Starflight Review

In a recent article, Ian Crawford briefly reviews the technical feasibility of starflight.  The main limit is economic: an ability to collect 50,000 tons of (deuterium/helium-3) nuclear fuel (the same weight as today’s total annual uranium mined, but vastly harder to collect) and launch that into space seems nearly sufficient. But of course with advanced robotics, nanotech, etc., that much may not be necessary:

The most technically mature concepts for achieving rapid interstellar travel are those based on nuclear fusion propulsion, of which the Daedalus study (Bond et al., 1978) is still the most detailed engineering assessment available in the literature. Daedalus was designed to accelerate a 450-tonne scientific payload to 12% of the speed of light. … This would permit a travel time of 36 years to the nearest star, although the resulting vehicle would be very massive (requiring approximately 50,000 tonnes of nuclear fuel) and far beyond present capabilities to construct. …

In the decades following the original Daedalus study, technical advances in a number of fields have occurred which may make fusion-powered vehicles of the Daedalus type more practical. … Developments in miniaturization … would ensure that a much less massive payload would be required … The National Ignition Facility … is, albeit unintentionally, building up technical competencies directly relevant to the development of fusion-based space propulsion systems. …

Impacts with interstellar grains will be potentially damaging for space vehicles. … However, the problem is not as severe as [many fear]. … The size of typical interstellar grains … in the solar neighborhood are expected mostly to be submicron in size. … The mass of a 1 um [= 10^-6 m] radius grain of silicate composition is 10^-14 kg, and its kinetic energy at 0.1c is 4.5 J. …

Until recently, it would have been expected that 1 um would be an absolute upper limit for the size of interstellar grains in the warm (T * 6000 K), low-density (n * 0.1–0.2 hydrogen nuclei cm^-3) interstellar cloud [LIC] which surrounds the Sun. However, recent … measurements … have identified a high-mass tail to the local interstellar grain population extending to perhaps as high as 10^-12 kg (i.e., 4.5 um radius). …

Martin (1978) … adopted beryllium as a potential shielding material. … Adopting an interstellar dust density of 6.2 x10^-24 kg m^-3 (i.e., that determined by Landgraf et al., 2000), we find that erosion by interstellar dust at a velocity of 0.1c would be expected to erode of the order of 5 kg m^-2 of shielding material over a 6-light-year flight. The need to provide such shielding will certainly add to the mass of an interstellar probe, but it hardly seems to be the showstopper. …

Interstellar particles even larger than the largest inferred from the spacecraft dust detectors (of the order of 40 um in size) may have been detected by meteor radar observations, and the possibility of colliding with even larger particles over the course of a voyage of several light-years cannot be discounted. Indeed … crudely extrapolating to higher masses, we would infer the spatial density of 100 um grains to be about 4 x10^-17 m^-3. Thus, over the 6 light-year flight … we might expect of the order of two impacts per square meter with such large particles. …

It is … certainly possible to envisage appropriate counter-measures. … By far the simplest solution, developed by Bond (1978) for the Daedalus study, would be for the spacecraft to be preceded by a fine cloud of small dust particles (ejected from the vehicle and thus traveling at the same velocity but a small distance ahead), such that any incoming large grains would be destroyed by collisions within this artificial dust cloud before they have a chance to reach the main vehicle. …

Journeys to the nearer stars with travel times of decades (necessitating velocities of the order of ten percent of the speed of light) will be a considerable technological (as well as economic and political) undertaking. The magnitude of the difficulties should not be underestimated, but neither should they be exaggerated. There is a large technical literature which demonstrates that rapid interstellar space travel is not physically impossible and is a legitimate technological goal for the centuries ahead. (more)

Of course if you want to slow down your ship at the other end instead of flying by at 0.1c, you need a much larger ship, with roughly 10 million tons of fuel.

Added 11a: Apparently, many of you need to review the rocket equation.

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  • Steve X

    Why do you need a larger ship to slow down? Why can’t you turn the ship around and decelerate using the engine?

  • Steve X

    Apologies, I see that project Daedelus then jettisons much of the ship on take off.

    But a project Orion style ship would have similar characteristics wouldn’t it?

  • David

    0.1c seems to me needlessly fast, and at that speed, even small collisions will be incredibly energetic. I would prefer the slower alternative, which I picture like this:

    A huge, ark-like ship that may take about a century or more to get to its destination. For power, we might use something as conventional and cheap as H-bombs, Project-Orion-style. To be honest, I don’t foresee fundamental advances in propulsion in the near future. But I do foresee massive advances in the following areas: Computing, AI, robotics, data storage density, and bio-medical science. It’s this stuff, which we’re about to be really good at, which we should leverage to the maximal degree.

    I picture sending out a “dead” ship, with a whole bunch of frozen genetic material (of humans, animals, plants, etc.). Once the ship parks in orbit around a suitable planet, the fist generation of fertilized eggs would be gestated in an artificial womb. [This is not crazy! Already, we're making great progress towards this from both ends: We can keep cells dividing for quite a long time without implanting them in a person, and incubator technology is also advancing rapidly. I expect that the two could meet in our lifetimes.]

    Once the first generation is born, they would be raised by AI “parents” – which might sound monstrous, but consider this: Do you think that if you gave a bunch of geniuses a bit of funding and ten years, the AI parents they would produce would be worse than the the sadly common bad human parents? TV raises many kids today- not a customized, needs-sensitive expert system with bottomless knowledge. I think that’s the correct test for judging the morality of this. As all this is happening in orbit, autonomous landers are building the infrastructure for a colony on the surface. When this is ready, the inhabitants move in. For the first few generations, they use the other frozen genetic material to guarantee a sustainable diversity, and behold, we have a colony for pennies on the dollar.

    This raises many interesting questions, like: What language will the colonists speak? A natural language, or an optimized one? What sort of ideology should they be indoctrinated into? Protestantism (for work ethic reasons – ha)? How much data should they bring from Earth? Everything? Even porn? Bringing data wouldn’t be difficult. I imagine that the massive shielding could be made of millions of ultra thin layers of Beryllium, all of which would be etched in redundant ways with data. When computers are built from local material, that data would be unspooled and read in orbit.

    Given our strengths, isn’t this the best approach to interstellar colonization?

    • lemmy caution

      “I picture sending out a “dead” ship, with a whole bunch of frozen genetic material (of humans, animals, plants, etc.). Once the ship parks in orbit around a suitable planet, the fist generation of fertilized eggs would be gestated in an artificial womb. [This is not crazy! Already, we're making great progress towards this from both ends: We can keep cells dividing for quite a long time without implanting them in a person, and incubator technology is also advancing rapidly. I expect that the two could meet in our lifetimes.]”

      I agree that we should do this. It would be good to cut down the risk of the extinction of mankind.

  • http://whyiamnot.wordpess.com Salem

    I find it fascinating that in a place like this we can ask so many questions interstellar travel – but not why we would wish to send colonists to another star system at such expense. It seems that the future is very shiny indeed.

    • http://williambswift.blogspot.com/ billswift

      Colonies around other stars, and even further away is better, are the ultimate survival refuge. That said, I don’t think it is likely to happen at all for biological humans.

  • Randaly

    @ Steve X: You also need to double the fuel you take, and that leads to corresponding increases in volume and mass that lead to greater starting requirements as well.

    • Anonymous from UK

      No you *square* the fuel/payload ratio, not double. That’s the killer of the rocket equation.

  • Buck Farmer

    Thanks, Robin, for bringing this up. Starflight is one of the most important mitigations (and sources of) to existential risk for our little pocket of low entropy.

    The top problem is marshalling enough economic resources to make this a reality. Additionally, there are still technical hurdles to building sufficiently sustainable lifesystems as well as psychologically healthy confined societies.

    I’m generally in favor of sending out von Neumann probes and of ark ships to carry our sentient forms and biological heritage to the stars.

    However, I’m very concerned about the risk that our spores (whether biological or technological) catch the sight of any of the universe’s weed-killers. See David Brin’s Lungfish for an excellent exploration of the evolutionary ecology of a universe with multiple types of von Neumann probes from multiple civilizations.

  • Hedonic Treader

    I still don’t understand the morals of this. Successful space colonization without successful (and improbably stable) bioethical abolitionisim will increase the expected value of the total amount of suffering in the universe by orders of magnitude. It will most certainly also increase the expected value of the total number of sentients who are forced to experience a subjective net-negative existence without prior consent to astronomical levels.

    Even if more good life than bad life is created, the victims of malice, or merely unfortunate circumstances, will suffer without the rewarding experience of the pleasures that might be created for other more lucky sentients.

    Now the egoist can say, “I don’t care, I believe in my personal consciousness, and I want to maximize my personal expected utility.” This is why most transhumanists care for longevity research – they want to personally live in a fancyful future. But can the egoist realistically say they will benefit from interstellar travel? The probability seems very low to me. Compared to the expected value of near-term or mid-term hedonistic resource use, the expected utility for the egoist from space colonization is sub-par. So the egoist should prefer to use the resources in the here-and-now (or the mid-term future for themselves), and the negative utilitarian should prefer less life rather than more life (because it implies much less suffering).

    As for the classical utilitarian, maybe they should re-check their philosophical premises? Why is it ok to let even a few suffer, so that (more) others can be happy? The suffering and the happiness don’t overlap, they exist as existentially distinct regions of space-time. Even if you don’t believe in the illusion of persistant individual consciousness, or essential self, this is still true. And if eternalism is true, that suffering is timelessly ontologically real, the “passage” of time will not undo it. The egoist might not care about this, but again, the egoist would not favor space colonization either.

    • Buck Farmer

      This is why I’m not a hedonist (either of the utilitarian or egotistical persuasions).

      I may be an Anti-enthropist…or perhaps a batshitcrazy Kantian…mostly my moral/practical philosophy seems incoherent, but comments like yours help me clarify (or complexify) it.

    • http://infiniteinjury.org/blog/ Peter Gerdes

      The reason it’s called a *premise* is because you accept it prior to analysis. We have a brute judgement when we consider the situation that some amount of additional suffering is acceptable to increase the average.

      Indeed, if we *didn’t* take that view we’d be morally compelled to do our utmost to extinguish life on earth. If you actually bite that bullet I salute you for your consistency but hope someone does away with you soon before you can make progress on that project.

  • scott

    Hedonic Treader: Successful space colonisation will dramatically increase the number of living humans and dramatically reduce the risk of extinction of the species. The ‘morals’ behind it – if any – are evolutionary. We are survival-optimisers first and suffering-minimisers second. Thence comes its appeal. I should note that this desire almost always trumps speculation about the ethical implications of the future.

    • Hedonic Treader

      I agree with the prediction that the actual decisions shaping the future will probably not be based on rational ethical reasoning. But projects such as going to the moon or eventually to the stars are born out of prestige-seeking, not a survival instinct. Most people don’t feel that their individual survival, or that of their family, depends on space colonization. In practice, most people probably don’t care that much about the long-term survival of the species, either.

      Regarding “evolutionary ethics”, it’s not true that everyone is a survival-optimizer. The “right to die” movement wouldn’t make sense otherwise. I personally have an intuitive hedonic calculus in the back of my mind when I make my decisions; I can easily see myself “cut off” sub-par parts of my future by committing strategic suicide at some point or another.

      • Robert Koslover

        The future belongs to those who choose to: (a) survive, and (b) reproduce. All the philosophical arguments in the world, no matter how clever or seemingly-persuasive, simply can’t change that.

  • Finch

    > Of course if you want to slow down your ship at the other end instead of
    > flying by at 0.1c, you need a much larger ship, with roughly 10 million tons
    > of fuel.

    There are technologies, like magsails, which would function as a brake without consuming fuel. Stopping might be easier than accelerating. Magsails aren’t exactly here today, but they are probably less advanced than large fusion engines.

  • Finch
  • Hedonic Treader

    @Robert Koslover:

    “The future belongs to those who choose to: (a) survive, and (b) reproduce.”

    More accurately, what you say is, “The future will contain those replicators that perpetuate themselves into the future.”

    That is true, trivial, and utterly meaningless for normative ethics.

    “All the philosophical arguments in the world, no matter how clever or seemingly-persuasive, simply can’t change that.”

    I didn’t say they could. Nevertheless, there’s nothing moral about creating more non-consensual torture for the sake of “the survival of the species” or “the greater good of humanity”, or whatever phrase people may use to invoke inspiration and draw funding.

    Whatever happens, happens.

    • Matthew

      Hedonic Treader and others who buy into the anti-natalist philosophy, really need to look into meditation, Buddhism and the nature of the “suffering self” you keep coming back to.

    • Jeffrey Soreff

      More accurately, what you say is, “The future will contain those replicators that perpetuate themselves into the future.”

      Well said…

      I once read Why the Reckless Survive by Melvin Konner – the bottom line is that they don’t only their genes survive. And this has the effect of polluting both the social and the genetic environment for everyone else.

  • William Newman

    Robin Hanson writes “Of course if you want to slow down your ship at the other end instead of flying by at 0.1c, you need a much larger ship, with roughly 10 million tons of fuel.”

    Or willingness to roughly double the travel time, using only half the 50k-ton spacecraft’s delta-V to accelerate, and the other half to decelerate.

  • Tony

    The mass of a 1 mm radius grain of silicate composition is 10^-14 kg, and its kinetic energy at 0.1c is 4.5 J. …

    That is a very low density for “silicate”. I would have guessed something more like 10^-6 kg, or about a milligram. Are you sure that isn’t 1 um (micrometer) instead?

    • http://hanson.gmu.edu Robin Hanson

      You are right! I’ve corrected the error now.

  • Brandon Reinhart

    It seems likely to me that any ship that took 36 years to reach its destination would be overtaken by more advanced ships that take fewer years. Say 15 years into that ship’s flight we develop a ship that can reach the destination in 10 years. Your 36 year crew might arrive at their destination to find a base already in place. This raises the question as to whether there’s a sweet spot for launching a mission and whether we should wait until we hit that. If we have a good idea that we’ll one-up our technology before X years, should we wait that long before launching or launch now? Etc.

    • David

      Yeah, I also thought about this “sweet spot” question. One difficult thing about it is that there will be a different sweet spot for mission cost and for arrival time. (Fast missions will be costly, but waiting for technology to develop further will drive down fast-mission costs – so how long should you wait?) This sweet spot will be very different from the sweet spot of “earliest possible arrival”.

      But when it comes to interstellar colonization, I think there’s another important milestone, which is: The earliest affordable launch that actually has a good chance of generating a successful colony. This seems to me more important than hitting one of the sweet spots.

      Even if that mission takes centuries and has a good chance of being preempted by another, I think it’s worth doing. We don’t have enough information about the future to project indefinite exponential improvements arbitrarily far in time. We can sit on our hands and wait for a time when we can do this “better”. The sweet spot might always seem to be in the future. But who knows what black swans are lurking in our future? I say we launch colonists as soon as we possibly can, and live with the fact that their mission will be very slow. At least they will already be on their way if something unexpected happens to us terrestrials. If they’re overtaken, that’s no great harm. If humanity misses its colonization window, it is (arguably).

  • http://www.crossfirefusion.com/thruster Brian

    Helium-3 is extremely rare in Earth’s crust, but boron(p-B11) is plentiful, which could make interstellar starship powered by aneutronic reactor more technically feasible.

  • http://WhatNextBlog.net Levi M -“what next”

    To answer the comment about more life = more suffering I would argue that life is always preferrable to non-life no matter the circumstances.

    We exist in a world of darkness. Our life is a window into the light through which we see the dance of colors, and forms, and patterns, and it’s beautiful, and it’s ugly too, but after a time the window closes again and we return to the darkness…forever. How can one be anything but utterly grateful to be given this window to peer through even for but a moment.

    And also this speculation is based on the nearest star, however the nearest hospitable planet is likely to be much farther away. Thus in order to get there would require many generations of humans.

    I think a better solution to creating one huge “ark” ship is to send out thousands of probes with life seeds (more like extremophile bacteria and spores) to numerous locations throughout the cosmos to ensure the endurance of life, even though most likely it has arisen elsewhere anyways, but we dont know that for sure.

    • Hedonic Treader

      Your metaphor of light and darkness is common, but rather misleading: Non-existence doesn’t imply the experience of darkness. We’ve evolved to be aversive to certain states such as being cold, alone, in the dark, or in spaces that are very tight and constraining, or very open, making us vulnerable. It is unsurprising that memes would evolve that associate being dead with these unpleasant states – religious hell is probably the best example: the aversion to fire is an evolved integrity protection mechanism, and religion merely hijacked its psychological salience. But of course, these experiences are only mental states of a sentient entity while it exists; there is no such thing as darkness for non-life.

      Non-existence is not bad, but utterly neutral. Of course, good life is preferable to non-life because it is interesting and it feels good. But to say it’s prefereable to non-life no matter the circumstance probably underestimates the negativity of severely unpleasant experiences – that would be an epistemic failure.

      The only good argument I can see is that “if there’s more good than bad experiences, then it’s net-positive”. But that implies that consciousness is aggregative in its nature. I think it’s more plausible to see distinct mental states experienced by different organisms (or by the same organism at different times) as structures in distinct space-time locations: Where the suffering exists, the pleasure doesn’t exist. This is timelessly true. If you are tortured tomorrow, but superhappy the day after, an aggregative calculus may be net-positive, but the torture and the happiness are not experienced by the same entity; once you change mental states, you’re no longer the “same” consciousness.

  • anonymous

    Why not just build a big gun in space? Shoot them out of that. Then they’d only need a way to slow down. Leading to less mass to travel with. A long gun could lessen the initial gs and accelerate them without scrambling them.

    Perhaps I watched too many cartoons as a kid?

    • David

      Unless it’s a very long gun (many times the size of the solar system), I suspect that nothing worth transporting could survive that kind of acceleration.

  • http://WhatNextBlog.net Levi M -“what next”

    Oh man, I know non-existence doesnt mean literal darkness. I was speaking metaphorically.

    “But to say it’s prefereable to non-life no matter the circumstance probably underestimates the negativity of severely unpleasant experiences – that would be an epistemic failure.” – hedonic treader

    I will give you this one example to prove my point. Even in concentrations, arguably the worst state of existence for human beings, most people didn’t commit suicide (which would have been easy) even without hope of ever being freed. There was no definite end or relief in sight, it wasn’t hope that kept them going, but simply the basic desire to exist, even if it meant to continue existing in their aweful circumstances. My source is the first hand account of actual prisoners, (If this is a Man/The Truce – Primo Levi)

    • Hedonic Treader

      I think I remember witness reports claiming that people touched electrified fences in order to commit suicide. Be that as it may, suicide obviously exists, if it were self-evident to all people that mere existence is an intrinsic good, there would be no such phenomenon, and there would be no ethical anti-natalist positions either.