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Antimatter Propulsion: Birth of a Concept

I spent this past weekend poking into antimatter propulsion concepts and in particular looking back at how the idea developed. Two scientists — Les Shepherd and Eugen Sänger — immediately came to mind. I don’t know when Sänger, an Austrian rocket designer who did most of his work in Germany, conceived the idea he would refer to as a ‘photon rocket,’ but he was writing about it by the early 1950s, just as Shepherd was discussing interstellar flight in the pages of the Journal of the British Interplanetary Society. A few thoughts:

Prof. Saenger - Bild

Sänger talked about antimatter propulsion at the 4th International Astronautical Congress, which took place in Zurich in 1953. I don’t have a copy of this presentation, though I know it’s available in a book called Space-Flight Problems (1953), which was published by the Swiss Astronautical Society and bills itself as a complete collection of all the lectures delivered that year in Zurich. If you like to track ideas as much as I do, you’ll possibly be interested in an English-language popularization of the idea in a 1965 book from McGraw Hill, Space Flight: Countdown for the Future, which Sänger wrote and Karl Frucht translated.

Greg Matloff has speculated that what may have drawn Sänger to antimatter is specific impulse, which reaches surreal heights if you can produce an exhaust velocity equal to the speed of light (see The Starflight Handbook for more on Matloff’s thinking). The speed of light being about 3 X 108 m/sec, Matloff worked out a specific impulse of 3 X 107 seconds. Recall that specific impulse measures engine efficiency. In other words, a higher specific impulse produces more thrust for the same amount of propellant.

Sänger must have been dazzled by this ultimate specific impulse, which he conceived possible only through the mutual annihilation of matter with antimatter. But recall that when Sänger was developing these ideas, the only form of antimatter known was the positron, or positively charged electron, which had been discovered by Carl Anderson in 1932 (he would win the Nobel for the work in 1936). When you bring positrons and electrons together, you produce gamma rays, an energetic form of electromagnetic radiation that moves at the speed of light.

Antimatter propulsion solved? Hardly. What the Sänger photon rocket had to do was to create a beam of gamma rays which could be channeled into an exhaust, somehow overcoming the problem that the gamma rays produced by the matter/antimatter annihilation emerge in random directions. They are highly energetic and would penetrate all known materials, a lethal problem for the crew and a showstopper for directed thrust unless Sänger could develop a kind of ‘electron-gas mirror’ to direct the gamma rays. Sänger never solved this problem.

The Radiator Problem


Writing in 1952, Les Shepherd went to work on antimatter equally limited by the fact that only the positron was then known — the antiproton would not be confirmed until 1955 (by Emilio Segrè and Owen Chamberlain — Nobel in 1959). Shepherd was a nuclear fission specialist who helped to found the International Academy of Astronautics and served as president of the International Astronautical Federation (see my obituary for Shepherd from 2012 for more). And his 1952 paper “Interstellar Flight” remains a landmark in the field.

Even without the antiproton, Shepherd would have known about Paul Dirac’s prediction of its existence and doubtless speculated on the possibilities it might afford. As Giovanni Vulpetti told me just after Shepherd’s death:

Dr. Shepherd realized that the matter-antimatter annihilation might have the capability to give a spaceship a high enough speed to reach nearby stars. In other words, the concept of interstellar flight (by/for human beings) may go out from pure fantasy and (slowly) come into Science, simply because the Laws of Physics would, in principle, allow it! This fundamental concept of Astronautics was accepted by investigators in the subsequent three decades, and extended/generalized just before the end of the 2nd millennium.

Vulpetti himself has been a major figure in that extension of the concept, with papers like “Maximum terminal velocity of relativistic rocket” (Acta Astronautica, Vol. 12, No. 2, 1985, pp. 81-90); and “Antimatter Propulsion for Space Exploration” (JBIS Vol. 39, 1986, pp. 391-409). Many back issues of JBIS are available for a fee on the journal’s website (http://www.jbis.org.uk/), though I haven’t yet checked for this one. But be aware that Dr. Vulpetti is also making his papers available on his website (http://www.giovannivulpetti.eu/).

Looking back at Shepherd’s “Interstellar Flight” paper is a fascinating exercise. Assuming that we could solve the Sänger problem, Shepherd saw that there were other issues that made antimatter extremely problematic. Obviously, producing antimatter in the necessary amounts would be a factor, as would the key problem of storing it safely, but Shepherd had something else in mind when he wrote “The most serious factor restricting journeys to the stars, indeed, is not likely to be the limitation on velocity but rather limitation on acceleration.”

The paper then moves to examine what happens as we unleash the power of matter/antimatter annihilation. Have a look at this:

We see that 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…

The result is an emitting surface that would reach temperatures of about 100,000 K. We need, in other words, to dispose of waste heat in the form of thermal radiation. Even assuming a way of channeling the gamma rays of positron/electron annihilation (or looking ahead to other forms of antimatter and their uses), Shepherd could see that accelerations high enough to shorten interstellar flight times drastically would have to solve the thermal dissipation problem.

The real difficulty, always assuming that we can find suitable energy sources for the job, lies in the unfavourable ratio of power dissipation to acceleration as soon as we become involved with high relative velocities. The problem is fundamental to any form of propulsion which involves non-conservative forces (e.g., the thrust of a rocket jet) to produce the necessary acceleration. The only method of acceleration which one can conceive that would not be subject to this difficulty, would be that caused by an external field of force.

So can we produce radiators that can handle temperatures of 100,000 K? Perhaps there are ways, but Shepherd could only note that the matter was so far beyond existing technologies as to make the speculation pointless. Sänger’s photon rocket — or any vehicle somehow creating an exhaust velocity near the speed of light, has to reckon with the radiator problem.

Remarkably ahead of their time, both Les Shepherd and Eugen Sänger helped define the problems of antimatter propulsion even before we had found the antiproton, a form of antimatter that offers new possibilities that would be explored by Robert Forward and many others. But more on that tomorrow.

The Sänger references are given above. Les Shepherd’s ground-breaking paper on interstellar propulsion is “Interstellar Flight,” JBIS, Vol. 11, 149-167, July 1952. For more background on this issue, see Adam Crowl’s Re-thinking the Antimatter Rocket, published here in 2012.


Comments on this entry are closed.

  • Neil S August 1, 2016, 12:52

    I recall an article in the ’50s on a “photon rocket” in a popular-science type magazine. Upon Googling it must have been the February, 1954 Mechanix Illustrated. Here’s a link to pictures. Go here then search the page for “photon”. http://www.projectrho.com/public_html/rocket/spaceageposter.php

    See also http://www.projectrho.com/public_html/rocket/deckplans.php#id–Tinsley_Photon_Rocket

  • Michael August 1, 2016, 15:39

    Might be possible to use a shield of material that surrounds most of the AM/M explosion focusing the light backwards and protecting the crew. The ionised material is then captured by a large magnetic field by following the magnetic lines of force all the way around back to the other end as it does so it radiates it’s heat to space. There may also be a way of getting directional gamma ray ejection by using polerised Bose- Einstein AM/M condensates as they behave as single atoms when ultra cold.

  • Adam August 1, 2016, 15:48

    Hi Paul
    Nice summary. Of course matter-into-energy propulsion has been around for longer than the professional discussion in SF. Olaf Stapledon’s “sub-atomic energy” being released as coherent beams of radiation is one example, in “Last and First Men” and “Starmaker”. Stapledon knew the top speed would be a fraction of lightspeed – he opted for 0.5c. Doc Smith used similar hand-waving in “Skylark of Space” and other tales, though with ludicrous speeds resulting.

    Of course a photon rocket is incredibly destructive through its exhaust alone, a fact Doc Smith, then Larry Niven have used to good effect in their tales. A more benign exhaust would be neutrinos, which – to my knowledge – only Bob Forward has featured in an SF tale.

  • James M Essig August 1, 2016, 16:49

    Perhaps one way of using greatly reduced temperatures is to convert electrical energy generated by the photon rocket mechanism to optical frequency light and back reflect the light by membranous inflatable concentrators. The concentrators could be inflatable, inflatable then rigidizable, rheo-elastic non-pressure deformed membranes, photo-electric non-pressure deformable membranes, and the like.

    Back in the day when my brother John and I were inventing and patenting things related to ultra-low mass reflector technology, we achieve a mass specific power output of 10,000 watts per kilogram using off the shelf reflective membranes. This works out to 10 megawatts per metric ton.

    Using reflector material on the order of 0.001 mils thick (one millionth of an inch thick) would enable power handing by the reflectors of 10 gigwatts per metric ton. Going down to materials on the order of one nanometer thick would enable reflectors to handle 100 gigawatts per metric ton.

    Using grid style reflective membranes which do not involve pressurized inflation could enable mass specific reflective power densities as high as 10 terawatts per metric ton. The good news is that such reflectors would only need to handle power flux densities equal to that of solar radiation as experienced here on the surface of the Earth.

    Grid style reflectors would offer strong drag reduction from massive species in the background and might have nano-technology or other-wise self-assembly repair features.

    My brother John and I came up with a wide variety of inflatable or other-wise pressure deformable reflectors.

    Another option would be to use super-emissive radiators. Super-emissive materials have either been demonstrated and/or are theoretically possible. These materials would radiate much more efficiently than black body emitters.

    Still other methods would involve thermal-diodic materials which would wick heat away very quickly in principle and very efficiently.

    Tantalum-Hafnium-Carbides have melting points as high as 4,400 K so these materials might make good emitters.

    A great facilitator would be a mechanism to convert matter-antimatter annihilation energy directly into electrical current which would then be transmitted to the reflector apparatus via extreme super-conductors.

    A method of entangling the matter-antimatter reactor generated energy, thermal, electrical, or in the forms of non-thermal quasiparticles, directly to the radiators might also be possible. A mechanism for entangling and transporting energy was theoretically proposed as a possibility. The caveat is that the velocity of light transmission would still be a limit for teleporting the energy.

  • Gerry August 1, 2016, 17:22

    So, 3 terawatts per ton for 1-G acceleration, and 100,000 K. What if the acceleration were reduced to 0.1 G? How would that affect the temperature of the radiating surface per square meter? Since the amount of energy being expended per unit of time is reduced by a factor of 10, would that reduce the heat to a perhaps more manageable 10,000K (still very hot); the tradeoff being a longer time in flight, but the longer the cruise phase relative to the time accelerating/decelerating, the less of an issue that would be.

    • Eniac August 2, 2016, 10:49

      Unfortunately, the power emitted by a black body increases with the fourth power of temperature, according to the Stefan Boltzmann law. That means a reduction in acceleration to, say, 1/16th, would only reduce the temperature to 50,000 K.

  • Charlie August 1, 2016, 17:54

    Instead of an enclosed chamber in which the gamma photons would have to become dissipated into surrounding chamber walls, how about a magnetic field confinement chamber consisting of nothing but magnetic field lines, but surrounded by the vacuum of open space ?
    I’m a little head of myself, but I’m speaking of about the proton-antiproton annihilation reaction in which you have some gamma rays, but also mainly charged particles as your primary exhaust.

  • Alex Tolley August 1, 2016, 17:57

    In Willy Ley’s 1964 book, “Beyond the Solar System”, illustrated byChesley Bonestell, the spaceship to make the journey to the stars uses photon propulsion. It was assumed that the ship would accelerate at 1/2 g and reach 0.5c. Bonestell’s illustration of the ship in flight looks like this:

    The radiators are glowing red hot, but perhaps not nearly hot enough. In the forward to the book, Von Braun notes that a photon drive is not yet feasible, but suggests we might eventually get there after a long technological journey.

  • Alex Tolley August 1, 2016, 17:59
  • Vince Teofilo August 1, 2016, 19:27

    The principal reason I initiated a program to study the creation of vacuum energy for space propulsion in 2004 was the obvious fact that ant-matter at a facility such as CERN requires costs of a trillion dollars per mg to produce. The study of methods to produce negative or vacuum energy for warping space that have been performed over the last 10 years indicate costs of the order < $1M per gram.

    • Astronist August 2, 2016, 19:29

      $1M per gram of what? And how many joules of negative or vacuum energy have been produced by those studies so far?

  • Andrew Palfreyman August 2, 2016, 2:52

    The necessary breakthrough for an economically viable matter-antimatter drive hinges on the ability to create the antimatter in situ at decent efficiency. In situ because storage is fraught with difficulty and danger. I see no signs of significant development in efficiency of production. The outlook is gloomy.

    • ljk August 2, 2016, 10:45

      That is why we need to be grateful there is an actual interstellar probe being considered at all:


      No one is producing antimatter in anything resembling usable quantities nor do those facilities feel they have a reason to. This is why antimatter will cost insane amounts of dollars until the antimatter drive equivalent of Breakthrough Initiatives happens.

      Any super wealthy Russians reading this?

    • Eniac August 4, 2016, 10:19

      In-situ? This makes no sense. The whole point of antimatter is to store energy. If you produce it in-situ, where do you get the energy?

      If, somehow, you have the energy, why not use it for propulsion directly, rather than convert to anti-matter and then back to energy at a great loss in efficiency?

  • Jim Early August 2, 2016, 9:33

    The other fundamental problem is to get the energy released converted to a collimated beam. No material can reflect photons at these energies. One would have to convert the energy into kinetic energy of charged particle by Compton scattering before turning the charged particles or to use stimulated emission for the matter-antimatter reaction. I’m sure someone must have written papers about why gamma-ray lasers are extremely difficult or impossible. There is an extensive literature about the “much easier” problem of x-ray lasers.

  • xcalibur August 2, 2016, 10:10

    I see others have brought up magnetic fields. My speculative line of questioning is: how effectively could magnetism be used for controlling a matter/antimatter reaction? Would it be possible to completely shape the flow of energy, to the point of using magnetic fields as an adjustable rocket nozzle? I imagine this would be difficult, but in theoretical discussions, we are concerned with the possible versus the impossible.

  • Dani Eder August 2, 2016, 10:15

    > Sänger must have been dazzled by this ultimate specific impulse, which he conceived possible only through the mutual annihilation of matter with antimatter.

    Assume you have a powerful laser located near the Sun, and a relay mirror about 800 AU opposite the direction you vehicle wants to go. The beam is focused by using the Sun as a gravitational lens. Because the Sun provides an enormous aperture (about 2 million km), the beam can stay focused on your vehicle at interstellar distances.

    The beam is used to power a particle accelerator, which emits propellant particles at relativistic velocities. The exhaust velocity is near c, and the particle mass is *greater* than what you start with in your tanks. The specific impulse can then be greater than what matter-antimatter annihilation can provide. This is possible because the energy is coming from an external source, and is therefore not limited to mc^2. You also can avoid the complexities of making and storing antimatter.

    In more realistic designs, storing antimatter is likely to be highly inefficient. I would not be surprised that the storage system masses more than 1000 times the antimatter mass. In that case, antimatter propulsion is inferior to fusion or beamed energy.

    • Andrew Palfreyman August 3, 2016, 10:57

      I love the idea of using a Sol gravitational lens for propulsion! Claudio Maccone has thus far proposed the use of a Sol gravitational lens as both a telescope and an interstellar communications node.
      There are some good ideas here which deserve fleshing out. Perhaps an arXiv paper or a website/blog description with diagrams?

    • Eniac August 4, 2016, 10:32

      I don’t think it is feasible to use the gravitational lens for power transmission. This is because the gravitational “focus” is not a point, but a line. Unless your receiver can cover the whole line (it can’t), most of the energy will necessarily be wasted.

      You are right, though, if you had, somehow, unlimited power available, a relativistic particle beam would give you much better ISP than an antimatter rocket. The problem here is that particle accelerators tend to have a pitiful power density (beam power/accelerator mass), so acceleration would be extremely low.

    • Eniac August 4, 2016, 10:41

      Actually, you might get a much better power density from a laser than an accelerator, which requires zero propellant and brings us back to the photon rocket.

      A common flashlight is in fact a photon rocket of high efficiency. With LEDs, more than 50% of the power is converted to photon thrust. You can do no better with relativistic particles. It is the power density that kills you.

      Of course, if you are going to beam the energy using light, a regular light-sail will be the simplest and most efficient solution.

      • Michael August 5, 2016, 17:36

        Do you mean specific power, the amount of power to the mass of the drive? Particle accelerators have enormous power and can be very, very light, the reason they are ‘heavy’ on earth is due to the need to keep the air pressure out, in space it is not a problem. The problem with focusing laser light using the Sun is a lot of the beam is still lost due to divergence, but it is still a lot better than without, rapid acceleration is the way to go.

      • Robert August 6, 2016, 14:30

        The ratio of thrust over power is 1/c or 3.33E-9 Newtons per watt for a photon rocket. Photon recycling may increase it by 20,000X or more but Shawyer’s Emdrive promises to increase it to about 0.3N/W which is in
        the range of chemical rockets yet without running out of mass. That’s a
        90 million fold increase in thrust for the same power.

  • ljk August 2, 2016, 10:42

    In 2012 the Icarus Interstellar folks said they were going to name the drop probes for their starship Shepherd Probes after Les. The original post on that from their site seems to be missing, but let us hope they stick to this nice tribute to one of their true pioneers.

  • Eniac August 2, 2016, 10:53

    The key to the photon drive is to realize that the heat we are radiating is not waste heat, but it is the propellant. In a photon rocket, we don’t need to be efficient, we can turn all the energy into heat and then use the black body radiation to propel us.

    The real problem is how to arrange things such that no gamma rays or neutrons or neutrinos or any other particles are generated that would escape and cause losses, or worse, damage. In other words, the usual engineering of “usable power vs. waste heat” is reversed: “usable heat vs. waste radiation”

    So, I think the photon rocket comes down not so much to reflecting gamma rays, but to thermalizing whatever the power source is and keep it contained in a magnetic field as a very hot plasma. The temperature has to be kept low enough such that most of the blackbody radiation can be reflected by a parabolic mirror. It seems 100,000K is not unreasonable, the spectral peak would be in the UV range. I think it is clear that at such high power levels, there is no way to “pipe in” the power, it has to be generated right there and then inside the ultra-hot ball of plasma.

  • Joseph R. Hale August 2, 2016, 10:59

    The frequent use of the word “impulse” in discussing antimatter propulsion makes me wonder whether Gene Roddenberry was familiar with Sanger’s work when he dreamed up “impulse power” for Star Trek.

    • ljk August 3, 2016, 9:27

      I am not entirely certain but Roddenberry did say early on that he wanted a realistic looking starship. Looking into a semi-plausible means of getting around the galaxy at FTL speeds would be part of it I assume.

      Here are some blueprints of the USS Enterprise’s warp engines, so we can get cracking on making this happen:




    • Christopher L. Bennett August 6, 2016, 21:05

      No, “impulse engine” in Trek was intended as just a fancy word for “rocket,” since all rocket propulsion involves impulse. And it’s the warp engines that are powered by antimatter, not the impulse engines (which are nuclear-powered; the second pilot of the original series mentioned “impulse packs” with “points about decayed to lead,” implying uranium fission, although THE NEXT GENERATION established that impulse engines are fusion-powered). In fact, according to THE MAKING OF STAR TREK, the producers and their technical consultants didn’t settle on antimatter as the ship’s power source until the show was already in production, some time after impulse engines were established in the second pilot. (The first mention of antimatter power was in episode 7, “The Naked Time.”)

      However, I do wonder if the term “photon rocket” for an antimatter engine inspired the term “photon torpedo” for ST’s antimatter weapons. I’ve always wondered about that odd usage.

  • Michael August 2, 2016, 16:05

    If we had a clump of anti mater and then hit it with an intense beam of electrons, protons and neutrons the gamma rays would have to fight back against the beam of normal mater which would shield or deflect the worst of it from the rest of the starship.

  • Al Jackson August 2, 2016, 23:09

    A footnote on Eugen Sänger:
    His wife and scientific collaborator Irene Sänger-Bredt wrote in 1977* about Sänger. He made out a list about 1930 of the research he wanted to accomplish, it was short.
    Understand rocket propulsion.
    Hypersonic flight at extreme altitudes.
    Manned space flight.
    Interstellar Flight.
    He accomplished all these at the highest technical level.
    Sänger wrote several papers about relativistic spaceflight and derived many of the results that others had published. There is one computation of his I can’t find any earlier reference to. That is if one studies the relativistic dynamics of accelerating at 1 g for approximately 50 to 60 years , ship time, one can ‘circumnavigate’ the ‘size’ of the universe.
    Carl Sagan repeated this calculation later; I think he gave Sänger credit. Of course Poul Anderson used this as a plot pivot for his novel Tau Zero.

    * I. Sänger-Bredt, The silver bird story: a memoir, R. Cargill Hall (Ed.), Essays on the History of Rocketry & Astronautics, Proceedings of the Third to the Sixth History Symposium of IAA, Vol. 1, NASA, Washinton, DC, 1977.

  • Robert August 3, 2016, 13:31

    3 terra watts per ton! Young Bae’s photon recycling scheme may help a lot. So far in tests Bae has achieved recycling factors around 20,000. That’s 20,000 times the momentum for the same beam energy. Unfortunately, you have to have mirrors close enough to recycle the photons but it’s a start. We can hope Shawyer’s Emdrive works because that effectively increases the momentum by millions of times. Two new peer reviewed theory papers by others have proposed mechanisms to explain how momentum is conserved in the Emdrive.