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Antimatter: The Heat Problem

My family has had a closer call with ALS than I would ever have wished for, so the news of Stephen Hawking’s death stays with me as I write this morning. I want to finish up my thoughts on antimatter from the last few days, but I have to preface that by noting how stunning Hawking’s non-scientific accomplishment was. In my family’s case, the ALS diagnosis turned out to be mistaken, but there was no doubt about Hawking’s affliction. How on Earth did he live so long with an illness that should have taken him mere years after it was identified?

Hawking’s name will, of course, continue to resonate in these pages — he was simply too major a figure not to be a continuing part of our discussions. With that in mind, and in a ruminative mood anyway, let me turn back to the 1950s, as I did yesterday in our look at Eugen Sänger’s attempt to create the design for an antimatter rocket. Because even as Sänger labored over the idea, one he had been pursuing since the 1930s, Les Shepherd was looking at the antimatter prospect, and coming up with aspects of the problem not previously identified.

Getting a Starship Up to Speed

Shepherd isn’t as well known as he should be to the public, but within the aerospace community he is something of a legend. A specialist in nuclear fusion, his activities within the International Academy of Astronautics (he was a founder) and the International Astronautical Federation (he was its president) were legion, but this morning I turn to “Interstellar Flight,” a Shepherd paper from 1952. This was published just a year before Sänger explained his antimatter rocket ideas to the 4th International Astronautical Congress in Zurich, later published in Space-Flight Problems (1953).

Remember that neither of these scientists knew about the antiproton as anything other than a theoretical construct, which meant that a ‘photon rocket’ in the Sänger mode just wasn’t going to work. But Shepherd saw that even if it could be made to function, antimatter propulsion ran into other difficulties. Producing and storing antimatter were known problems even then, but it was Shepherd who saw that “The most serious factor restricting journeys to the stars, indeed, is not likely to be the limitation on velocity but rather limitation on acceleration.”

This stems from the fact that the matter/antimatter annihilation is so mind-bogglingly powerful. Let me quote Shepherd on this, as the problem is serious:

…a photon rocket accelerating at 1 g would require to dissipate power in the exhaust beam at the fantastic rate of 3 million Megawatts/tonne. If we suppose that the photons take the form of black-body radiation and that there is 1 sq metre of radiating surface available per tonne of vehicle mass then we can obtain the necessary surface temperature from the Stefan-Boltzmann law…

Shepherd worked this out as:

5.7 x 10-8 T4 = 3 x 1012 watts/metre2

with T expressed in degrees Kelvin. So the crux of the problem is that we are producing an emitting surface with a temperature in the range of 100,000 K. The problem with huge temperatures is that we have to find some way of dissipating them. We’d like to get our rocket operating at 1 g acceleration so we could tour the galaxy, using relativistic time dilation to send a crew to the galactic center, for example, within a human lifetime. But we have to dispose of waste heat from the extraordinarily hot emitting surfaces of our spacecraft, because with numbers like these, even the most efficient engine is still going to produce waste heat.

Image: What I liked about the ‘Venture Star’ from James Cameron’s film Avatar was that the design included radiators, clearly visible in this image. How often have we seen the heat problem addressed in any Hollywood offering? Nice work.

Now we can look at Robert Frisbee’s design — an antimatter ’beamed-core’ starship forced by its nature to be thousands of kilometers long and, compared to its length, incredibly thin. Frisbee’s craft assumes, as I mentioned, a beamed-core design, with pions from the annihilation of protons and antiprotons being shaped into a stream of thrust by a magnetic nozzle; i.e., a superconducting magnet. The spacecraft has to be protected against the gamma rays produced in the annihilation process and it needs radiators to bleed off all the heat generated by the engine.

We also need system radiators for the refrigeration systems. Never forget that we’re storing antimatter within a fraction of a degree of absolute zero (-273 C), then levitating it using a magnetic field that takes advantage of the paramagnetism of frozen hydrogen. Thus:

…the width of the main radiator is fixed by the diameter of the superconductor magnet loop. This results in a very long main radiator (e.g., hundreds of km in length), but it does serve to minimize the radiation and dust shields by keeping the overall vehicle long and thin.

Frisbee wryly notes the need to consider the propellant feed in systems like this. After all, we’re trying to send antimatter pellets magnetically down a tube at least hundreds of kilometers long. The pellets are frozen at 1 K, but we’re doing this in an environment where our propellant feed is sitting next to a 1500 K radiator! Frisbee tries to get around this by converting the anti-hydrogen into antiprotons, feeding these down to the engine in the form of a particle beam.

Frisbee’s 40 light-year mission with a duration of 100 years is set up as a four-stage antimatter rocket massing millions of tons, with radiator length for the first stage climbing as high as 7500 kilometers, and computed radiator lengths for the later stages still in the hundreds of kilometers. Frisbee points out that the 123,000 TW of first-stage engine ‘jet’ power demands the dumping of 207,000 TW of 200 MeV gamma rays. Radiator technology will need an extreme upgrade.

And to drop just briefly back to antimatter production, check this out:

The full 4-stage vehicle requires a total antiproton propellant load of 39,300,000 MT. The annihilation (MC2) energy of this much antimatter (plus an equal amount of matter) corresponds to ~17.7 million years of current Human energy output. At current production efficiencies (10-9), the energy required to produce the antiprotons corresponds to ~17.7 quadrillion [1015] years of current Human energy output. For comparison, this is “only” 590 years of the total energy output of sun. Even at the maximum predicted energy efficiency of antiproton production (0.01%), we would need 177 billion years of current Human energy output for production. In terms of production rate, we only need about 4×1021 times the current annual antiproton production rate.

Impossible to build, I’m sure. But papers like these are immensely useful. They illustrate the consequences of taking known theory into the realm of engineering to see what is demanded. We need to know where the showstoppers are to continue exploring, hoping that at some point we find ways to mitigate them. Frisbee’s paper is available online, and repays a close reading. We could use the mind of a future Hawking to attack such intractable problems.

The Les Shepherd paper cited above is “Interstellar Flight,” JBIS, Vol. 11, 149-167, July 1952. The Frisbee paper is “How to Build an Antimatter Rocket for Interstellar Missions,” 39th AIAA/ASME/SAE/ASEE Joint Propulsion Conference and Exhibit, 20-23 July 2003 (full text).


{ 49 comments… add one }
  • djlactin March 14, 2018, 12:57

    The essay did not even consider the dangers to the human passengers or the difficulty in maintaining an ecology to support them. I think the idea of sending a huge generation ship (with a population sufficient to maintain viable genetic diversity) is just impractical, now and forever. Interstellar exploration vehicles will be small, perhaps even microscopic.

    • Harold Daughety March 14, 2018, 14:14

      I am made to wonder what the topic would be if Star Trek had never been broadcast or even written. There is ample reason to return the idea of antimatter propulsion to the realm of fantasy. We have a complete star system to explore and develop, and that should keep us busy for several generations. We do not have a valid reason to try to reach the stars. Maybe in generations to come the how-to knowledge will be developed. Maybe in that distant time we will have been found by E.T. Maybe our wanderlust is simply a manifestation of our species’ immaturity.

    • Thomas Goodey March 14, 2018, 16:11

      The question as to what the payload should be, in ecological terms and societal terms, indeed was not addressed. But many possible approaches have been considered in the science-fiction literature. I would point out that the question of genetic diversity is a non-problem, because frozen human sperm and frozen human eggs weigh almost nothing, and a large number could be preserved in a container shielded from radiation by all the spare water carried for the trip.

    • Jason Sewell March 16, 2018, 12:13

      Concur. The idea that we would send human passengers and their requisite ecology across interstellar distances makes as much sense to me as a 19th-century scientist training a horse and jockey to leap to the moon. Substrate independence will allow “humans” to comfortably cover these distance in tiny crafts much more easily than their flesh-and-blood equivalents ever could.

  • Andrew Palfreyman March 14, 2018, 13:51

    The black hole space drive (Crane & Westmoreland, reviewed here https://www.centauri-dreams.org/2009/11/27/a-universe-optimized-for-starships/ ) is probably the most efficient sublight space drive after antimatter production efficiencies are accounted for. In fact I extended the concept using a staggered collection of Kugelblitzer and obtained vastly improved performance over that of the published paper, using a non-linear solver to optimise the BH masses.

    • Lepton March 15, 2018, 11:15

      Sure, as long as we can produce or obtain those Kutties.

  • Charley March 14, 2018, 13:56

    “The full 4-stage vehicle requires a total antiproton propellant load of 39,300,000 MT. The annihilation (MC2) energy of this much antimatter (plus an equal amount of matter) corresponds to ~17.7 million years of current Human energy output. At current production efficiencies (10-9), the energy required to produce the antiprotons corresponds to ~17.7 quadrillion [1015] years of current Human energy output. For comparison, this is “only” 590 years of the total energy output of sun. Even at the maximum predicted energy efficiency of antiproton production (0.01%), we would need 177 billion years of current Human energy output for production. In terms of production rate, we only need about 4×1021 times the current annual antiproton production rate.”

    So why not use the energy output of sun ??

    • Alex Tolley March 14, 2018, 15:35

      So why not use the energy output of sun ??

      Because it would take nearly 600 years to harvest the anti-matter using the total resources of the sun, presumably from some sort of Dyson sphere/swarm. That alone will take a very long time to build. So by the time the ship launches, star ships using far more economical propulsion systems will have reached teh stars long before.

      • Charley March 14, 2018, 17:04

        what economical propulsion systems ??

        • Jim Baerg March 14, 2018, 21:41

          Some sort of beamed propulsion seems a lot closer to plausibility. Lasers or particle beams to accelerate the ship. A magnetic sail of some sort to slow the ship at the destination.
          There are lots of difficulties to work out, but not as outrageously hard as an antimatter rocket.

      • Charley March 14, 2018, 21:30

        Somehow everything that you just said Alex concerning it, taking 600 years to get the energy equivalent of the antimatter completely rubbed me in the wrong way. Now I haven’t done any calculations on this except in extremely rough manner, nor have I given it any deep, deep thought on the particular energy collection systems that you seem to be proposing, but I have to think right off the top of my head that you’re dead wrong on saying it takes all those centuries to collect the necessary fuel.

        To whit, if I remember my numbers correctly (and I’m not saying I have), but correct me if I’m wrong, but doesn’t the sun convert 4 million tons of hydrogen into helium every second ?
        And is in the energetics ratio for fusion/antimatter somewhere in the neighborhood of 1:100 ? If so, then that would mean that the same total energy output of the sun per second would yield you 4000 tons of antimatter in the same period of time.

        The proposed ship, I think, necessitated the use of 40 million tons of just antimatter along which would mean that assuming total collection of energy would require only approximately 40,000 seconds (approximately 11 hours) to get what you need.

        Now before you jump all over me. You got to realize that I used very rough numbers and I’m working from memory on all of these facts that I believe are true, but I do think that what you stated above does not in any way match the reality of the situation.

        • Lepton March 15, 2018, 11:19

          Have you factored in the 1e-9 inefficiency in antimatter production?

          • Brett Bellmore March 17, 2018, 10:50

            It needs to be remembered that inefficiency is derived from machinery that’s actually optimized for producing science, not antimatter. An accelerator optimized for antimatter production could presumably knock off a few zeros.

  • Michael March 14, 2018, 14:27

    Just wondering if we could use electrons from the heated surface and use a very fast axial rotating magnetic field to throw them out along the magnetic lines of force where they radiate energy synchronously. They arrive back at say near the front of the craft much cooler. Electrons can have a huge cooling effect not only because they are light weight but they will create a large area to radiate energy.

  • Anton M. March 14, 2018, 14:51

    I am just wondering if a hundreds km long structure can withstand 1 g (-ish) stress or crumble.

    • galacsi March 14, 2018, 16:51

      So the only way to the stars is Quantum mechanics. All these Rube Golberg contraptions will never work. This is steal punk SF.

      • galacsi March 17, 2018, 11:17

        Steal Punk : funny ! It could have been “Steel Punk” , but it was “Steam Punck” of course !

  • Brett March 14, 2018, 16:07

    I didn’t even realize it was that bad. 590 times the output of the Sun over the course of a 100-year voyage – that’s wild. Same goes for a planet-diameter radiator array (you’d be struggling mightily just to get the heat to the radiator panels without a ton of it radiating out into your pipes and rocket).

    Yet another reason why I don’t think we’ll do it with antimatter. We’ll star hop between the nearest possible stars, going at comparatively slow speeds (5-10% of the speed of light, or maybe even less), and then set up lasered arrays at sending and destination systems to make it easier to send future ships.

    • Antonio March 15, 2018, 10:36

      To me, going to the other side of the galaxy in just one step didn’t make much sense to begin with. Why would someone want do that, having billions of interesting places to visit in between?

  • Law March 14, 2018, 20:18

    With a mere superlaser output of 60 petawatts, Robert Forward’s countrysized 2stage laser sail (with a decelerator) looks pretty good now.
    Have you heard of the “Valkyrie” tractor configuration for an antimatter-propelled spacecraft (engine in front)? It claims to be able to achieve comparable performance to Frisbee’s monster in a much more compact package, but appears to be based on a much less rigorous analysis. Ditto for the 2stage laser sail using the first stage mirror as a paraboloid reflector to redirect laser light to decelerate the second.
    Also, antimatter rockets are unlikely to achieve bussard ramjet like performance (this seemed to be what you were insinuating in your article?), for they are limited by having to carry their own fuel and propellant. The rocket equation is altered at relstivistic velocities, so an exhaust velocity of 0.5c might not be able to get you up to 0.9999c without a ludicrous mass ratio.

    • Paul Gilster March 14, 2018, 20:57

      Yes, Valkyrie is Charles Pellegrino’s concept. I haven’t had the chance to look at it deeply but need to do that. Thanks for the reminder!

      • Law March 15, 2018, 11:02

        Is Forward’s 2 stage decelerator sail concept considered viable in current expert opinion?

      • Andrei March 15, 2018, 11:35

        Even though Valkyrie starship is an interesting idea.
        There is several parts we have no idea on how to build.

        • Law March 15, 2018, 23:43

          Yep. That was my source too. ☺

          • Andrei March 16, 2018, 15:29

            Aha! A funny but very correct site.
            The original purpose to give information to science fiction writers. When something is fantasy and will not work Mr Chung tell so very clearly.

      • Charley March 16, 2018, 14:36

        what is Charles Pellegrino’s Valkyrie concept ?, in words please

  • Charley March 14, 2018, 21:38

    With regards to the radiator questions being bandied about above in the article, is the situation quite as dire as has been portrayed ?
    What I mean here is that the dumping of all the 200 MeV gamma rays could seem to be sent directly from the reaction zone (in the magnetic nozzle that is) directly passing through the magnetic field into free space and in turn only (hopefully) and extremely small amount could be intercepted by the semi conducting magnet that created the magnetic nozzle as well as the 7500 kilometer support structure that would be sent from the main body of the ship. Again, no calculations here, but hopefully all those terawatts of power could simply passing to free space and might avoid any type of heating issues. Anyone agree, or disagree ?

  • Charley March 14, 2018, 21:40

    Sorry, not some a conducting, and to say superconducting magnet …

  • ole burde March 15, 2018, 9:23

    When we understand just how difficult it will be to reach more than a very small fraction of lightspeed , it is just one more reason to fall back on the less demanding idea of a Generationship , and the general idea of learning to LIVE in space …..and from that perspektive it is strange to observe the almost complete lack of interest in the most urgent , practical babysteps leading in that direction …such as experimenting with rotating space habitats , closed-cycle agriculture , and the prospecting for rawmaterials outside our planet

    • Antonio March 15, 2018, 10:41

      I think suspended animation or cyborgs will make generation ships obsolete even before they are built.

  • James Stilwell March 15, 2018, 11:57

    This post seems to make it probable that humanoid artilects will be going to the stars long before humanity, barring a FTL breakthrough…We need navigation charts anyway if flesh and blood is going to go out at 2% light speed. The Webb telescope and others might enlighten us on what’s out there. Curiosity will overwhelm the wish for absolute safety. Meanwhile human population growth is going to start making demands impossible to meet here on earth. Hawking aid that much years ago.

    • Mephane March 19, 2018, 4:35

      > Meanwhile human population growth is going to start making demands impossible to meet here on earth.

      Mass migration is not going to happen any time soon, even if we actually do colonize Mars at the optimistic time scales projected by SpaceX. Their idea is to have 1 million people living on Mars by the end of this century – which is only a 7000th of the current world population. And even of that million a significant portion, probably the majority, would be born on Mars anyway, and not immigrants from Earth.

  • Gary Wilson March 15, 2018, 12:20

    Let’s get back to discussing more likely scenarios for short and medium term development of propulsion systems. If the technologies discussed here happen at all given our human propensity for self destructive behavior it will be far in the future. Sending robotic probes to nearby star systems is an engineering problem of massive proportions and should give us many new, and possibly revolutionary ideas without bringing up issues such as how to generate and control enormous quantities of anti-matter. Re-visit the idea every thousand years or so to see how much progress has been made.

    • ole burde March 16, 2018, 8:08

      ”Let’s get back to discussing more likely scenarios for short and medium term development of propulsion systems. ” True , but why limit our involvemet to propulsion systems `? At any point in time there are ”vacum areas” of technology where a relatively small effort can give meaningfull results ….right now some of the most empty spaces are 1. rotating habittats with variable gravity , 2. closed cycle agriculture , 3. prospecting for useful materials on the moon , 4 longdistance teleoperation using virtual reality

  • Gary Wilson March 15, 2018, 12:23

    In fact, I’m already looking forward to Paul’s 3018 report on improvements in the manufacture and storage of anti-matter!

  • Brett Bellmore March 15, 2018, 18:44

    So, would it be feasible to simply build an antimatter bomb version of Orion? Granted, it wouldn’t be a photon rocket, but it would still be potentially much better performing than a mere fusion based Orion.

    Alternatively, how about using antimatter to create a sort of mini-Sun behind your craft, and sail on the light from it? The obvious question would be what you could power with the antimatter, that would remain functional, while being in the right temperature range to radiate something you could still reflect, maybe vacuum ultraviolet? Perhaps a magnetically contained plasma?

    Though I must admit I am persuaded that mass beam propulsion is the way to go, allowing you as it does to circumvent the rocket equation, and simplify the propelled craft to the maximum extent. If only it were better suited to slowing down at destinations you don’t already have equipment at.

  • Charley March 16, 2018, 16:03

    In looking through the thread of discussion here on these particular problems associated with the heating difficulties with such an engine I must confess that I have reached the point of somewhat great amusement into what I would cede to be a relatively non-difficult problem, to be honest.

    Why am I so relatively sanguine ? Simply because of the fact that the paper dealing with the particular antimatter rocket that was referenced above seem to ignore some obvious truths on any type of vehicle that would be committed to a serious interstellar mission profile. As well as upcoming and inevitable technological developments.

    What I referring to is the robotic revolution that is even upon us now, and is transforming our life here on the Earth. I hope everyone is familiar with the relatively famous mathematician Johnny von Neumann who had a considerable impact in the 19 forties and fifties on computer technology. He is the father, in a matter speaking of the idea that robotic machines could expand outward into interstellar space, and upon arrival at the destination star they would in fact a program to build further probes that can move out into further interstellar regions. Thus you have an expanding sphere of human driven interstellar explorers who live off the land, so to speak that furthers the interstellar exploration.

    It is not necessary in an interstellar vehicle to carry sufficient amount of fuel to make a journey to the star system, as well as make the return journey back to earth (if that’s the intention).
    Rather, what is done is that you would send only enough fuel to allow you to accelerate and then de-accelerate into the target star system.

    Once you arrive into the target star system your robotic slaves would begin to construct a antimatter factory in orbit about the sun to allow you to collect sufficient fuel, such that you can make the return journey to earth. If that is your intention. This of course would be conducted in concert with your own exploration efforts of that interstellar system which, if you are realistic is probably going to take decades if perhaps not even centuries to thoroughly comb through. Otherwise, what’s the use of even going ? After all, your entire purpose is to INVESTIGATE a new star system and if you are going to investigate a star system, then actually INVESTIGATE IT !!
    Meanwhile your robotic slaves are all the while gathering fuel, such that you can then either continue your journey to another star system are to go home.

    Using such logic and based upon the projected starship that was in the article your fuel mass does not come out to be 882 times your initial payload mass. Rather, it becomes a mere 30 times the dry payload mass of the ship. This would be far, far more manageable and it also just by the virtue of the fact that you do not require such a enormously powerful engine vastly reduces heat load (not to mention the radiation exposure) that would be otherwise necessitated to move all the dead weight fuel back and forth between your destination and your return journey to earth.

    Let’s be honest and practical about all these things. The entire endeavor of going into a new interstellar system is one of the most fraught with danger adventure that any spacefaring species can undertake in the name of science and exploration. To only go with half the fuel load required to make a round-trip journey simply is a another (and should be acceptable) risk that people who would sign up far this journey would be (and should be) willing to take. Thus, given the wide eyed expectation that you are going to take a risky journey should be motivation enough for you to risk having only half the fuel required to make a round-trip journey.

    • Alex Tolley March 17, 2018, 14:35

      If you just send robots on the 1-way journey, they can do much of the investigation, and construction of whatever a succeeding human colonist will need. Transmitting finds back to Sol might be all that is needed, and simpler and cheaper too.

      I prefer the idea of using other drives, like fusion or beamed sails for the journey as these seem more feasible.

      I think Jason Sewell is probably more on the right track. Send entities that can be transported easily. This might be downloaded minds for in situ constructed machine bodies, or possibly embryos for rearing. If we can get past the need for adult meat humans to be the payload, then machine entities are the most suitable way to go, transported in a minimal state and using local resources to “grow” or be constructed. In the few hundred years (or more) before we reach the stars, our technological options for such alternative entities is going to be staggering.

      • Charley March 17, 2018, 17:59

        Not just robots ; robots AND men ( if you wish)

        • Mephane March 19, 2018, 4:41

          I am quite confident that by the time we will actually be able to construct the first interstellar spaceship, that distinction will have long since become meaningless.

          • Brett Bellmore March 19, 2018, 12:22

            I wouldn’t be that confident; In my opinion we’re not all that far from a “Manhattan project” moment for self-reproducing factories. Once we have that technology, we’ll be past K1 and headed towards K2 within decades, and have all the resources necessary to launch starships using beam propulsion.

            The scientific and social developments necessary to erase the distinction between human and robot will take quite a bit longer to arrive.

      • Charley March 19, 2018, 14:38

        I’d like very much to address the issue brought up above by Alex Tolley that proposer requirements might be more easily met by the use of beamed power as a answer to propulsion for traversing starships. While I completely agree that the being power has the obvious advantage that it is not a slave to the Rocket Equation and therefore doesn’t suffer the mass penalties that may be inherent in any kind of onboard fuel scheme, I hasten to point out that that is not the only consideration within this given subject.

        You notice that in the discussion that formed the basis of this article that there was a talk of shielding requirements, not just far the GammaRay heating, but for passenger cruise that would be on the journey, and this would apply to purely robotic missions as well. Shielding will be a major factor. If you are going to go into relativistic flights; ultimately, and who knows when that will be a vessel in transit is going to encounter various degrees, which can range in any size whatsoever, but virtually all of them will have the net effect of possibly creating deleterious if not possibly catastrophic damage to the vessel. It is ultimately unavoidable.
        The problem here is that beamed power craft whether under acceleration or not will be at the mercy of any obstacle that will be in its path. Even if there is a thruster system that can allow the craft to alter its course, so as to prevent collision that introduces a another problem for a beamed spacecraft: reacquiring the beam or the ballistic flight path, such that you can continue your journey onward.

        Obviously, a fueled craft will have the capability of performing engine firings which will simplify reacquiring the trajectory with greater ease than in the former case. As it is not knowing just how far one must deviate from your planned course to avoid collision, it becomes much more difficult to plan and implement this re-acquirement of your flight path. That should give a strong pause to any interstellar proposal using the beamed power method.

        As long as we are contrasting various methods, I’ve often wondered whether it might behoove us to turn to higher anti-matter elements (Helium?, Lithium?, Etc.) as a type of potential fuel for an antimatter drive. Obvious question is why.
        It is not a certainty, but possibly higher atomic number elements may interact with their normal matter counterparts such as that there may be little (or possibly none) gamma emissions among the decay products of their interactions. Thus, it would be well worth the effort to perhaps synthesize these higher atomic number elements with the idea of reducing potential heat stressing problem is on our proposed engine. It might be well worth investigating. As perhaps a surprise made lie down the road of such an types of investigations.

        • Brett Bellmore March 19, 2018, 20:58

          “The problem here is that beamed power craft whether under acceleration or not will be at the mercy of any obstacle that will be in its path.”

          The answer to that is that we already know that the density of matter in interstellar space is remarkably low. And that, furthermore, there’s a power law at work, such that as objects get larger, they get much less common.

          Furthermore, the objects you’re considering dodging are extremely small, and have to be detected at great distance to have any chance of dodging them at all. Let’s consider the case of 5% of light speed, and a ship only 10 meters wide, capable of 1 g acceleration. Just to shift it’s own diameter sideways, it would require about a quarter second. Even with instantaneous response, you’d have to detect the obstacle at least four thousand kilometers away.

          Are you going to detect a pebble at that distance? Instantaneously?

          So it makes sense to resolve this issue by reducing the swept cross section of the craft, providing shielding suitable for gas and dust particles, and otherwise just accepting that interstellar travel is a dangerous enterprise.

  • Michael March 20, 2018, 10:22

    “The problem here is that beamed power craft whether under acceleration or not will be at the mercy of any obstacle that will be in its path.”

    By ‘tacking’ the sail the spacecraft can avoid the obstacle or if it has reflectors or lens the beam power could be concentrated to destroy an obstacle if it is not too big. There are a few methods that can be employed to reduce or remove dust and gases that would destroy the spacecraft.

  • Charley March 20, 2018, 15:54

    At @Michael, @Brett Bellmore
    So what about
    “Even if there is a thruster system that can allow the craft to alter its course, so as to prevent collision that introduces a another problem for a beamed spacecraft: reacquiring the beam or the ballistic flight path, such that you can continue your journey onward.”

    • Brett Bellmore March 21, 2018, 6:24

      Well, of course my position is that at even a few percent of the speed of light, detecting something in time to dodge it is basically impossible. You’re looking for tiny, low albedo objects, thousands of kilometers away, and in interstellar space they’re not conveniently illuminated, either.

      So you’re using something like radar or lidar, and that’s subject to 4th power attenuation. (Because it has to make a round trip.) At the power level necessary to detect a pebble far enough away to have any chance at all of making a course correction in time to matter, you’d likely just be vaporizing the obstacles, instead of detecting them. If, that is, you had the power budget for that.

      But, fortunately, we know that the density of matter in interstellar space is extremely low, (About 1 atom per CC in this area of the galaxy.) because we can see clearly through many lightyears of that space. 99% of that is gas. Of the remaining 1%, almost all of it is dust. Particles you could call “pebbles” are very rare indeed.

      You are, I’ve concluded, best off just setting up a system to deal with gas and dust, and just take your chances with the pebbles. The odds aren’t actually that bad, as long as you make the frontal area of your ship small.

    • Michael March 21, 2018, 17:05

      Due to the size and masses of gas and dust destruction or displacement of the material to disperse it would be better so manoeuvring is not needed. Making yourself as small a target as possible makes a lot of sense. A flat on sail would be torn to pieces a lot sooner than an edge on configuration. A larger craft could use a conduit all the way through the ship from the main engine, it would be mass negative in a sense you would not need a laser or particle beam system to clear gas and dust ahead of the craft. The raw energy of gamma/x-ray and UV and even some charged particles being shallow reflected off the inside of the conduit would have a powerful gas and dust removing ability.

  • Charley March 22, 2018, 15:14

    I’m afraid that I’m going to have to disagree with all the beamed power advocates that are on here and which this particular website seems to heavily lean for.

    In any of these scenarios in which beam power is used to propel substantial loads which would be required for a heavily instrumented and/or manned spacecraft, they are usually talking about a sail that sometimes has a diameter that can cover a good portion of the state of Texas. Texas ! Think about it; a area that covers one of the largest states in the union. And from this, we’re supposed to believe that such a gossamer sail, which has the strength of a spider web, whether under powered or cruise mode is not going to have the living tar beat out of it by not just by any silicate particles or dust (much less some pebble), but simply the interstellar medium of one hydrogen atom per every cubic meter is not going to play havoc with your sail concept.

    Everybody on here seems to have their eyes blinded with some type of sparkly light blinding them to the ludicrousness of such a proposal as this for any relativistic and/or long distance interstellar voyage. While I can understand the mesmerizing aspect of this with regards to the mass savings by the rocket equation, I cannot honestly entertain the idea that this craft could be substantial enough or hearty enough to endure such a voyage. My criticisms of losing the beam path and reacquiring it if some type of obstacle avoidance is needed still stands , what I consider a reasonable criticism of the entire venture.

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