: who has an unfairly large share of intellect and wit – well, I think it’s unfair – is unequalled in the clarity of his writing, charming and genuine in his politeness, a fine Christian exemplar – and incredibly hairy]; Emma Hibling [who thinks faster and better than any computer ever invented – and plays Go with vicious skill]; Paul Humphreys [logic cuts fine, Paul, and you taught me that even deep differences in opinion need not damage friendship]; Allan Jupp; Dmitriy V. Kokiyelov [whose jokes I dare not use]; John Latham [John, I wish I had your skills in study and business organization]; Rachel Latham; Veronica Lawson [who finally convinced me that language translation is difficult, and we often ignore its complexities]; Paul Lewis [who thinks differently]; Des Maisey [who was a hard taskmaster in Systems Programming – thank you, Des]; Raphael Mankin [colleague and college friend who taught me more about computing (and Hebrew and Judo and horse-riding and recorder-playing and… insert long list here) than I thought it was possible for me to know]; Ann Marsden; David Marsden; Dr. Margaret Masterman; Rev. Caryl Micklem [Reverend and musician, who always made an inspired choice of musical notes (even though organists don’t like playing tunes in the key of D-flat), and the perfect choice of contemplative words]; Romilly Micklem; Timothy Morgan; Alan Pool; Rowena Poppe; Peter Priechenfried; George Purvis; Alan Rabjohn; Rajan Harishankar; Sanjeev Richarya; Paul Schooling [dear friend and unforgiving logician, who struggled to teach me discipline of thought – poor pupil that I am!]; Jane Skinner; Noel Skinner; Coco Smith; Phil Smith; Tony Spooner [R. E. Noopsca]; Edmund Stephen-Smith [ESS]; Tom Stockwell [brilliant teacher of physics and scientific method: alas, I have forgotten much of both]; Poh-Teen Tang; Tim Upton; Vince West; David Wigg; Tom Wil­son [wonderful teacher of mathematics, musician, and gentleman]; Andy G. M. Wood [AGMW] (not “Woods” – there’s only one of him). And [non-alphabetically] my dear family: my patient, beautiful, charming, irreplaceable wife Gay [How do put up with me, Gizzie? I am thrilled that you do.]; our fantastic children Benjamin and Miranda; and my wonderful, wonderful parents, Percy and Vera, whom I can never thank enough.

If there are names omitted from this list of friends and absent friends, it is from my stupid forgetfulness, not spite. Thank you all for your ideas, help, patience, friendship and love over the years.

Summary Contents

The Garden of Earthly Delights (doors): Hieronymus Bosch

Detailed Contents

The Asteroid Belt


The overall argument is that Interstellar Travel (“IT”) is possible, and necessary, and costly – and interesting. I show it is possible by describing the first part of how it could be achieved; with a discussion of mankind’s future annihilation I show it to be necessary; and by referring back to the possible techniques of its achievement, give it an initial costing. I am passionately interested in the topic, and I hope you too will be when you have read this book.

You don’t need a lot of prior knowledge to start reading. True, this book goes through ar­eas of sociology and economics and physics and astrophysics and biology and as­tronomy and mathematics. There are sections about chemistry and computers and nuclear energy and cooking and education and politics (both local and interna­tional). But I assume, throughout, that you are the ordinary, non-technical reader – everyone can read this.

What is meant by “Interstellar Travel”? It means transporting an appreciable number of living human beings beyond from the confines of the Solar System as we know it. This is not talking about just ex­ploring parts of the Solar System – just going to the other local planets – but a journey that is very much longer than that. The size of the Solar System is measured in (at most) a few “light days” (the distance light would travel in a couple of days): the nearest star is more than four light years away1. The journeys considered here are several hundreds – or thou­sands – of light-years in length. If the Interstellar Ships are designed correctly, the journeys might even be millions of light years in length.

Looking at how viable Interstellar Travel (IT) might be achieved, we see that the IT project would not be small, and would not be easy. One possible technique of cre­ating a ship would be to choose an existing rock in the Solar system and make it habitable, and then consider how it can be moved over very long distances with human traveller. So the possibility is considered of taking a large asteroid, hollowing out its core as the living space and projecting that asteroid by ejecting (in small bursts) its holl­owed-out core. Other possibilities are also considered. There are discussions on how much energy and what timescales would be re­quired. There are also enquiries on how long the result­ing environment (habitat) might remain stable, and how long an isolated community could live in such an environment – scientific and sociological stability.

Why is this IT project necessary? Because mankind is doomed. There are various – numerous – disasters waiting to happen. Some of these disasters will extinguish mankind. Note – “will extinguish” not “might extinguish”. We – humankind – are in serious danger, here on Earth, and unless there is a subset of mankind that gets away from this planet then the annihi­lation clock is ticking for us – and ticking loudly2. It is we ourselves that have helped wind this clock.

If we are going to hollow out an asteroid, we have to get to the asteroid, and we have to take equipment to the asteroid, and we have to have advanced manufactur­ing at the asteroid. We have to take people to that work area, and we have to take all the basic ingredients for a “bio dome”. Being a large project, it is costly. We dis­cuss just how costly, and the economic impact (and ad­vantages) of this project. If we choose any of the other techniques we have to have the same discussions.

Finally we ask “When can it be started?” So: Can we? Must we? How? What scale? When?


Where are we Going?

This is about interstellar travel: it is not science fiction docu­ment, but science and sociological fact – if you wish to make fiction from it, be my guest! My train­ing as an engineer has been incomplete, and others can fill the (many) holes in this discussion – but the idea’s core is here.

Per Ardua ad Astra is the motto of the Royal Air Force – “by effort to the stars”. I do not be­lieve that the bulk of what I describe here will be achieved in my lifetime. I urge you, though, to think about it. Every generation needs its cathedrals, its great works, its potentially imposs­ible dreams. Travel to the stars is, for us now, the ultimate physical frontier. And I will argue it is an essential part of our future.

Answering the question as to which stars we should first visit is complex.

The Questions

There are five main questions to consider, with many subordinate questions hang­ing off them. The five main questions are:

Can it be done – is it possible for us to go to the stars?
Can we?

Do we have to go to the stars? Is there compulsion?

Must we?

How can we get to the stars – what is the engineering involved?


What is the scale of the operation, and its cost? How much?

What scale?

How long will it take, and how often can we repeat it?


Which stars do we visit?


The main sections of this document consider these six main points, with a number of the subor­dinate questions.

The conclusions cannot be summed up in one sentence – but the summary of the sum­mary is:

  1. yes, we can travel to the stars, and

  2. yes, we must travel there, or we – all of mankind – are utterly doomed, and

  3. there is engineering that will get us there – even though there are some de­vel­opments that have to be made, and

  4. the scale is considerable, the cost is great (and discussed in much more de­tail here) – but the negative costs are even greater, and

  5. we could (and must) start the first journey3 within 100 years – or within a shorter pe­riod, if we can get the political and social will.

Can we? Yes. Must we? Yes – or we are doomed as a species. How? By making an existing celestial body habitable (for example, by hollowing out an asteroid and impelling it forward4). What scale? At a cost of $3×1014 spread over 100 years5. When? We could – and we should – start right now, and repeat it many times, but need political and social will to do so. Where? There are a large number of possibilities.


The sixth question – the one not fully posed here – “Why?is difficult, as there are many levels of reason. There is no way adventure can be ex­plained to a non-adventurer. “Because it is there” is one reason enough, or perhaps “be­cause we have to”. You may personally believe that this is something we – man­kind – must do and will do, or the contrary. I cannot persuade you it is a good idea if you are con­vinced oth­erwise – or too scared to accept its necessity.

If we do not think of our future amongst the stars, we – hu­mankind – are doomed to soon die on this planet, with abso­lute certainty. If we do go beyond our tiny Solar System, then there is a chance – just a chance – that we, through our descendants, will sur­vive be­yond the 8 million years that (currently) seems to be the real outer limit of our expected spe­cies lifetime6 (Ref: [Gott2001] p.210).

Scientists are encouraged to consider questions that start “What?” or “When?” or “How?” or “Who?”, and the question “Why?” is sometimes called “The Devil’s Question”, and it is not always easy to determine which sort of “Why?” is meant, when it is asked.


In the short term (at most a few hundred years) if we do nothing we are doomed to run out of space and food. Growing at only 1% per year would mean that in 100 years the world’s population would be well over 16 thousand million (16,000,000,000) – assuming we are starting with “only” about six and a half thousand million now. In 200 years it would be 43 thousand million (43,000,000,000) and in 500 years it would be (gulp!) 868 thou­sand million (868,000,000,000). With over 6.5×109 (6,500,000,000) we are already over­crowded – at 8.6×1011 people we would (metaphori­cally) be standing on each other’s shoul­ders.

So we have to stop before then. Overcrowding will kill us7.


And there is pollution. The number of people with (for example) asthma is grow­ing – not just be­cause the population is growing, but in proportion to the popula­tion. We have (inevitably world wide) an increase in the carbon dioxide and meth­ane and dust being pumped into the atmosphere by man’s actions. My family in Ireland tell me that they will not eat fish caught in the seas between Ireland and Britain as they fear that those fish will be contaminated with radioactivity. And you personally have met and heard of many other examples more serious than that

In the Western world – the arrogantly named “first world” – there is8 an increase in life expec­tancy. But this is not always the case in the truly-named third world. We have, in countries like Zaire and Zimbabwe, terrible mortality arising from AIDS and political theft of food. We have, all over the African continent, the rape of na­ture, and the destruction of natural resources. We have, in Russia, hideous mis-management of water resources – to the extent that a complete sea has been dried up because of man’s intervention. We have in China, and the far East, seemingly unre­strained pollution poured into the atmosphere out of “economic necessity”. You can see in South America many instances of river pollution, forest destruction, the surface degradation of the land – again from “economic wishes”

And, no, they are not economic necessities – they are just wishes. This is not a po­lemical tract, tell­ing you how mankind should reorganise so that all can be fed, all can be clothed, all can be healthy – but it is a matter of fact – of known fact – that there is already sufficient food on the planet to feed us all, but that food is unfairly distributed.


In the longer term, Sol9 (the Sun) will go nova. We are pretty certain of this10. So even if we do not die from overcrowding (starvation, atomic war, biological annihi­lation, etc.), we will – in less than 5.5×109 years – die from an exploding sun. Just 1 AU from an exploding star is not a healthy place to be11. If at that time mankind is confined to that point in space, then mankind is doomed. Doomed with complete, unquestionable finality.

We have to do something. We have to do something big. If we do not, we will all perish – the whole of humanity. The stakes could not be higher.


When should we commence the work towards interstellar travel?

In once sense, we have already started it. Although at the time I first wrote this para­graph (October 2003) there had been an interval of over 20 years, mankind has been to the Moon, Luna, and we are planning (though with political prevarication and budgetary weakness) manned trips to Mars12.

If the expected lifetime of the planet is five thousand million years we cannot wait four thousand million years before we start work – that would be far too late. If our expected species lifetime is 8 million years, we cannot afford to wait 7.5 million years before we start work – that also would be far too late. If we die of hideous overcrowding in 500 years, we cannot af­ford to wait 400 years before we start to work on the problem – that again would be too late.

Nor can we afford to wait for the technology to be right – it will not just come right by itself – we have to choose to do the work, choose to do the research, choose to do the de­velopment. By working on it, the technology will be forced to come right13. Much of the engineering required to reach Luna, for example, arose simply because there was a (political and social) will to get there: the technology followed the desire.

So we have to start work now – right now. For now – in the early years of the twenty-first cen­tury– we have the nucleus of the necessary technology, we have the nec­essary wealth, and we have breathing-space enough to work on hard problems before they become so pressingly urgent that we ei­ther panic into bad solutions or give up and submit to tragedy. But it’s a narrow time window.

Will we do it? That I cannot say. If it were just the choice of the engineers, just the choice of the geeks (us geeks!), then – yes – we would build interstellar craft. It is, however, a politi­cal choice, and politi­cians (for all their fine words) have only short-term views. And this is, above all, a long-term pro­ject. This is a decision, a project, that is too important for the petty politicians – what genius of social engineering will get it started, though, I can­not yet fathom.

The Destination

One major decision – at least from a psychological point of view is “are we going some­where, or are we just going?” That is “Where?: do we have a final destination – a specific place each ship is aiming for, or are we simply launching the ships to be worlds in them­selves, distant from Terra?

Although this is a major question, it is once whose answer we can change even after we have set out. A ship could, for example, set out with a specific destination in mind, but partway there decide to just continue travelling. A ship could suffer a disaster (an engi­neer­ing mishap) en route, and find that it no longer has the ability to reduce speed ade­quately to land (stop) anywhere – it is then compelled to con­tinue travelling at very high speeds, until (perhaps – and very improbably) it catches up with something that was initially moving away from it very fast.

To start with, in our engineering, we will assume that we are going somewhere – each ship has a destination, and that we have to try to retain the ability to slow down again. It is just as glorious and worthwhile a trip should we have no final des­tination, though, other than the journey itself. The travellers too must remember that. We are possibly designing ships with no final, fixed desti­nation but themselves – and the future.


In this book the figures are approximate. The whole subject is still very conjec­tural, and it is too early to make precise calculations. Accurate calculations, however, can be made – and, if we are to achieve our ends – must and will be made. When you are evaluat­ing these figures, do not ob­ject if they are few percent out – but (by all means) object of they are tens of percent or orders of magnitude out.

Where there are conjectures or uncertainties I also indicate my (personal) degree of cer­tainty as to the validity of the figures quoted. My certainty is expressed in one of two ways: (i) as a valid range for the figure (e.g. there is a better than 95% probabil­ity that I will live “more than one day from the time of my writing this sentence, but less than a hundred years from that time”)14 or (ii) my be­lief in the accuracy of the figure quoted (e.g. I am more than 99% certain that I am 58 years old, at the time of writing). Where these certainties are expressed they may be written in the forms “[range](probability)” or “value{certainty}”: e.g. “[1 day, 100 years](>95%)” or “58 years{>99%}”.

I assume in writing my formulæ that you have a grasp of only simple mathematics – what is now called “GCSE Level” in the UK (and used to be called “O Level”). I do not as­sume that you can understand integral calculus or non-Euclidean geometry or tensor calcu­lus or how to calculate the volume of water required to support a million frogs. If you can understand all of these, so much the better – but I doubt that the calculus or the geometry will be of any use. The frogs might. (See Ref. [Offw2005]).

Because the meaning of “million” is held in common on both sides of the Atlantic, and means 106 (or 1.0E+6) on both sides, that is a word that can be safely used. We will be talking about much larger numbers, however, and – alas – words like “bil­lion” and “tril­lion” cannot be agreed upon. To an old Englishman like myself, “bil­lion” is “bi-million” or 1012 (1.0E+12) – but in the USA it is a mere 109 (1.0E+9)15. Similarly for “trillion” which is either a “tri-million” of 1018 (1.0E+18) or – in the USA – 1012 (1.0E+12). Hence these larger words cannot safely be used without am­biguity.).

In these notations, the overall cost of this project could be written as [$2E+14,$1E+16].

I am also avowing my beliefs. Because I desire something and because I believe some­thing it does not follow that I am right. You have to judge. The arguments given here are (in­variably) one-sided. It is for you to find valid counter-arguments – to correct my fig­ures, tutor me in physics, criticise my engineering, and so on. Do not be fooled by my erudite quotations – quidquid in latine dictum sit, alta vide­tur16 – think for yourself. Tell me what you think by e-mail to Ian Kelly (interstel­lar) (address idkk@idkk.com) and from this document’s the­sis and your (collective) an­tithesis we will be able to synthesize a better plan for this, our most exciting – and I believe necessary – ad­venture.

If nothing else, think about what name the ship would have. It will be a technologically complex Ark carrying a saving remnant of humankind away from destruction of its homeland to an unknow­able far destination.

I would dearly love to travel in space, but I know I won’t. I really want to live long enough to see men on Mars, but I probably won’t17. If we want our children’s grand­chil­dren for many genera­tions to continue survival, however, we absolutely must, as a spe­cies, move off this planet. The non-negotiable cost of not doing so is annihilation. The (monetary) cost of do­ing so is great – but, I believe, necessary. The cost of not doing so is greater than any other cost we have ever had to consider in the his­tory of mankind18. We simply have to make the effort19 to get out there.

Per Ardua Ad Astra.


Everything that exists, exists in some quantity, and can – in principle – be measured.”


This is consideration of the question “Can we?”. That is, we are asking whether it is pos­sible to send humans safely from Terra to a star, or a planetary system around a star other than Sol, and (possibly) back again. Can we do it?

To answer this question we have to consider the other questions in more detail, but we can come up with a first layer of answers here. It is my opinion that we can do it. That is, I believe that it is possible – right now – to design and construct inter­stellar travel de­vices for humans. My belief sys­tems, however, do not of themselves show that it can in reality be done – I have to justify what I am stating, and try to give some de­scription of how it can be done, what it will cost, and when it can be done – the other main questions in this document.

As part of the first layer of consideration of the question “Can we?” we have to con­sider the dis­tances involved, the time-scales involved, and what the (sketchy) de­signs could be for interstellar travel devices. We have to consider the physics that we know now – with­out inventing new physics – and the engineering that we know now – though we may have to assume some (reasonable) fu­ture development in engineering. We must not, at any time, assume any “Silver bullets” – inven­tions that make it all easy, or principals of physics not yet discovered which make (for example) faster than light travel trivial. We must assume the problem is hard, and face it accordingly. Let us see how hard …

Problem Size

This is not a small or simple problem. The distances are enormous, the logistics complex, the timescales long. The engineering is not obvious, and (at the very least) extremely ex­pensive. We do not have any “Silver bullets” to make the problem sim­pler, quicker, eas­ily tractable. I would have liked to assume just one “Silver bullet”, in the means of energy production (see page 62 below) – I would really, really have liked to – but even though this is something being currently researched – with great avidity – it is still too far-fetched20. We can not assume any magic breakthroughs – just hard slog, and error, and great cost, and long times, and social disagreement – all the things that have character­ised large engineering projects from the Pyramids through the Great Wall and the Cathedrals and the Lunar landings.

If the solution of this problem is to be undertaken then we have to be ready for the costs. If we do not undertake to travel beyond Terra, however, we are doomed. Terra has a finite lifetime, and we are rather sure that we have about another five thousand million years21 before Sol goes nova. That, kids, is The End – the end of the human race (unless, some­how, we have learnt how to sur­vive in plasma – a rather doubtful prognostication!).

It is against this immensely high – and non-negotiable – cost of not solving the problem of inter­stellar travel that we have to consider the real costs of travelling. It is also against this very high abstinence cost that we have to measure the difficulty of the problem. I would suggest that the negative costs are so high that almost any expenditure is justifi­able: if we don’t do it, we – the Human race – don’t survive. Periodi.

All of this is considered in more detail in the section on Costs (page Error: Reference source not found below).


The distance of the nearest star to Terra is about 93 million miles. That star is, of course, Sol22. The next nearest star (Proxima Centauri), however, is about 4.5 light-years away23 – or about 31 by 106 (the number of seconds in a year24) by 300,000 (the speed of light in kilometres per secondii) by 1000 (metres in a kilometre) by 4.5 (the num­ber of light-years) metres (gulp!) – which is about 4.18×1016 meters25. For a planetary system we have to con­sider up to 100 light years as the destination – about 5×1018 meters – and probably more. We will – at the upper extreme – also consider distances of up to 1,000 light years – about 5×1019 meters. Journeys of inter­galactic distances (of the orders of up to millions of light years or 5×1019 to 1021 metres and be­yond) are not considered in this documentiii - we are considering only what we could – just possibly – do: this is all science fact, not science fiction.

These distances imply very long journey times. We must assume (as physics teaches us) that faster-than-light travel is impossible. Faster Than Light (FLT) travel is sci­ence fiction – that’s not what we need to have here. Indeed, getting to appreciable fractions of light speed is difficult. If we have very long periods of sustained accel­eration we can get to very high speeds – ac­celeration at about 1g for a year gets you to about 80% of the speed of light, for example. But we have to justify being able to produce accelerations of this magnitude for time-scales of this magnitude before we can consider journeys at such high speeds. Even in space the mass of the projectile is directly related to the force required for a given accel­eration.

Let us (initially) assume that for the trip we are considering we need to travel 100 light-years. Let us also assume (and this will be later justified) that it is possible to achieve continuous accelerations of up to 0.1g, or 1 m/s/s. If we had the energy (which is enormous) and if we could keep up this ac­celeration for 2.5×108 seconds or under 3000 days, then we would reach a speed of 0.85c – that is, 85% of the speed of light. This means that (with the times required for acceleration and decelera­tion) the outward journey time is over 140 years. This is rather a long time, and does mean that we have to construct a vessel (or vessels) that can carry breeding colo­nies. (We are not assuming that we will find the engineer­ing to produce suspended ani­mation – as beloved by so much science fic­tion!). Remember – we accept that this is a hard problem, and has to be faced accord­ingly.

This initial assumption of scale has to be considered against the background – and very real – pos­sibility that we are designing a machine for eternity. That is, there is no final stopping place for the interstellar ship, and the inhabitants need support for all the time they can reasonably be expected to survive. The engineering and biological and sociological (etc.) systems must be designed with extremely long times in mind. Systems that last for, say, a hundred thousand years are not just dif­ferent in scale from those designed to last one or ten years, they are different in kind. I shall be as­suming, in my final figures, that we are designing for a ship that works for at least one million years – and if we can achieve that, we may well have achieved the longest reasonably possible for humankind. After a million years will the travellers still be “human” as we recognise humanity? We cannot say.

Because of the various relativistic effects, it may be best to limit our speeds to under 0.5c, (half the speed of light), where there is a relatively small effect on the mass and time dilation caused by the motion. The standard multiplier of 1/√(1-v2/c2) (called gamma, γ, or the Minkowski Factor) (Ref: [Ein­s1955] and [Born1962]) is, for 0.5c, only about 1.15 (reciprocal is about 0.86). This makes only a small ef­fect upon the energy required for acceleration, and upon the differences between the time-percep­tion of the travellers compared to the time-perception of Terra. In fact, we are (for reasons of propulsion) likely to limit our speed to under 0.23c and much lower, which gives a gamma (γ) of only 1.0275.

I will present figures for two possible maximum speeds – 0.01c or one percent of the speed of light, and 0.001c or a tenth of a percent of the speed of light – both very high figures, but nowhere near what is described in Science Fiction26.


Because we have to transport a breeding colony, there is a certain minimum size, below which we cannot sustain both (a) genetic diversity, and (b) the required mix of skills for long-term civilised survival. For the sake of this discussion we are going to assume that the base population27 for the first, trial ship is 120,000 – we are going to set off with one hundred and twenty thousand people. We do not assume that the popu­lation size will remain static, but that we can support up to 500,000 (five hundred thousand – half a million) inhabitants / colonists / trav­ellers in a single ship – or maybe even a million.

Other scenarios are possible – we could, for example, have a cluster of ten ships, each initially housing just 12,000 people, that set off in convoy. This is the same num­ber of peo­ple, but in smaller ships. If these ships remained within ten million kilo­metres of each other28 – 1010 metres – there could still be communication and ex­change between the ships, but the chances of a single disaster’s destroying the com­plete crew (i.e. the crews of all ships in the fleet) would be much re­duced. “Cluster or singleton” has both engineer­ing and psychological/social implica­tions – but these are for consideration elsewhere. Henceforth (initially) we will consider only a singleton – one vessel. But we have to re­member there are alternatives.

A single vehicle to transport over a hundred thousand people would have to be rather large. We assume in the first sketch design here that it is, at the very least, seven­teen kilometres long by eleven kilo­me­tres in diameter, and that it is (roughly) cy­lindrical in shape. We go into a more detailed descrip­tion later. The walls of such a vehicle would have to be “adequately” thick. Ade­quate for what? Well, adequate for at least a hundred thousand years of service, and possibly more (up to a million years). If we are considering dis­tances of up to 1,000 light years, it is very much more than 1,000 elapsed years – possibly 10,000 elapsed years. That’s a lot of heavy engineering! In this first design, however, that means that the outer walls themselves should be at least 25m thick, assuming that the construction mate­rial is nickel-iron (Ni-Fe). For a hollowed aster­oid of greater size, for example, we could usefully – and easily – have outer walls that are over 100m thick. Indeed, for a “large enough” asteroid (one whose diameter is over 75 km.) we could have an outer wall of 1 km. thick. And for the upper timescales we are considering (one million years) even 1 km. might not be enough – we will be considering several kilometres of wall.

These sizes may seem ridiculously large – but we are talking about a very long journey, and of incar­cerating generations of people for their complete lifetimes. And we are talking about extreme (though rare) stresses – see the section on engineering, and meteor im­pacts. Since, also, an inter­stellar vehicle would have to be built in space – it cannot be built on Terra and then launched – the extremely large size need not be an obsta­cle – provided that the basic materials for its construction can be easily found in space already, and do not have to be all transported there.

Other sizes are possible – larger and smaller. The smaller our initial crew, the more we risk losing of our human culture. The larger, the more populous, we make our initial vessel, then the larger and the more various the tranche of life that we are preserving. I have spent some time working in the city of Exeter, Devon. This is a delightful place, well set, beautifully contained, with a wide vari­ety of humankind in it. Within this city – which has about 120,000 inhabitants – there are ex­perts on mathematics and music and cooking and mediaeval carving and calligraphy and bee-keeping and medicine and psychology and engineering and building and public hy­giene and school-teaching and cheese-making. Within the city there are players of Bridge and Chess and Go and Football and Rugby and Cricket. There are church organs and their organists. There are theatres with actors, writers and readers of poetry, artists, scientists, and sundry folks of many and varied skills all supporting each other and the community. If we could transport a body of people this large on our interstellar ship, then we would have preserved a very great deal. If we could transport an even larger initial number we would have an even greater genetic mix, and the possibility of pre­serving (by transporting) the arts of, say, cake decoration, fine wood carving, lute-making, hedge-laying, computer design, mathematical re­search, philosophy, Flamenco dance, contortion, librarian­ship … and thousands of other arts and practical bodies of knowledge that might not exist in a smaller popu­lation, and which (in a smaller crew) might be considered superfluous.

And these example lists (above) show how we have to choose our skills carefully – librarianship, fine wood carving, hedge-laying and computer design are, in fact, important skills to preserve, and expertise on mediaeval carving and playing Go not as important.


Part of the measure of possibility is the measure of effect. There are two effects – the effects upon the participants (and their descendants) and the effect upon those who do not participate – the rest of the human race.

This is a project that can be undertaken only if we – collectively – decide it is worthwhile. The costs, after all, are huge – both financial and social. I argue in this document (and I am not the first, nor the last to so argue) that the costs of not mak­ing this adventure are even greater – the certain annihilation of the complete hu­man race. I am (at the time of writing) 60 years old. I know that I, personally, will not take part in interstellar travel in any physical way. But I will die more con­tented if I can be persuaded that humankind will at least try to save itself from de­struction. Interstellar travel is part – but only part – of what is required. Also we need to be more forward-looking, more respectful of others (including the future generations), less greedy, more careful, more caring. We need to be – as all the relig­ions have told us for a long time – more loving.



We need to consider:

  • What are the possible designs for a ship? (See “Possible Designs” page 37 be­low)

  • How can a ship be powered? (See” page Error: Reference source not found below)

  • How can a ship be transported – what is its mode of propulsion? (See “Propulsion” page 74, and “Speed” page 78)

  • How can a ship be constructed? (See “Construction” page 42 below, and “Getting There” page 84 below)

  • What do we do with a new planet if we get to one? (See “” page Error: Reference source not found below)

This section, then, is concerned with the hardware engineering aspects – “every­thing you can kick”. The biological and social questions are considered in other sections.

This is an engineering project – so this section on engineering is (inevitably) one of the longest.

Possible Designs

Here my engineering ignorance shows through. I am going to suggest two possible de­signs. The first – for lack of a more subtle suggestion – is a long cylinder with rounded (hemispherical) ends. The second design is a hollowed-out asteroid.


This cylinder would rotate about the long axis of the cylinder, to give pseudo-gravity (centripetal force). The direction of motion would be along the long axis of the cylinder, so that any accelera­tion effects are in a con­stant direction (and the floors could be slopediv to compensate for this.

This design gives an obvious location for the application of thrust, and for a “col­lecting scoop”, to gather space-material (of which more anon). This is also an easy design to il­lustrate in calculations.

For all the apparent advantages of a structure like this, it would be impossibly costly to construct. The matter for building it would either have to be lifted from Terra (costly in energy) or found al­ready in space and processed there – perhaps using Luna as a materials source, and taking advan­tage of its low “gravity well”. If we are going to house 120,000 people29 at the density of the 23rd densest country cur­rently on Terra – Netherlands – which is 392/km2, then we need an area of at least 120000/392= 306 km2.

If we are constructing this as the interior of a single cylinder (the ex­treme, worst, case) then from the formula 2 πrl for the area (where r is the radius and l the length), then we need a cylinder about 17.5 km long, and 5.6 km radius (ignoring the area of end caps) – and this is just the “bio-dome” section for the initial crew. If we were to consider using the density of France (110/ km2), and a final population of 750,000 (three-quarters of a million), then we would have to start with a bio-dome cylinder of internal area 7.5×105/110 = 6818 km2 implying a cylinder of perhaps 110 km long by 10 km radius. Such a long cylinder (width:length::1:5) might flex, so a more stubby shape (shorter, but wider), or a multi-layer structure might be considered.

If we consider (in the first sketch design) the cylinder to be 10,000 metres long, and 2,000 me­tres in diame­ter then it has a total surface area of30 about 1.256×108 m2 (again, ignoring the caps). Atmos­pheric pressure is about 14 lb/in2, or about 14/2.2 kg per (2.54)2 cm2 or about 1 kg/cm2. This is about 107 g/m2 or 104 kg/m2 Thus from atmospheric pressure alone we have to consider a total of about 1.25×1012 kg. Note this is 104 kg/m2 above any other strength required.

The pseudo-gravitational pressure of the contents may be assumed to be the equiva­lent of a col­umn of water 500m high over each point. This is a mass of 5×105 kg. If we add ten times the atmospheric pressure this gives 6×105 kg/m2 or 600 tonnes per square metre.

If we multiply this by three, to give a reasonable margin of error, we get about 1.8×103 tonnes per square metre. Allowing 1cm thickness for each tonne31, we have 1.8×103 cm, or (mini­mum) 18 me­tres – just to withstand the basic pressures32. Hence the suggestion of the mini­mum skin thickness of 25 metres, with (possibly) 50 me­tres in places. (As we have observed earlier, even greater thickness – very much greater – would be used if hollowing out an asteroid.)

With an overall density of 2.5 (a not unreasonable figure: see Ref. [Brit2001]) this means that the mass of the skin alone, at 25 metres thickness, is 7.81×1012 kg or about 7.81×109 tonnes or about 7.81 thousand million tonnes.

But this is a lot of construction from raw materials – and it still does not allow for all the matter that will have to be ejected as the rocket exhaust in propelling the ship.

Hollowed Asteroid

All of the above leads us to suppose that perhaps we should look to another source of large, co­herent masses in space, which could be fashioned into ships, and which do not involve constructing, from small com­ponents, such large structures.

So the potential second design is to (partially) hollow out an exist­ing asteroid.

If we compare the mass of our first sketch (the cylinder) with the mass of known asteroids, 215 Kleopatra has a mass of at least 8.6×108 million tonnes or 8.6×1017 kg. (see Ref: [USNO2004] which gives (1.0 ± 0.1) × 10-12 solar mass, [Brit1988] gives the solar mass as 1.99×1030 kg., hence im­plying 215 Kleopatra has mass of about 2.0×1018 kg.) which is a factor of more than 105 greater than this.

Some of these asteroids, in natural orbit between Mars and Jupi­ter, can be quite large. For example 1 Ceres is about 8.7×1020 kg. in mass, and more than 930 km. in diameter33 and there are numerous (hundreds) known aster­oids that are lar­ger than a 100 km. diameter sphere. This is likely to give us larger ships, that are easier to con­struct. There is, al­ready, enough con­struction matter up there.

For all designs, the thickness of the outer skin has to be able to withstand:

  • the internal pressure of the atmosphere, and

  • the pseudo-gravity, and

  • the longitudinal thrust, and

  • reasonable lateral thrust, and

  • the expected impacts (up to a reasonable limit) during the journey, and

  • the gravitational/tidal effects experienced in the gravita­tional field of a nearby star or planet,


  • the ship has to be to withstand this for at least a hundred thousand years – and more, as we are consider­ing extremely long journeys.

In fact we should be considering even longer – much, much longer periods. Planning for a hundred years is hard. Planning for a thousand years is something we rarely do. For this project we should plan for ten, fifty or a hundred thousand years – something we have never done. But – collectively – we can do it. Now, kids, set your minds on a million years.

There is also a lot of stuff that wants to bump into us. The Earth (Terra) grows in mass by at least two hundred tonnes (two hundred thousand kg.) per day (Ref: [Aste2004], which quotes 300 tonnes per day – a substantially larger figure than we are using here) from the meteoric dust and particles it accumu­latesv. This is partly because the Earth (Terra) is a gravitational sink, attracting matter to it, and it is rea­sonably large, compared to our ship. The ship, though, will still suffer impacts – few because of gravitational attraction, but many by happenstance. And these impacts, even if they are from rather small particles, will be at extremely high relative ve­locities. Being hit by a bullet that travels at 2,000 km. per hour (which is 555.55 m/s) is one thing – but being hit by a bullet that travels at 2,000 km. per second is quite another. And that sort of rela­tive veloc­ity is perfectly feasible.

Remember that kinetic energy is proportional to the square of the velocity – so the relative energies of these two projectiles is not 3600:1 but 12960000:1 – not 3.6×103:1 but 1.296×107:1. If we envisage moving (ultimately) at 0.1c that is about 30,000 km. per second – striking an encountered, stationary, bullet-sized ob­ject at that relative speed is equivalent to ≈ 3.0×1010 or about thirty thousand million times as powerful as a bullet of the same mass. Ouch.

Hence I suspect that we need to think of a skin much thicker than intuition initially suggests. I propose a skin of at least 10 km. Yes, ten kilometres – 104 metres. We need to make more detailed estimates of (a) what the probabilities are of being hit by objects of various sizes, and (b) what the relative velocities of the ship and the impacting object are likely to be, and (c) what degree of shell strength is required to withstand what degree of impact. Once these estimates have been made we will be better able to choose the skin thickness.

When a bullet (or other projectile) hits its target, the entry hole and the exit hole are very different. The entry hole tends to be small, and similar in shape to the projectile. On passing through the outer skin of the target both the target and the projectile are damaged, and energy is transferred. Hence the exit hole (if there is one) tends to be larger and more diffuse.


The techniques of construction depend upon the detailed engineering. The main question which has the greatest influence is:

  • Is there one vessel or many vessels? (Cluster or Singleton).

One hollowed-out asteroid and ten nickel-iron asteroids in convoy are very different engineer­ing tasks.

Cluster of Iron Ships


This seems to be a set of smaller problems than the single ship – each ship is its own nexus of engineering prob­lems, and the cluster of ships is a cluster of these. And despite the title, there is no specific need for the ships to be of iron – though nickel-iron alloy may well turn out to be (a) plentifully available in space, without having to be transported from Terra, or some other deep gravity well, and (b) adequately strong, and (c) ade­quately durable (remembering the specialist long-term meaning of “adequate” in this context).

But it is not simpler than a single ship: it is the same as a single ship – but with the solution repeated a number of times. A cluster is not an avoidance of complexity.

There are sociological considerations to be taken into account for clusters of ships: there must be cooperation, not conflict between them; there must be a means of interchange between them; they must not be too close together, so that some could avoid disasters that overtake others, and so on. Some of this is discussed elsewhere in this document. Here – in this section – we are concerned only with the building of the ships.

Building such ships would be a lot more “fiddly” (and possibly more energy expensive and more time-expensive) than hollowing a single asteroid – but it would allow more choices to be made. For example, one ship could be desert, another could be humid, another could be tropical, another temperate, another arctic.


I have suggested in the title “Iron”, but the body of a ship may be of anything that is strong enough – any mixture of materials. As we do not want to lift huge quantities of matter from the bottom of the gravitational well which is the Earth (Terra), but use readily-avail­able materials, we may well be persuaded into using nickel-iron (Ni-Fe) from some M-type asteroid(s) that we “capture”. Ni-Fe is strong. Given the ab­sence of corrosive substances (on the outside of the ship) we do not have to be too con­cerned about external corrosion (rusting, oxida­tion). The in­side, however, will be warm, oxygen­ated and damp – the sorts of climates and con­ditions human being like to live in – and we do have to be sure that the hull does not rust through from the inside. Remember we have to design for a long period of time – a machine that lasts just a year, or just a cen­tury is very different from one designed to last millennia.

There will have to be means of transferring energy within the ship. This may be radiant energy – which means we need glass or transparent plastics, or current en­ergy – which means we need con­ductive substances (e.g. copper, gold or steel, etc.) and insulators (e.g. rubber or plastic or waxed paper etc. – whatever is appropriate).

We have to be able to grow plants within the ship – and hence we need a good deal of organic matter. Some of this can be manufactured in situ, by living organisms fixing the elemental or inor­ganic compounds they are fed. But we cannot perform transmutation of matter, changing atoms from, say iron into carbon. That is, we will have to ensure there is enough carbon, and nitrogen and oxygen (etc.) to sup­port the large life-mass (biome) we are proposing.

From the section on biology, this means we need 2E+7 kg ( 20,000 tonnes) of car­bon, 1.5×109 kg (1.5 million tonnes) of oxy­gen etc. (see the table 192), which means a total of half a million tonnes of organic base to be transported to the ship(s) before they start (assuming that much of the hydrogen and oxygen can be gathered from the asteroid rocks themselves). <<


There is a minimum size for a ship. That size is controlled by three things (i) the stability of the internal biome, and (ii) being large enough to allow the inhabitants to have a rea­sonable life, remem­bering that this is the only environment that they will have, and (iii) being large enough to support a crew from which a balanced human population can de­scend. Thus these criteria are Stability, Variety, and Qual­ity.

It is size that dictates a lot of the engineering difficulties. Unfortunately, a large size is a require­ment. A small biome is not stable. A small environment would not al­low a large enough population to be sustained – a population large enough to main­tain the engi­neering and culture and develop­ment of a worthwhile human envi­ron­ment. My opinion is that (initially) 10,000 people is the very minimum population that we should consider. The upper limit is constrained only by the engi­neering – from the point of view of this essay we will start by assuming it to be just 500,000 peo­ple. This gives us a broad band for initial consideration.

A population of, say, 100,000 to 120,000 gives us the flexibility of a small city like (for example) Exeter – which would allow us to transport from Terra a wider vari­ety of our skills and learning. This (I suspect) might not be the size of our first IT project – but it is a size we should (at some point) consider.

Since this is a project which concerns saving the whole of the human race (as a spe­cies) we have to consider more that one IT ship – a minimum of three (IMHO) or perhaps “one per century” (with no numerical upper limit) until we run out of Terra’s resources (or come to our senses).


As part of our engineering we will also have to try things out. New bits of engi­neering have to be tested. It might be reasonable, for example, to take a smaller structure (such as of the size of, say 3554 Amun or 1864 Daedelus), and put it into a Pluto orbit, at (say) 20AU. This would be a reason­able test-bed for the large-scale engineering required for the real ship.

A prototype in distant Solar orbit would allow us to check out the technology for very long-term habita­tion in closed, artificial environments. It would allow us to make modifications – perhaps radical ones – whilst still relatively “close” to Terra – al­though 20AU might not normally be considered “close”, it certainly is close com­pared to the ultimate dis­tances for the interstellar ship.

Such a prototype would not be cheap, if by “cheap” you mean “costing only a small amount of money” (the British English meaning of the word), but would (IMHO) be a really necessary first run. Discover­ing that this or that system does not work is not something you want to do at more than 100 AU from Terra, and moving away from it at an appreciable fraction of light-speed.

This prototype would, in itself, be a worthwhile scientific base, allowing us to measure aspects of space not easily accessible from close to Sol. It would also allow us to measure the effects of long-term cultural isolation upon groups of technically sophisti­cated people – but there is more discus­sion of that in a subsequent section of this work (see page ??? et seq. below, and also Ref. [Hall2001], [Karu2003] ) <<<<< INSERT PAGE REFERENCE


Of course the location is in space! But where exactly? If we are hollowing an aster­oid, then we have to start in that asteroid’s orbit. When finally fitting out the ship prior to final launch, however, we may make use of more distant orbits – to get a feel of what it is like to be out of touch of Terra for a long period.

One suggestion, mentioned previously, is to put the ship into a Pluto orbit, or an even more dis­tant orbit at (say) 40 AU. It is reasonable that the starting location is in the as­teroid belt (be­tween 2 and 3 AU from Sol) – if we are hollowing an asteroid, rather than build­ing from scratch, this is the necessary starting location34 - though we may find a usable Aten or an Amor or an Apollo, which intermittently may bring us closer than 2 AU. We may also consider a testing location, much closer to Terra: there is a stable orbital point in the shadow of Terra, about 1,500,000 Km (930,000 miles) further out. This is one of the Lagrangian points35, and it has the advantages of being:

  • close to Terra – it would not take long to get there and back, during the building phase

  • in (partial) shadow – it is a good test location to determine whether the internal systems actually work, with the chance of fixing them from Terra before the ship sets out.

The mean distances of the major planets from Sol, in AU and meters are:






























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