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The Cereal Box

“No matter how these issues are ultimately resolved, Centauri Dreams opts for the notion that even the back of a cereal box may contain its share of mysteries.” I wrote that line in 2005, and if it sounds cryptic, read on to discover its origins, ably described by Christopher Phoenix. I first encountered Christopher in an online discussion group made up of physicists and science fiction writers, where his knack for taking a topic apart always impressed me. A writer whose interest in interstellar flight is lifelong, he is currently turning his love of science fiction into a novel that, he tells me “incorporates some of the ideas we talk about on Centauri Dreams as a background setting.” Today’s essay examines the ideas of a physicist who dismissed the idea of interstellar flight entirely, while using a set of assumptions Christopher has come to challenge.

by Christopher Phoenix


“All this stuff about traveling around the universe in space suits — except for local exploration which I have not discussed — belongs back where it came from, on the cereal box.”

Over fifty years ago, physicist Edward Purcell penned the boldest dismissal of interstellar flight on record in his paper “Radioastronomy and Communication Through Space”. In that paper, Purcell uses the elementary laws of mechanics to refute the possibility of starflight in total. There are many people, of course, who share his belief that we will never reach the stars.

Keeping a firm grounding in the laws of physics is absolutely necessary when researching interstellar travel. A healthy skeptical attitude can help keep researchers honest with themselves. Certainly, not everything we imagine is possible. Nor can we hope to ever reach for the stars if we do not keep our feet firmly planted in reality.

However, sometimes such extreme skepticism deserves some healthy skepticism itself. Even though Purcell’s equations aren’t wrong, he didn’t prove that starflight belongs back on the cereal box. Instead, he defines the problem of interstellar travel in such a way that it seems to be insurmountable.

Radioastronomy and Communication Through Space


Before we begin, I want to quickly introduce Purcell and this paper. Edward M. Purcell made important contributions to physics and radioastronomy. He shared the 1952 Nobel Prize in Physics for discovering nuclear magnetic resonance (NMR) in liquids and solids. Later, Purcell was the first to detect radio emissions from neutral galactic hydrogen, the famous “21cm line”. Many important developments in radioastronomy resulted from his work.

“Radioastronomy and Communication Through Space” was the first paper in the Brookhaven Lecture Series. These lectures were meant to provide a meeting ground for all the scientists at Brookhaven National Laboratory. In this paper, Purcell argued that traditional radio SETI, not interstellar travel, is our only way of learning about other planets in the galaxy.

Image: Edward Mills Purcell (1912–1997). Credit: Wikimedia Commons.

Purcell builds to his conclusion in three sections. The first section discusses then-recent discoveries in radioastronomy. Purcell tells how astronomers mapped the galaxy by observing radio emissions from neutral galactic hydrogen (the 21cm line). He notes in particular that we gathered all this information by capturing an astonishingly tiny amount of radio energy from space. Over nine years, the total amount of radio energy captured by all 21cm observatories added up to less than one erg (10-7 J).

The paper then jumps from radioastronomy to more speculative topics. In the second section, Purcell takes on the idea of interstellar travel and runs some calculations on relativistic rockets. He concludes that interstellar flight is “preposterous”. In the final section of his paper, Purcell argues that radio messages can be sent between the stars for relatively little energy cost, while the energy required for interstellar travel is unobtainable.

I shall primarily discuss the second part of this paper, where Purcell argued against the possibility of interstellar travel.

“This is preposterous!”

From the start, Purcell considered fast interstellar travel as our only option. Purcell noted that relativity is not the obstacle to reaching another star within a single human lifetime. We cannot travel faster than light. However, if a we travel at speeds close to that of light, time dilation becomes an important factor, reducing the amount of time that passes for us on our trip. You will age much less than your friends back home if you travel to the stars at relativistic speeds.

This is perfectly correct, in my view, so far as it goes. Special relativity is reliable. The trouble is not, as we say, with the kinematics but with the energetics… Personally, I believe in special relativity. If it were not reliable, some very expensive machines around here would be in deep trouble.

The problem, Purcell says, is building a rocket capable of carrying out this mission. He develops this argument by examining a particular example flight.

Let us consider a trip to a place 12 light years away, and back. Because we don’t want to take many generations to do it, let us arbitrarily say we will go and come back in 28 years earth time. We will reach a top speed of 99% speed of light in the middle, and slow down and come back. The relativistic transformations show that we will come back in 28 years, only ten years older. This I really believe… Now let us look at the problem of designing a rocket to perform this mission.

So, Purcell has defined the problem in a certain way. The starship must fly to another star and return to Earth within a human lifetime. To do so, it will reach a top speed of 99% the speed of light (C) in the middle of the voyage. The craft is a rocket, and it must carry all its propellant from the beginning of the trip. It cannot refuel anywhere. To reach 99% C within a short amount of time, the rocket must maintain an acceleration of one g for most of the trip.

Having laid out the starting assumptions for our trip, Purcell uses the relativistic rocket equation to calculate the amount of propellant the rocket will require to complete the trip. Remember that rockets are momentum machines. They throw a certain mass of propellant out the back, and the reaction force pushes the rocket. When that propellant is all gone, only the payload remains and the rocket has reached its final speed.

A rocket engine’s performance is determined by its exhaust velocity (Vex). This is the velocity at which propellant leaves the engine as measured by the rocket. The higher the Vex, the more efficiently the rocket engine uses propellant. Engineers refer to rocket efficiency as specific impulse (Isp). A rocket’s specific impulse is determined by its exhaust velocity.

If you have a rocket of a certain Vex, and you want to accelerate it to a certain maximum velocity (Vmax), physics imposes a certain relationship between the initial and final mass of the rocket. Engineers call this ratio a rocket’s mass ratio. This relationship is shown by the rocket equation. Unfortunately, if our Vmax is much larger than our Vex, mass ratios increase exponentially. This is because the rocket must not only accelerate the payload, but also all the as-yet unused propellant. To go faster, you need more propellant, but you need more propellant to carry that propellant- and so on.

So, our next problem is choosing an engine. We want to travel close to the speed of light, so we need an engine with the highest exhaust velocity (and thus highest Isp) possible. Chemical rockets have much to low a Vex to do this- they would require an unimaginably large amount of reaction mass to approach the speed of light. We need a far more powerful engine.

One type of engine that could perform far better than chemical rockets is the nuclear fusion rocket. So, Purcell first proposes using idealized nuclear fusion propellant. In this case, the rocket’s initial mass must be a little over a billion times its final mass to reach 99% C. A ten ton payload will require a ten billion ton rocket at the start of the journey. This is simply too much mass!

We need something far more potent. Purcell turns to idealized matter-antimatter (M/AM) propellant. Again, we assume the fuel is utilized with perfect efficiency. Matter annihilates with antimatter, and the resulting energy is exhausted as massless electromagnetic radiation (gamma rays), giving us a Vex of C. We can’t beat that.


Image: VARIES (Vacuum to Antimatter Rocket Interstellar Explorer System) is a concept developed by Richard Obousy that would create its own antimatter enroute through the use of powerful lasers. Credit: Adrian Mann.

The situation is vastly improved by M/AM propellant. To reach 99% C, the rocket’s initial mass must be only 14 times its final mass. But we must also slow down at the destination, and slowing down requires just as much effort as accelerating in the first place. After that, we must turn the ship around and return to Earth.

So, during the course of our flight, the rocket shall undergo four accelerations. On the trip away from Earth, the rocket will accelerate to 99% C, and then decelerate back down to rest at the destination star. After turning around, it will accelerate back to 99% C on the trip home and then decelerate back down to rest at Earth. To do this, the rocket must start with an initial mass 40,000 times its final mass. To send a ten ton payload on this round trip will require a 400,000 ton rocket, consisting half of matter and half of antimatter.

The starship must accelerate at one g for most of the trip. At the outset of its journey, this rocket must radiate 1018 watts of radiant energy to accelerate its 400,000 tons of mass at one g. This is a little over the total power that the Earth receives from the sun. Only this energy is in gamma rays, which presents a shielding problem for any planet near the ship. In addition, once the rocket achieves relativistic velocities, cosmic dust and gas present a shielding problem for the ship itself. At these speeds, even tiny specks of matter will behave like pinpoint nuclear explosions, and individual protons will be transformed into deadly cosmic rays.

Purcell concludes that these calculations prove that interstellar flight is “preposterous”, in this solar system or any other.

Rigging the game

There isn’t anything wrong with Purcell’s calculations. The problem is that Purcell wants to take this one set of calculations and prove that any form of interstellar travel is impossible. This isn’t very fair, since the starting conditions he picked in his example lead to his pessimistic conclusions. Let’s examine these assumptions.

Purcell’s first assumption is we must travel at 99% C. Why must we travel so fast? Even to complete a trip to a nearby star within a human lifetime, you can travel slower than that. Purcell is committed to these extreme relativistic speeds in order to take advantage of time dilation and complete the round trip in a decade.

If we are willing to travel much slower, perhaps 10% C, or even 1% C, and let multiple generations of crew make the trip, the difficulties are greatly reduced. At slower speeds, propulsion requirements are far more reasonable, and deadly collisions with cosmic dust would be easier to defend against.

Of course, there are many very difficult challenges to solve before we can launch such a ship. The travelers must recycle all their air and water, grow their own food, and build a stable society able to last for centuries. Some form of artificial gravity must be provided to prevent muscle and bone loss in microgravity. The habitable sections of the ship must be shielded from cosmic rays. But none of these represent hard physical limits arising from the laws of mechanics and nothing else.

This is all assuming humans are making the trip. Slow travel is made even easier if humans do not make the trip, just as we have done with our current robotic exploration of the solar system.

The second assumption is the starship must return to Earth. Particularly if we must carry all the propellant we use from the outset, a round trip mission is far more difficult than a one-way trip. But why must the starship return to Earth? There are many interesting missions that do not require the spacecraft to return to Earth. A colonizing expedition does not have or even want to return. Neither does a robotic probe. A fly-by probe like Daedalus doesn’t even need to carry propellant to slow down at the destination.

Purcell’s third questionable assumption is an interstellar vehicle must carry all its energy and reaction mass on board from the start of the trip. Is this really true? Think about in-situ resource utilization. An interstellar expedition could mine propellant from planetoids encountered at the destination. We can use propulsion systems that use the resources present in space, like gravitational assists, solar sails, or even interstellar ramjets. Granted, gravitational assists and solar sails could not get you anywhere near relativistic speeds, but they could work for slower travel.


Image: A Bussard ramjet in flight, as imagined for ESA’s Innovative Technologies from Science Fiction project. Credit: ESA/Manchu.

If the natural resources of space are not sufficient, there are other options. Rockets carry all their energy and reaction mass from the start. Beam-rider propulsion systems are an alternative that leave heavy engines, energy sources, and propellant back home. One such craft is a photon sail pushed by a laser. Another is a spacecraft propelled by a stream of relativistic pellets, each transferring momentum to the craft. As a cursory read of Mallove and Matloff’s excellent book The Starflight Handbook shows, we are not limited to rockets only.

Ultimately, Purcell’s conclusion that all speculation about interstellar travel belongs back “on the cereal box” simply doesn’t hold air in the space vacuum.

SETI vs interstellar travel?

Purcell’s paper underscores an unfortunate split in the ranks of scientists. Many scientists interested in SETI maintain that interstellar flight is simply not feasible for any civilization. They argue that we don’t need to physically travel to other planetary systems in order to learn about the rest of the universe. We need only turn our radio telescopes to the sky and search for broadcasts from more advanced civilizations. If we find them, these advanced civilizations will hopefully tell us everything we want to know. We might even find that mature civilizations in space have formed a galactic community of communicating societies. Perhaps they might allow us to join the conversation once we demonstrate enough maturity to engage in interstellar radio communications. This an exciting possibility, if a bit idealistic, and SETI deserves our support.

However, it is important to realize it is not an either-or question. We can research interstellar travel and carry out SETI searches at the same time. Even if SETI searches find communicative aliens to talk to, that will not negate the usefulness of interstellar travel. We will still need interstellar flight to investigate the countless solar systems where such civilizations are not present, and starflight is absolutely necessary for interstellar migration. But it seems like some SETI supporters don’t see it that way.

Denying starflight has become a fundamental tenant of the SETI worldview. It speaks directly to the question of whether it might be dangerous to contact alien civilizations. Many SETI supporters claim that we don’t have to worry about this question. If we assume interstellar travel is impossible, no civilization in space can physically threaten another. As Purcell claims in his paper:

It [communicating with ETI] is a conversation which is, in the deepest sense, utterly benign. No one can threaten anyone else with objects. We have seen what it takes to send objects around, but one can send information for practically nothing. Here one has the ultimate in philosophical discourse – all you can do is exchange ideas, but you do that to your heart’s content.

In my opinion, this is the real reason why Purcell argues so vehemently against the possibility of interstellar flight. In order for communication with ETI to be completely safe, interstellar travel must be impossible for any civilization anywhere in the universe. Contact with ETI becomes more complicated if there is a possibility of encountering them or their technology physically. Of course, we can’t be entirely sure messages from ETI will be entirely harmless either, if they contain instructions or information that might pose a danger.

I suspect that Purcell’s pessimistic arguments against starflight were driven more by his desire to believe that discourse with aliens comes without risks than a genuine interest in the future of space travel. Whatever the disposition of aliens, we can’t allow our personal hopes and dislikes to bias our conclusions. While interstellar travel is very difficult, we can already conceive of ways that a sufficiently motivated civilization could reach the stars.


Comments on this entry are closed.

  • Michael November 22, 2015, 16:23

    @Eniac November 22, 2015 at 15:31

    ‘How slow do the decelaration sails/pellets get? It seems to me that you could easily get into a situation where you need to have sent them millenia ago, which kind of defeats the purpose of a fast journey.’

    The last few deceleration sails/pellets will be at high speed and they will have to be slowed down in order to be used efficiently. Vaporising the fuel to allow it to be electromagnetically collected and absorb the high punch energy better may be the way to go. The concept looks to work best with the high speed flyby approach though.

  • NS November 22, 2015, 16:38

    “For all intents and purposes uploaded”.

    I agree that behavioral replicas of human minds (and maybe even if human individuals) may someday be possible, either through artificial brains or breakthroughs in computation as Eniac suggests. But while behavior is observable by third parties, it may be impossible to know anything about the consciousness (if any) of the replicas, which is part of what Eniac believes can also be replicated. Based on behavior we may assume consciousness in the replicas (as indeed we do in each other) but if the replicas’ “brains” are drastically different from human ones, it may forever be an assumption.

  • david lewis November 22, 2015, 18:53

    Given the energy to get an object up to 99% that of light, a fast ship is going to be far more restricted in the opportunities available to its crew. Every ounce will count, and that will mean cutting costs in things that would make life comfortable. It will also mean a smaller crew, meaning less opportunity for socialization.

    Sure, you might get to a place faster, but so what. When you get there you’re still x light years from home, and I doubt the children born to those crew will be able to return to earth; as has been mentioned, it takes a heck of a lot of energy to get something up to 99% the speed of light, which means no return.

    A fast ship also means the odds of a colony in another star system being successful is reduced. It will be started with less people and less resources with no redundancy when something goes wrong. With a world ship you can place a village on a world, and wait a decade. Should the village fail there’s the world ship to retreat back to where they can rethink their plans, and try again.

    The only benefit of a fast ship is getting there faster, but that doesn’t change the situation for any children born once you’re there. Those children would start a life under much harsher restraints than they would on a world ship, and there still would be no place for them to escape to. It’s not like we’re going to find a ton of planets perfectly suited to human life. They will be stuck with the much smaller ship, or a tiny base on a planet. It would be centuries before the population increased enough to properly call it a colony.

    Any argument made based on children born to a life they didn’t ask for can also be made to those born from the crew of a fast ship. Actually, more so. But then the same can be said of every child on earth; they didn’t chose the situation into which they’re born.

    But then I would expect that life on a world ship would be at least as good as the average life of someone living in Europe, or the US. Perhaps a fair bit better.

  • Eniac November 22, 2015, 21:53


    Based on behavior we may assume consciousness in the replicas (as indeed we do in each other) but if the replicas’ “brains” are drastically different from human ones, it may forever be an assumption.

    So it is. However, as you allude to, it has never been anything other than an assumption regarding other people. Philosophers have been discussing this for a long time, a conclusion is not in sight, and AI is not presenting anything radically new to this question.

    I myself believe that this is an altogether ill-posed question. If you define consciousness in any clear and rational way, there must be an amount of observation of behavior that establishes it to be present (or absent). Otherwise, the definition is meaningless. There is then no good reason a sufficiently complex machine could not live up to that standard.

  • John November 23, 2015, 8:31

    Perhaps the problems with the generational ship idea, as mentioned by Charlie and others, mainly arise because the idea does not go far enough. Why not go really big, and really slow: There are serious proposals for plans to move whole planets, and other stars are known to come within a lightyear of the Sun every 100,000 years or so – could a planet be made to jump to the passing star?

  • swage November 23, 2015, 11:02

    The way i see it there are three possible solutions to the problem, each one respective to the relationship of mass, time/energy and speed.

    The simplest solution is certainly to use very small vehicles with external propulsion, such as beamed propulsion solutions. If it works in our particle accelerators, it works also in space. The drawback is that this solution requires a level of sophistication in nanotechnology we have not reached at this time and maybe will never reach. The good news is we may not even have to reach the technological prowess to do so an could simply use constructs naturally occurring, aka real, living microbes. Its of course not manned spaceflight, but based on this concept we are very close to the ability to seed the entire galaxy with colonies… of microbes. Utilizing beamed propulsion, the light of creation may indeed be a rather literal thing instead of a theosophic construct. This raises a few interesting questions about the origin life on our planet, as well.

    The moderately difficult path is certainly using vast vehicles with comparably low power, not breaking the speed but the time limit. Generation ships are such a solution or maybe a hibernation solution could be employed with respect to cryogenics.

    The most difficult solution is the high-energy one, with the most viable propulsion technology either totally removing the propulsion from the spacecraft (again beamed propulsion on a light sail) or by using apocalyptic drive systems like Zubrins nuclear saltwater rocket. To be fair it has to be mentioned that 99% lightspeed is very energy demanding, but 25% is nothing to sneeze at, either. Of course the relativistic effects in aging at relativistic speeds DO increase the range a human can travel in a lifetime very considerably.

  • Mark Zambelli November 23, 2015, 11:11

    The objections so far brought up against generational ships are dealing with only one possible format for the ship… namely a picture of the worst version possible is being painted and while I’m all for the generational ship idea I certainly wouldn’t step foot aboard any ship that has been so incompetantly run so as to include the possibility of the crew going mad or the ship to break and not be fixable, or where the arrivees have to fight for survival like some medieval colony taking it’s chances for a better life.

    To be clear, we will only be launching a generational ship when we’re ready, ie after we have solved a lot of things, be that from a crew perspective or an engineering perspective.

    Psych-evaluations for volunteers, large numbers of crew, frozen genetic material, autonomous error correction and structural repair, 3d printing tech, total/partial virtual reality (‘Matrix’/’ST holodecks), total or partial cryosleep, large ship, total control of the biosphere, computer/narrow-AI redundancy backups, constant telecoms lock with the receding Earth so not cut-off, complete library of all human knowledge/books/movies… etc. All these and lots more would have to be included in the mission plan and none I see as being a fundamentally insurmountable problem.

    To refrain from making the same mistake Purcell did, we mustn’t be so narrow and cherry-pick the type of mission no one would ever launch. Any objections raised by early 21st century peeps like us would surely need to be adressed before even thinking of setting off. Neil, Buzz and Mike wouldn’t have been sent to the lunar surface if we thought they wouldn’t be coming back. Similarly the first generational ship will leave when the tech is mature and proven.

  • BRS November 23, 2015, 12:00

    400 million people will live past age 400 before (in the reference frame of the sun) any spacecraft of ours enters orbit around a star other than the Sun. At even odds I would bet heavily in favor of this proposition.

  • Michael November 23, 2015, 13:26

    @Alex Tolley November 22, 2015 at 13:00

    ‘Thank you. I see how the combination or fuel and sail works. Are there any published papers on this that I can study?’

    The idea surrounds the use of sails but not in the transfer of momentum but fuels. This article represents Kare’s concept crossed with a Bussard ramjet and pellet runway concept, there are doubtless many articles on the net about it or variants.


    And as discussed on CD before



    Fission fuels such as plutonium could work very well, not only are they very dense they have higher strengths and vaporisation temperatures than fusion fuels and could handle high temperature accelerations. The cone sails would need to be smart, perhaps controlling the distribution of light by controlling the rim of the cone electrically, magnetically or thermally would enable them to self control to remain on beam and at the correct speed.

  • Robert November 23, 2015, 15:23


    “It just seems inevitable that somebody would be unhappy and that would fester in their souls. As was said, there are people who live in small tiny villages on earth and can handle that, very, very well. Of course they can handle it very, very well because they know for a fact that whenever they wish they can leave that situation for a while – and later on, go back to their village refreshed and ready to go. NOT SO ON A STARSHIP. ”

    Generation starships would likely evolve out of centuries of experience with space colonies, many with little if any physical contact with other colonies. But throughout most of human history people lived in small isolated communities so that is nothing new. There will still be the possibility of communication with the larger civilization through radio. And it’s not like there are no unhappy people on earth.

  • Alex Tolley November 23, 2015, 15:39

    @Al Jackson – possibly this is an example?

  • Alex Tolley November 23, 2015, 16:28

    @Robert. I agree with you. Charlie’s comments do not apply to the vast majority of history when most people lived and died in their village. In medieval England the peasants were not even allowed to leave. Island colonies, especially in Polynesia would be similarly restrictive.

    My sense is that if we build O’Neill type colonies, especially the large Island 3’s, that these would be perfect vehicles to start slow boating to the stars. However since recycling is never 100% perfect, they might not be able to survive voyages of many thousands of years.

  • Alex Tolley November 23, 2015, 16:31

    @John – I am not clear what the advantage is of moving the Earth to another star. far better to move it within the solar system to maintain its habitability. After all, the sun will last billions of years before it burns out. One might even be able to move the Earth to survive the red giant stage.

    Perhaps better to send a small moon to the stars instead?

  • Eniac November 23, 2015, 23:42

    Alex & John: We’re agonizing over how much energy a starship would cost to move, so now we want to move a planet or a small moon? That costs about a million times more energy (or a million million, perhaps), and would probably take a million times as long, too.


    The simplest solution is certainly to use very small vehicles with external propulsion, such as beamed propulsion solutions. If it works in our particle accelerators, it works also in space.

    In space, yes, but only for particles. Anything larger is MUCH harder to accelerate, and the way we do it with particles simply won’t do. The reasons mainly have to do with the charge/mass ratio. For anything larger than a small molecule, let alone a vehicle, q/m is far too small and far too variable to allow efficient acceleration or accurate deflection in a magnetic field.

    BRS and others: I fail to see how a longer life span allows longer journeys, in any way. People change over time just as much as societies do, so the person who arrives will be as different from the person who left as a multi-generational family would be. Also, I doubt that a longer life will make people more patient in any way. Twenty years away from home is very close to forever, no matter how long you live.

  • John November 24, 2015, 3:48

    @ Alex Tolley My thinking was that (very long term) if we keep hopping then Earth’s habitable lifespan becomes indefinite – although which world we use isn’t important, as long as it provides enough space to avoid the objections raised about being in a closed environment. I figure that such a move would be an ultimate outgrowth of efforts to move inhabited worlds away from the Sun as it’s brightness increases with age, assuming we still need habitable wortlds as we recognise them today (and whoever ‘we ‘ is at that point), Don’t take this as being a well thought out master plan of mine, what I really wanted to illustrate was that a generation ship big enough to avoid the objections raised about people trapped in closed environments would need to be de-facto a world, not a ship, big enough to satidfy their human needs for space and fredom.

  • Harold November 24, 2015, 11:52

    I think the strongest counter to Purcell’s argument is the list of all the potential vectors for interstellar travel: manned, robotic, Ai, Al, generational, etc. The counter to his argument becomes weaker when we focus on any one vector. In the comments, I think this is best demonstrated by the discussion of generational ships and the exaggerated description of their emergent social stability. A persistently self sufficient space colony is no more equivalent to a tiny Earth than a bottle of water is the equivalent to an ocean. The scale of the Earth allows it to absorb war, terrorism, religion, psychopathy, sociopathy, murder, theft, rebellion, etc. A self contained space colony will be orders of magnitude more vulnerable to an individual citizen than Earth or any planet. As well, the more persistently self sufficient a space habitat is, the more likely a rebellion against the mission is to succeed. And there are rational reasons for the middle generations to rebel. A space colony traveling at 10% c is in more danger than a space colony not moving at 10% c.

    There are certainly ways to increase the success rate of a generational ship but they will not be free societies. A free society allows its citizens to choose their destiny. A generational ship must stay on mission.

  • BRS November 24, 2015, 11:57

    @Eniac: I brought up longer lifespans in response to discussion of the ethics of the “generation ship” concept. Asking people who have previously agreed to undertake a long mission to uphold their prior commitment is very different from requiring people to work in furtherance of a mission they never agreed to undertake and will never see to fruition.

    Everything else you said is absolutely contrary to my own experiences and intuitions. People don’t change as fast as societies do: just look at any elderly person to verify this. I am much more similar to myself 20 years ago than to my father, likewise the people I have known many years and their parents. Many apparent changes are due to a different station in life rather than a fundamentally different character. I think living for hundreds of years would make people much more patient (and temperate, and wise).

  • Alex Tolley November 24, 2015, 12:38


    My thinking was that (very long term) if we keep hopping then Earth’s habitable lifespan becomes indefinite

    Asimov pointed out teh fallacy of that argument a long time ago. The stars you are hopping to are aging as well, so there is little benefit. He made the analogy that it was like moving to a new tree in thunderstorm when the rain got through the leaves of the tree you are standing under. That next tree would also be similarly wet below.

    The best approach would be to pick a slow burning red dwarf for longevity.

  • Alex Tolley November 24, 2015, 12:40


    We’re agonizing over how much energy a starship would cost to move, so now we want to move a planet or a small moon?

    I think it is purely a thought experiment. I don’t think it makes much sense. But be aware that we have talked about moving suns by directing the energy output, so perhaps it is not such a fantastical idea.

  • Charlie November 24, 2015, 13:17

    “The most difficult solution is the high-energy one, with the most viable propulsion technology either totally removing the propulsion from the spacecraft (again beamed propulsion on a light sail) or by using apocalyptic drive systems like Zubrins nuclear saltwater rocket. ”

    You quote the following: “… by using apocalyptic drive systems …”; I’m sorry , an ‘ apocalyptic drive systems ‘. What exactly is an apocalyptic drive’ ? Apocalyptic ?

  • Charlie November 24, 2015, 13:30

    @ Michael

    in response to what you said concerning people living in villages their entire life. I point to you what was stated later on:
    @Harold … “A persistently self sufficient space colony is no more equivalent to a tiny Earth than a bottle of water is the equivalent to an ocean. The scale of the Earth allows it to absorb war, terrorism, religion, psychopathy, sociopathy, murder, theft, rebellion, etc. A self contained space colony will be orders of magnitude more vulnerable to an individual citizen than Earth or any planet. As well, the more persistently self sufficient a space habitat is, the more likely a rebellion against the mission is to succeed. And there are rational reasons for the middle generations to rebel. A space colony traveling at 10% c is in more danger than a space colony not moving at 10% c. ”

    ‘war, terrorism, religion, psychopathy, sociopathy, murder, theft, rebellion, etc.’ that says it all to me, and he’s right that the earth can absorb so much more in terms of unhappy people than a small tin can group of individuals. The repercussions of which is highly magnified by the close space and EXTREME vulnerability that such a ship would have alone and in the vacuum of space. The earth can sort of a lot of punishment possibly even nuclear war. But a tin can ? It could be an absolute disaster.
    With respect to the statement you made concerning plutonium and its profitability as a fuel, let me point out that a fusion system is pound for pound, eight times more energy dense than the equivalent wheat in fissionable material. Check that out. The volumes of the materials, of course, could be different, and that may make a difference, but in so far as energy density you can’t really at the present time, the nuclear fusion.

    I’m not a proponent of the solar sail business because of the fact that it would probably be far more costly than any kind of onboard fueling situation that could be devised and I do believe that cost is going to be a major factor. Solar sailing also has a extreme disadvantage in the fact that if you are using beamed power to allow yourself to accelerate you must retained the course you’re on, come hell or high water.
    Why is that important ? Say you’re on a collision course with some object, you must make a course correction, can you be certain that you can retain the beam that you were on to re-accelerate ? A tight focus being on a sale might not be as easy to re-obtain as you might think.

  • david lewis November 24, 2015, 16:57

    war, terrorism, religion, psychopathy, sociopathy, murder, theft, rebellion, etc.’

    The initial base of a fast ship will be much more vulnerable to those issues than a colony ship. The fast ship arrives in a new system, fine, but what then? If it’s not a one-way suicide mission by the crew then they will have children. Why would those children be any happier with their situation than children born on a world ship would be? Not only would they have less opportunities compared to a child born on earth, they would have less than a child born on a world ship. A single terrorist act by any one of those children could easily wreck the mission.

    Just because you get there faster doesn’t mean a thing when you’re carrying less supplies, less people, and less redundancy. A world ship isn’t an actual world, but it can survive a lot more damage than a fast ship and the initial base it sets up.

    Even doubling its population every 20 years, the base set up by a fast ship with 100 people would take 140 years to reach the population of a world ship that has a population of 10,000 – 15,000. Just because it’s on a planet doesn’t make that base secure. That world will be hostile in some way, and even minor damage to any base could doom that base.

    It would also probably be an order of magnitude, or two, harder to get a ship of 100 people up to 0.99c and then slow it down than to get a world ship of 10,000 people up to 0.01c and slow it down.

  • Eniac November 24, 2015, 20:17

    Alex Tolley:

    But be aware that we have talked about moving suns by directing the energy output, so perhaps it is not such a fantastical idea.

    I am well aware of that, and I recall from that same discussion a quick calculation that shows it takes on the order of a billion years to significantly change the velocity of a star this way.

    After all, such a star is nothing other than a photon rocket, with a very small mass fraction that can be burned, and a very long burn time.

  • John November 24, 2015, 20:42

    @Alex Tolley: I agree about the red dwarf stars being the best lng term homes, but I’m not sure I follow the rest – stars are different ages, with different lifespans. So, to make Asimov’s meterphour a bit more accurate: The ‘trees’ are dripping at different speeds, and started getting rained on at different times, so some will not yet be wet, some will stay dry longer than others. All will ultimately get dripped through, forcing a move to a tree that is more waterproof, or which has been being rained on a shorter time. No star could be home forever.

  • Michael November 26, 2015, 9:52

    I am not convinced we need stars to go about our business, if we can create our own mini stars using fusion we would not have to go to other star systems other than for fuel but we would go between them. A well insulated habitat would reduce the amount of energy substantially as heat would need to go downwards against a gradient limiting its loss.

  • Alex Tolley November 26, 2015, 13:19

    @Michael – I think John’s point was that we might need a planet sized biosphere for long term sustainability, and that O’Neill’s and their relations are unproven in that regards. It is a reasonable question to ask at this juncture in our technological development. Ecologists will often deride the idea of “small” colonies because of known issues of small ecosystems, and we have Biosphere II as an example of failure.

    I tend to agree with Eniac, that moving a star, or even a planet, makes little sense with the technologies we can envisage today. I prefer a very different approach that doesn’t require long travel times, uses relatively little energy, and doesn’t require people being confined to living out their lives in habitats for many generations.

  • Eniac November 29, 2015, 14:23

    A reminder: Red dwarfs live trillions of years, so moving from a dying sun to one of those buys you quite a bit. It’s like moving from a tree to a bus shelter, really.

    Another reminder: On these timescales, the galaxy is a swirling vortex mixing up everything. Stars that are neighbors now will be at opposite ends later, and vice versa. We only need to master the hop to the very next star to eventually find ourselves spread everywhere.

  • Michael December 1, 2015, 11:53

    @Eniac November 29, 2015 at 14:23

    ‘A reminder: Red dwarfs live trillions of years, so moving from a dying sun to one of those buys you quite a bit. It’s like moving from a tree to a bus shelter, really…’

    Eventually they turn Blue, so our future selves if ‘we’ are still around will have to rename them.

  • StanFL December 2, 2015, 0:49

    This discussion seems to have fallen into the trap of assuming that we advance in physics and starship engineering, and everything else stays about the same as it is now. Why not assume that genetics and biology are also moving forward, and making spectacular advances as well. By the time a starship is ready, we can simply grow people or other organisms when the ship gets to another planet, eons after leaving. No need for all the hotel load for a multi-generational crew.

  • Alex Tolley December 2, 2015, 11:48

    @StanFL – I think teh problem is that people tend to see the issues of rearing children without biological parents as much harder than building a worldship. The problems of designing and maintaining a closed recycling system seem more tractable than the unknowns of creating artificial wombs, raising children with machines however intelligent, and so on.

    This is probably simply due to the our particular vantage point in technological development. After all, O’Neill’s were proposed in the 1970’s, and we can always strap on engines and fuel tanks to make a slow boat to the stars using designs as old at the BIS’ Daedalus and current Icarus projects . This seems like more straightforward engineering and is certainly full of less unknowns.

    Given the pace of new biological knowledge, the situation may be mostly reversed in a generation or two.

  • Stevo Darkly December 2, 2015, 19:44

    Like many of you, I admit that I am not very comfortable with the idea of a generation starship traveling alone between the stars for decades, centuries or millennia. I am afraid that the society inside that tin can might feel very claustrophobic, isolated and fragile.

    However, these downsides of a generation starship might be mitigated by launching not just one such ship toward a particular target, but two of them traveling traveling together. Or half a dozen. Or a fleet of 20. The ships would be somewhat physically isolated from each other, but it would also be possible to take a shuttle from one to the other. They would also be able to exchange transmissions of news, arts and entertainments.

    Each ship could have its own particular ecosystem and perhaps also its own culture, and be able to experiment with different forms of government. It would be possible to visit a different generation ship from one’s own for a change of pace, at least once in a while, and even (up to the limits of the population size a ship could support) change residences from one ship to another.

    Multiple ships traveling in company, but with a certain degree of diversity and independence from each other, could do a lot to lessen the feelings of isolation and claustrophobia that might arise in a single ship traveling alone. It would also provide a degree of redundancy and source of nearby aid if one ship was struck by a physical or social catastrophe.

    And finally: I’m not sure what kind of government or economy you’d be able to set up here, but it might be possible to have the multiple ships in the fleet somehow “compete” with each other, while they travel, to provide the most attractive physical environment and society, and attract the smartest, hardest working, etc., immigrants from each other. That would be a driver of progress and help prevent the onboard societies from stagnating or degenerating to tyranny as they might if they were in isolation.

  • Eniac December 4, 2015, 23:58

    Multiple ships is a good idea. One thing to think about, though, is that competing societies might also go to war against each other, with disastrous consequences for the mission.

  • Michael December 5, 2015, 10:38

    @Eniac December 4, 2015 at 23:58

    ‘Multiple ships is a good idea. One thing to think about, though, is that competing societies might also go to war against each other, with disastrous consequences for the mission.’

    They also give an huge baseline for communication systems to and from Earth, multiple ships are the way to go, perhaps in a line so if the first one gets hit it will protect the others.

  • Stevo Darkly December 10, 2015, 8:39

    @Eniac December 4, 2015 at 23:58

    “One thing to think about, though, is that competing societies might also go to war against each other, with disastrous consequences for the mission.”

    Yes, something to think about. Even if the ships aren’t deliberately armed with weapons per se, they could probably rig something up to hurt each other with. Zapping each other with communication lasers at close range, maybe. Or using a remote-controlled shuttle or probe to ram another ship. Aside from fore (and aft?) shield against micrometeoroids in the direction of acceleration/deceleration, I would image the ships would have relatively fragile areas.

    However, I also think it would be relatively easy to set up something like, “If you attack my ship, I’ll attack yours.” A mutual assured destruction scenario, and hopefully mutual deterrence. I would hope that if two ship-societies decided they couldn’t coexist together, rather than physically attack each other — an attack that could so easily become mutually destructive — they would instead move physically away from each other to make both visits and attacks more difficult, and cease official communication.

    Although … before it got to that point, I could also see conflict between two ships taking this route: One sending agents to another to assassinate the targets leaders, or whatever, and take over … That could be destructive to the mission without being so physically destructive to the ships.

    But on the other other hand, you could still have that kind of conflict taking place between hostile factions within one isolated ship. It still seems safer to have multiple ships than just one ship. In fact, if conflict arises within a ship that’s part of a larger fleet, a possible solution would be for one faction to emigrate to another ship rather than live in the same ship as their enemies. At least they wouldn’t be trapped in the same vessel with their enemies, possibly having to see and interact with them every day, which might increase the chances of erupting into violent conflict. In a single-ship mission, there would be nowhere for enemy groups to go.