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An Interstellar Provocation

It had never occurred to me that there was something the Graf Zeppelin and the Saturn V had in common. Nonetheless, a re-reading of Freeman Dyson’s paper “Interstellar Transport” confirms the obvious connection: Like the great airships of the 1930s, the Saturn V was huge and carried a payload that was absurdly small. Dyson, writing in 1968 fresh off the end of Project Orion, the rise of Apollo, and the triumph of chemical propulsion, had thought at one time that the US could bypass the Saturn V and its ilk, offering a fast track to the planets at a fraction of Apollo’s cost. The Atmospheric Test Ban Treaty of 1963 was a major factor in putting an end to that speculation.

I mentioned yesterday that I thought Dyson set about to be deliberately provocative in this piece, that he hoped to reach people who would have been unaware that interstellar distances could conceivably be crossed (thus his choice of Physics Today as his venue). To do that, he had to show that even reaching the Moon was a stretch for chemical methods, which he characterized as “…not bad for pottering around near the Earth, but… very uneconomic for anything beyond that.” While an Apollo mission to the Moon demanded staging and a huge mass ratio, an Orion vessel was built with only one stage, its mass ratio well under 10 even for long journeys out and around the Solar System.

Image: Dyson’s largest concept, a ‘super-Orion’ carrying colonists on an 1800 year journey. Credit: Adrian Mann.

Orion could have managed this because the exhaust velocity of the debris from its nuclear explosions would be in the thousands of kilometers per second range instead of what the chemical rocket could offer with its paltry 3 kilometers per second. Dyson assumed the use of hydrogen bombs (“the only way we know to burn the cheapest fuel we have, deuterium”) and a conservative energy yield of one megaton per ton, going on to say this:

These numbers represent the absolute lower limit of what could be done with our present resources and technology if we were forced by some astronomical catastrophe to send a Noah’s ark out of the wreckage of the solar system. With about 1 Gross National Product we could send a payload of a few million tons (for example a small town like Princeton with about 20,000 people) on a trip at about 1000 km/sec or 1 parsec per 1000 years. As a voyage of colonization a trip as slow as this does not make much sense on a human time scale. A nonhuman species, longer lived or accustomed to thinking in terms of millenia rather than years, might find the conditions acceptable.

Anyone who has spent time in the absurdly pretty town of Princeton NJ, where Dyson has lived for years while pursuing his work at the Institute for Advanced Studies, knows why he coupled a familiar scene with something as joltingly unfamiliar as a starship. The choice is reflective of his method: Dyson expresses the results of his calculations in tableaux that are both publicly accessible and mind-jarring, as a look through almost any of his books will demonstrate (think, for example, of his idea of a life-form that might poke out from an inner sea onto the surface ice of a Kuiper Belt object, a kelp-like, mirrored being he christened a ‘sunflower’). Root one end of an idea in the everyday, the other in a mind-bending direction, and you make your point memorable, which is one reason Dyson has inspired so many young people to be scientists.

Remember, the intent here was to get the Orion idea into the public discussion, along with an interstellar implication that Orion’s original designers had never built into their thinking. Dyson always knew that if you put the idea out there, the next step is to get to work on the specifics, detail after patient detail, work that on the interstellar level would presumably involve many generations. When remembering Dyson’s involvement with Project Orion, I think about something he once told Stewart Brand (in a Wired interview):

You can’t possibly get a good technology going without an enormous number of failures. It’s a universal rule. If you look at bicycles, there were thousands of weird models built and tried before they found the one that really worked. You could never design a bicycle theoretically. Even now, after we’ve been building them for 100 years, it’s very difficult to understand just why a bicycle works—it’s even difficult to formulate it as a mathematical problem. But just by trial and error, we found out how to do it, and the error was essential.

It’s the same method we would have used for Orion if the project had proceeded, but the number of factors working against it proved insurmountable, and here one of Dyson’s greatest strengths — his ability to engage the public — was running up against a growing public distrust of nuclear technologies. But the point is that theory always couples with engineering practice, hammering on a problem until the best solution is reached. Unless, of course, the kind of bureaucracy that Dyson so disliked steps in to muzzle the research early on. A bit of that dislike comes across in the conclusion of “Interstellar Transport,” as he ponders what a starship would achieve:

By the time the first interstellar colonists go out they will know a great deal that we do not know about the places to which they are going, about their own biological makeup, about the art of living in strange environments. They will certainly achieve two things at the end of their century-long voyages. One is assurance of the survival of the human species, assurance against even the worst imaginable of natural or manmade catastrophes that may overwhelm mankind within the solar system. The other is total independence from any possible interference by the home government. In my opinion these objectives would make such an enterprise worthwhile, and I am confident that it will appear even more worthwhile to the inhabitants of our overcrowded and vulnerable planet in the 22nd century.

Dyson looked at questions of cost and energy production and assumed a continued economic growth of what today seems like a sizzling 4% per year. Working out the cost of the Orion starship (he figured 1011 dollars), he concluded that such a mission would be as economically feasible in the future some 200 years off as a Saturn V was in 1968. We can argue about such numbers (and be sure to check the comments from yesterday, where a fruitful discussion on the implications of exponential economic growth is continuing) but I suspect they are the first instance of a methodical prediction on when starflight will occur that most readers of Physics Today had ever encountered.

The paper thus comes into focus as a landmark in introducing a pulsed fusion concept to a wide audience, explaining its deep space potential, and calculating when an interstellar future might be possible. I can see why Greg Matloff considers it a key factor in the growth of the interstellar movement because of its broad audience and energizing effect. But tomorrow I’ll make the case for a slightly earlier paper’s even more profound effect on the public perception of interstellar flight, one that has played into our media imaginings of traveling among the stars ever since its publication.

Dyson’s paper “Interstellar Transport,” ran in Physics Today October 1968, pp. 41-45, and is available online.


Comments on this entry are closed.

  • david July 11, 2012, 10:35

    Politics! A century ship would have to have its own government probably based on the government from which it hails. It seems like that “space government” would have to actually start here on earth during the construction and commencement of the project. A government inside a government focused on getting off the planet. Or even a culture inside a government with its own by-laws. The question is how to do the testing for the nuclear engine. Maybe SpaceX will figure it out….

  • Alex Tolley July 11, 2012, 10:58

    It would have been an interesting exercise had Dyson’s proposal followed the even more successful trajectory of O’Neill’s space colonies idea that was launched in exactly the same publication, almost a decade later. We would have had books, conferences and some serious study, a body of work that would have jump started the idea over a quarter century ago.

    One could argue that they were complementary ideas, as worldships are space colonies with engines.

  • Interstellar Bill July 11, 2012, 13:21

    Just a little quibble about the Saturn V:
    it’s payload wasn’t just the teeny Apollo capsule
    but the entire third stage, the SIVB with LEM,
    for a total 250,000 pounds delivered to LEO,
    which is a payload fraction of 5%,
    vs. the Shuttle’s 1% (now that’s ‘absurdly small’).
    At $100M each, we should have just kept making the Saturn V.

  • FrankH July 11, 2012, 15:18

    Dyson’s Orion (or it’s modern equivalents) is still the only realistic, achievable propulsion method that can open up the solar system (if not beyond) to human exploration.
    One solution would be to build and launch the spacecraft in Lunar orbit. Not only does it give us a reason to go back to the Moon, it minimizes the nuclear waste impact on the Earth’s atmosphere/magnetosphere.
    The Nuclear Test Ban Treaty can be broken, or interpreted to allow Orion-type spacecraft; it’s not testing nuclear explosives in space – its using them for propulsion.

  • Christopher Phoenix July 11, 2012, 17:20

    @Interstellar Bill

    Redefining the word “payload” as meaning the whole upper stage of the Saturn V does not make chemical rockets efficient. Staging is exactly the problem. Saturn V weighed 3,350 tons and could send 2 tons to a soft landing on the Moon. An basic interplanetary Orion vehicle would weigh 4000 tons and could send 1,200 tons to a lunar soft landing. With a single stage. And a mass ratio well under ten. Which do you think has better performance?

    Imagine if you got on an airliner that was completely filled with fuel and had seats for only three passengers, and then you threw away pieces of the plane all the way to your destination. Do you think airlines could turn a profit if jetliners worked like that?

    Why should we keep investing in inefficient- if majestic- chemical rockets when nuclear propulsion would be so much more powerful, efficient, and cost-effective? It is too late for Saturn V, anyway, since most of the contractors are no longer in business and the craft would have to be entirely recreated from scratch. It would be cheaper to just build a new rocket.

  • Astronist July 11, 2012, 20:04

    I wonder whether it was Dyson who was first to link starship-building together with the idea of escaping a catastrophe on Earth or in the Solar System, exemplified in two of the quotations above?

    (As I may have mentioned before, such a link appears to me to be both unnecessary, and harmful to the public image of the interstellar enterprise: we want to run away while the rest of you die horribly.)

    Oxford, UK

  • David J. Williams July 12, 2012, 1:21

    Thus far, this is the great Missed Chance of our species.

  • ljk July 12, 2012, 8:43

    Let me say again as I have elsewhere in this blog: Which nation has an advanced space program, five space launch facilities (and building a sixth), a sophisticated nuclear program both for power and the military, the humanpower, the resources, and a huge barren desert to test and launch an Orion from Earth’s surface? Oh yes, and a lack of squeamishness when it comes to nuclear energy.

    Now, will they ever actually build or even have plans to do Orion? I do not know, but I am just saying they are probably the best bet right now for Orion to ever become a reality. I might have included Russia in this a few decades ago, but most of their future space plans seem to be mainly talk. The private space industries cannot focus on anything involving nuclear engines or bombs, at least for the forseeable future.

  • Holger July 12, 2012, 11:27

    Is the “10 to the 11 dollars” price tag for Orion correctly quoted? That’s only 100 billion dollars and would be economically feasible already today, not just in 160 years, it’s hardly more expensive than the Apollo program was.

  • Paul Gilster July 12, 2012, 12:54

    Holger, 1011 is indeed the figure Dyson uses.

  • Randy McDonald July 13, 2012, 9:25

    That doesn’t take into account inflation. This online calculator


    suggest that 1 1968 US dollar would be the equivalent of 6.66 US dollars now in 2012.

  • Holger July 15, 2012, 13:13

    @Paul Gilster:
    Thanks for the confirmation. But then the 200 years timeframe claim doesn’t work: according to Wikipedia, a single Saturn V cost about 400 million $ in 1968; if Dyson assumed real economic growth of 4 % (i.e. adjusted for inflation), it takes “only” some 125 years (i.e., till 2093) to make 100 billion $ the same percentage of GDP as 400 million $ was in 1968. (And it’s not necessary to get the Orion mission that cheap: Apollo consisted of 15 Saturn V’s plus lunar modules etc., while a single Orion ship would already be quite an achievement.)

    Actually, I “over-accounted” for inflation in my post: the nominal price for the Apollo program back in the sixties was only ~25 billion $. So if economy grew by 4 % above inflation (8.4% nominally, according to your calculator’s 4.4% average inflation) for the last 44 years, Orion would actually be cheaper (in proportion to GDP) today than Apollo was back then.