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

LR_Shepherd

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.

tzf_img_post

Toward a Beamed Core Drive

If you didn’t see this morning’s spectacular launch of the SpaceX Falcon 9, be sure to check out the video (and it would be a good day to follow @elonmusk on Twitter, too). As we open the era of private launches to resupply the International Space Station, it’s humbling to contrast how exhilarating this morning feels with the great distances we have to traverse before missions to another star become a serious possibility. We’ve been talking the last few days about the promise of antimatter, but while the potential for liberating massive amounts of energy is undeniable, the problems of achieving antimatter propulsion are huge.

So we have to make a lot of leaps when speculating about what might happen. But let’s assume just for the sake of argument that the problem analyzed yesterday — how to produce antimatter in quantity — is solved. What kind of antimatter engine would we build? If everything else were optimum, we’d surely try to master a beamed core drive, the pure product of the matter/antimatter annihilation sequence. Protons and antiprotons are injected into a magnetic nozzle, blowing out the back at a substantial percentage of the speed of light. This is the kind of rocket analyzed by Ronan Keane (Western Reserve Academy) and Wei-Ming Zhang (Kent State University) in the paper I’ve been skirting around the edges of these past few days.

Channeling Antimatter’s Energies

The paper, headed for publication in the Journal of the British Interplanetary Society, has the provocative title “Beamed Core Antimatter Propulsion: Engine Design and Optimization,” and it deals with the particle stream emerging from proton/antiproton collisions. What you get when you put the two together are gamma rays and pions, some of the latter charged and some neutral. Almost immediately the pions decay into positrons and electrons, which meet each other and produce gamma rays. But the tens of nanoseconds the pions take to decay gives us long enough to channel the charged pions through a magnetic nozzle to produce the needed thrust.

Image: Antimatter promises fast transportation throughout the Solar System and the opportunity for interstellar probes, but only if we can master its production and storage. New work is explaining how efficient an antimatter engine might be. Credit: Positronics Research, LLC.

The beamed core engine, then, is all about channeling the pions into a focused flow. Get this right and you’ve got a lot of energy to work with. In fact, Keane and Zhang note that the energy released per kilogram of annihilating antimatter and matter is 9 X 1016 joules, which is two billion times more than the thermal energy from burning a kilogram of hydrocarbon, and over a thousand times larger than burning a kilogram of fuel in a nuclear fission reactor. But while the beamed core engine is attractive because of the high relativistic velocities of the charged particles produced by the annihilation reactions, the situation is not ideal.

For one thing, much of the energy of the reaction goes into producing electrically neutral particles, which are impervious to the workings of a magnetic nozzle and thus cannot contribute to thrust. The other problem is that the nozzles we’ve been able to analyze have efficiency problems of their own in terms of creating the tight beam of thrust we’d like to produce. What Keane and Zhang do is to use software called Geant4 from the CERN accelerator laboratory to produce simulations of the interactions of particles with matter and fields. They want to bring previous studies of beamed core concepts up to date especially in terms of magnetic nozzles.

Robert Frisbee has performed rigorous studies of beamed core concepts in which magnetic nozzle efficiency is only about 36 percent, which means that while you’re dealing with pions that are initially moving at 90 percent of light speed and above, the exhaust velocity of the rocket would be just a third of that amount. Keane and Zhang derive an efficiency that is better than twice that, and manage to reach charged pion exhaust speeds of 69 percent of c. They also show that the initial speed of charged pions in a beamed core engine is actually closer to .81c than Frisbee’s 90 percent-plus. Despite the lower initial speed, the nozzle efficiencies make quite a difference depending on the kind of mission being attempted:

Frisbee’s papers explain in depth the needed generalization to account for emission of uncharged particles… When loss of propellant is taken into account, Frisbee has shown that ve ~ 0.3c leads to a beamed core rocket facing daunting challenges in reaching a true relativistic cruise speed on a one-way interstellar mission where deceleration at the destination (a “rendezvous” mission) would be involved.

Fuel requirements become critical with lower nozzle efficiencies:

… with a payload of 100 metric tons, a 4-stage beamed core rocket designed for a cruise speed of 0.42c on a 40 light-year rendezvous mission would require 40 million tons of antimatter fuel. If the cruise speed were limited to 0.25c or less, only two stages might be needed, and Frisbee envisaged viable interstellar missions with as few as one beamed core stage; in such scenarios, fuel requirements would be dramatically lower.

All of this at 36 percent nozzle efficiency. The new numbers change the picture, with Keane and Zhang stating “With the new reference point of ve =0.69c provided by the present Geant-based simulation, true relativistic speeds once more become a possibility using the highest performance beamed core propulsion in the distant future.”

Note the ‘distant future’ caveat, highly significant when you consider our problems in producing antimatter (or harvesting same) and the perhaps even more intractable issues involved in storage.

On Software and Methodology

But even if we can’t put a timeframe on something as futuristic as a beamed core rocket, we can continue to study the concept, and it’s heartening to see Keane and Zhang’s conclusion that the simulation software at CERN has proven robust in meeting this challenge and updating our numbers. Whether or not Keane and Zhang’s methodology is on target may be another issue, as Adam Crowl noted recently in a post to a private mailing list of aerospace engineers. Crowl hastens to add that his computations are provisional, but let me quote (with his permission) where he is right now on the magnetic nozzle efficiency issue:

There’s a problem with using just the exhaust velocity given to *part* of the fuel/propellant. It means the actual mass-ratio for a given delta-vee is quite different to a naive computation using the classic Tsiolkovskii equation. A more useful figure of merit for rockets with mass-loss in addition to reaction mass is specific impulse – momentum change per unit mass of fuel/propellant. Using the equations derived by Shawn Westmoreland and the rather vague particle energies in Zhang & Keane, the effective specific impulse is ~0.28c. Even with a perfect jet efficiency the Isp is just 0.31c.

The antimatter reaction, then, may not offer as much as we hoped:

The 0.81c average particle speed quoted in the paper isn’t as useful as the spread of kinetic energies in the particles produced, or the total kinetic energy in the distribution, but they don’t report either figure. What it does imply is that an antimatter-matter reaction puts about 11% of the mass-energy into the charged particles. Not exactly spectacular.

The chance to go to work on concepts through papers in the preprint process is invaluable, and we’ll see how Crowl’s thinking, as well as Keane and Zhang’s, evolves with further study of the issues in this paper. One thing is for sure: Given the manifest problems of antimatter production and storage, we’ll have no shortage of time in which to consider these matters before the question of actually producing this kind of antimatter rocket becomes pressing.

The paper is Keane and Zhang, “Beamed Core Antimatter Propulsion: Engine Design and Optimization,” accepted by JBIS (preprint).

tzf_img_post

Antimatter: The Production Problem

Antimatter is so tantalizing a prospect for propulsion that every time a new slant on using it appears, I try to figure out its implications for long-haul missions. But the news, however interesting, is inevitably balanced by the reality of production problems. There’s no question that antimatter is potent stuff, with the potential for dealing out a thousand times the energy of a nuclear fission reaction. Use hydrogen as a working fluid heated up by antimatter and 10 milligrams of antimatter can give you the kick of 120 tonnes of conventional rocket fuel. If we could get the cost down to $10 million per milligram, antimatter propulsion would be less expensive than nuclear fission methods, depending on the efficiency of the design.

But how to reduce the cost? Current estimates show that producing antimatter in today’s accelerator laboratories runs the total up to $100 trillion per gram. But when I was researching my Centauri Dreams book, I spent some time going through the collection of Robert Forward’s papers at the University of Alabama-Huntsville, where several boxes of materials are stored in Salmon Library. Forward was constantly working in a number of different fields, always keeping his eye on the latest research, and as part of that effort he produced a series of newsletters on antimatter developments that he circulated among colleagues.

Image: A Penn State artist’s concept of an antimatter-powered Mars ship with equipment and crew landers at the right, and the engine, with magnetic nozzles, at left. Credit: PSU.

Reading through these materials, I came to see that when we quote the $100 trillion per gram figure, we’re talking about antimatter as produced more or less as a byproduct. Forward understood and appreciated the science requirements of particle accelerator labs but also saw that they were hardly the most efficient place to produce antimatter in any quantity. They were not, after all, in the propulsion business. He proceeded to do a study for the US Air Force looking at what might happen if an antimatter facility were actually designed for no other purpose than the creation of antiprotons, finding that the energy efficiency could be raised from one part in 60 million to a part in 10,000, or 0.01 percent.

The cost of building the factory, meanwhile, could be lowered dramatically, to the point where Forward believed our $10 million per milligram would be within reach. This is an interesting figure in several ways. As noted above, it makes antimatter feasible for certain kinds of space missions (assuming equivalent advances in our methods of antimatter storage. But as the price begins to drop, we can expect to find new applications in other areas of research, which should drive demand and spur further work on efficient production. It’s worth remembering that even at today’s prices, antimatter has proven its worth in scientific research and medical uses.

What about other ways of lowering the cost? One possibility is to look beyond slamming high-energy protons into heavy-nuclei targets. Writing with Joel Davis in a book called Mirror Matter: Pioneering Antimatter Physics (Wiley, 1988), Forward looked at options like heavy ion beam colliders, in which beams of heavy ions like uranium could be collided to produce 1018 antiprotons per second (with acknowledged problems in creating large amounts of nuclear debris). He also considered new generations of superconducting magnets to create magnetic focusing fields near the region where the beams collide, which should make tighter beams and greater antimatter production possible.

I bring all this up because the possibility of harvesting antimatter from natural sources in space, which we talked about last week, has to be weighed against boosting production here on Earth. But Forward’s ideas actually coupled the two notions. He wanted to move antimatter production by humans into space in the form of huge factories. Here’s what he has to say on this in an essay in his book Indistinguishable from Magic (Baen, 1995):

Where will we get the energy to run these magic matter factories? Some of the prototype factories will be built on Earth, but for large scale production we certainly don’t want to power these machines by burning fossil fuels on Earth. There is plenty of energy in space. At the distance of the Earth from the Sun, the Sun delivers over a kilowatt of energy for each square meter of collector, or a gigawatt (1,000,000,000 watts) per square kilometer. A collector array of one hundred kilometers on a side would provide a power input of ten terawatts (10,000,000,000,000), enough to run a number of antimatter factories at full power, producing a gram of antimatter a day.

We’re a long, long way from producing a gram of antimatter a day, of course, which is why studies like the recent one performed by Ronan Keane (Western Reserve Academy) and Wei-Ming Zhang (Kent State University) have such a futuristic air. But it’s important to learn the theoretical constraints on propulsion systems even if the required antimatter isn’t available, and on that score, Keane and Zhang are thinking ahead to the most advanced kind of antimatter of them all, a beamed core drive. To make it work, assuming you have the antimatter available, you need to inject protons and antiprotons into a magnetic nozzle, one that channels charged pions from the matter/antimatter annihilation into a focused beam of powerful thrust.

Although charged pions decay quickly, they can start out at 90 percent of the speed of light. Unfortunately, earlier magnetic nozzle calculations have proven inefficient at channeling these energies, dropping the exhaust velocity down to a third of this value. Tomorrow we’ll look at how a more efficient magnetic nozzle can produce better results, as Keane and Zhang have analyzed using CERN software to simulate what would go on in the hellish interior of a beamed core antimatter engine. But we also need to consider other ways of using antimatter for propulsion, assuming that Forward’s space-borne factories aren’t going to be coming online any time soon.

tzf_img_post

Antimatter: Finding the Fuel

In Stephen Baxter’s wonderful novel Ark (Roc, 2010), a team of scientists works desperately to come up with an interstellar spacecraft while epic floods threaten the Earth. The backdrop gives Baxter the chance to work through many of our current ideas about propulsion, from starships riding a wave of nuclear explosions (Orion) to antimatter possibilities and on into Alcubierre warp drive territory. I won’t give away the solution, but will say that it partly involves antimatter used in an unorthodox way, and because Baxter’s is a near-term Earth, there simply isn’t enough antimatter to go around. That means getting to Jupiter first to harvest it.

Antimatter in space is an idea that James Bickford (Draper Laboratory) analyzed in a Phase II study for NASA’s Institute for Advanced Concepts, for he had realized that high-energy galactic cosmic rays interacting with the interstellar medium (and also with the upper atmospheres of planets in the Solar System) produce antimatter. In fact, Bickford’s calculations showed that about a kilogram of antiprotons enter the Solar System every second, though little of this reaches the Earth. To harvest some of this incoming antimatter, you need a planet with a strong magnetic field, so Jupiter is a natural bet for Baxter’s scientists, who go there to forage.

The odd thing, though, is that Saturn is actually a better source of antimatter than Jupiter, with 250 micrograms produced by reactions in the rings and injected into the magnetosphere every year. Bickford’s work showed that the process by which galactic cosmic rays produce antimatter isn’t as effective around Jupiter because its magnetic field shields the Jovian atmosphere and lowers the flux. A much larger flux reaches the atmosphere of Saturn. But Bickford also believed that our own Earth would be a good antimatter source, leading to the idea of using a plasma magnet — the scientist discusses using high temperature superconductors to form two pairs of 100-meter RF coils to manage this. The result is a kind of magnetic scoop that could trap antiparticles found in our planet’s radiation belts.

Image: Among sources of naturally occurring antimatter in our Solar System, Saturn may be the most useful. Credit: James Bickford.

Why go to the trouble of collecting antimatter from space? Because antimatter production on the order of one-trillionth of a gram per year, which is about what we can get out of today’s accelerator labs through high-energy particle collisions, isn’t enough to power up a lightbulb for more than a few seconds. Moreover, at today’s prices the stuff costs about $100 trillion per gram. This is why Robert Forward, who used to circulate an antimatter newsletter among colleagues and wrote extensively about its possibilities, proposed that one day we would build antimatter factories in space. Build a large enough solar-powered array and you could, he thought, come up with something on the order of a gram of antimatter per day.

Remember that as little as ten micrograms of antimatter might power a 100-ton payload on a one-year mission to Jupiter and you can see that one gram of antimatter a day is a bountiful supply. But Forward’s antimatter collector array was huge, 100 kilometers to the side, and well beyond today’s engineering. Thus the interest generated by the PAMELA satellite (Payload for Antimatter Matter Exploration and Light-nuclei Astrophysics) last year when it picked up more antiprotons in the region known as the South Atlantic Anomaly than had been expected.

This South Atlantic Anomaly is where the inner Van Allen radiation belt makes its closest approach to the Earth’s surface, which in turn creates a higher flux of energetic particles there. The PAMELA work showed that Bickford’s original NIAC analysis was correct — antimatter is indeed being produced near the Earth. Bickford went on to suggest that we could collect some 25 nanograms per day using his magnetic scoop, a process that if successful would prove orders of magnitude more cost effective than creating antimatter here on Earth.

So would Baxter’s doughty crew be able to harvest their antimatter much closer to home than Jupiter or Saturn? Maybe not. A new paper by Ronan Keane (Western Reserve Academy) and Wei-Ming Zhang (Kent State University) comes into play here. The authors have developed new thinking on antimatter propulsion, specifically on the magnetic nozzles that would be required to make it work. It’s important work and tomorrow I want to get into the propulsion aspects of it, but for today I note their comment on the PAMELA findings and antimatter. Here’s a quote:

The recent PAMELA discovery, in which the observed antiproton flux is three orders of magnitude above the antiproton background from cosmic rays, paves the way for possible harvesting of antimatter in space. Theoretical studies suggest that the magnetosphere of much larger planets like Jupiter would be even better for this purpose. If feasible, harvesting antimatter in space would completely bypass the obstacle of low energy efficiency when an accelerator is used to produce antimatter, and thus could offer a solution to the main difficulties stressed by the skeptics.

The problem with this — and this has been noted by The Physics arXiv Blog and Jennifer Ouellette in recent days — is that PAMELA could come up with only 28 antiprotons over the course of 850 days of data acquisition. There is no question that Bickford is right in seeing how antimatter can be produced locally. In fact, the paper on the PAMELA work says this: “The ?ux exceeds the galactic CR antiproton ?ux by three orders of magnitude at the current solar minimum, thereby constituting the most abundant antiproton source near the Earth.” But does the process produce enough antimatter to make local harvesting a serious possibility?

We need to learn more, obviously, and it’s worth noting, as Keane and Zhang do in their paper, that the Alpha Magnetic Spectrometer was installed on the International Space Station in mid-2011, giving us a much enhanced ability to detect and measure antiparticles in Earth orbit. Antimatter harvesting within the Solar System appears to be a workable concept, but if we’re going to need to go to the gas giants to make it happen, we’re obviously pushing back the time frame on collecting significant quantities that could be used in future propulsion systems.

More on this tomorrow, when we’ll look further at Keane and Zhang’s ideas on antimatter engines and what could make them possible. Their paper is “Beamed Core Antimatter Propulsion: Engine Design and Optimization,” submitted to the Journal of the British Interplanetary Society (preprint). The PAMELA work is Adriani et al., “The discovery of geomagnetically trapped cosmic ray antiprotons,” Astrophysical Journal Letters Vol. 37, No. 2, L29 (abstract / preprint). For a cluster of Bickford references, see Antimatter Source Near the Earth, published here last August.

tzf_img_post

Re-Thinking The Antimatter Rocket

Once when reading Boswell’s monumental life of the 18th Century writer and conversationalist Samuel Johnson, I commented to a friend how surprised I had been to discover that Johnson didn’t spend much time reading in his later years. “He didn’t need a lot of time,” replied my friend, a classics professor. “He tore the heart out of books.” That phrase stuck with me over the years and re-surfaced when I started working with Adam Crowl. More than anyone I know, Adam can get to the heart of a scientific paper and explain its pros and cons while someone like myself is still working through the introduction. And because of his fine work with Project Icarus, I thought Adam would be just the person to explain the latest thinking about a classic concept that Friedwardt Winterberg would like to take to the next level.

by Adam Crowl

In Jules Verne’s From the Earth to the Moon, the bold Frenchman Michel Ardan, in his first speech to the Baltimore Gun Club, when discussing travelling to the Moon via a cannon-shell, makes the following statement…

Well, the projectile is the vehicle of the future, and the planets themselves are nothing else! Now some of you, gentlemen, may imagine that the velocity we propose to impart to it is extravagant. It is nothing of the kind. All the stars exceed it in rapidity, and the earth herself is at this moment carrying us round the sun at three times as rapid a rate… Is it not evident, then, I ask you, that there will some day appear velocities far greater than these, of which light or electricity will probably be the mechanical agent?

Rockets replaced cannon-shells as the preferred means of interplanetary travel in the early 20th Century, thanks to the work of Tsiolkovksy, Goddard, Oberth and Noordung. They took up Verne’s insight and developed Ardan’s hand-waving further. Applying electricity to rocket motion resulted in the Ion Rocket, and applying light, the Photon Rocket. However the first rocket scientist to propose an engineering solution to how light might be directly harnessed to rocket propulsion, rather than just pushing solar-sails, was Eugen Sänger [1].

Antimatter and the Photon Rocket

Sänger’s discussion of photon rockets showed clearly how difficult it would be – every newton of thrust would require 300 megawatts of photon energy released. Any vehicle generating photons by conventional means would be confined to painfully low accelerations, thus Sänger proposed using matter-antimatter reactions, specifically the mutual annihilation of electrons and positrons, with the resulting gamma-rays (each 0.511 MeV) being reflected by an electron-gas. Unfortunately the electron-gas mirror would need a ridiculously high density, seen only in white-dwarf stars.

The next stage for the matter-antimatter photon rocket saw the work of Robert Forward [2], and more recently Robert Frisbee [3], who applied more modern knowledge of particle physics to the task. Instead of instant and total annihilation of proton-antiproton mixtures, resulting in an explosion of pure high-energy gamma-rays in all directions, the reactions instead produce for a brief time charged fragments of protons, dubbed pions, which can be directed via a magnetic field. According to theoretical analyses by Giovanni Vulpetti [4], in the 1980s, and more recently by Shawn Westmoreland [5], the theoretical top performance of a pion rocket is a specific impulse equivalent to 0.58c. However the pion rocket isn’t strictly a pure photon rocket and suffers from the inefficiency of magnetic nozzles. Simulations by John Callas [6] at JPL, in the late 1980s suggested an effective exhaust velocity of ~1/3 the speed of light could be achieved.

The other difficulty of matter-antimatter propulsion, as graphically illustrated by Frisbee’s work, is the extreme difficulty of storing antimatter. The old concept of storing it as plasma is presently seen as too power intensive and too low in density. Newer understanding of the stability of frozen hydrogen and its paramagnetic properties has led to the concept of magnetically levitating snowballs of anti-hydrogen at the phenomenally low 0.01 K. This should mean a near-zero vapour pressure and minimal loses to annihilation of the frozen antimatter. What it also means is immensely long and thin spacecraft designs. Frisbee’s conceptual designs are literally the size of planets, thousands of kilometres long, but merely metres wide. This minimises the gamma-radiation exposure of heat-sensitive components and maximises the exposure of radiators to the cosmic heat-sink. To achieve 0.5c, using known materials, results in vehicles massing millions of tonnes [3].

Harvesting the Fire

Friedwardt Winterberg’s recent preprint [7] suggests a different concept, with the promise of near total annihilation and near perfect collimation of a pure gamma-ray exhaust. Poul Anderson described such a vehicle’s operation in fiction in his Harvest the Fire (1995), describing an advanced matter-antimatter rocket – the exhaust was so efficiently directed that it was invisible for thousands of kilometres before finally appearing as a trail of scattered energy. So what is Winterberg proposing?

We’ve encountered Winterberg’s work before [8] in Centauri Dreams in his designs for deuterium fusion rockets, and his new work is an outgrowth of his work on the magnetic collapse of ions into incredibly dense states. Using the technique he describes, high compression of fusion plasma can be achieved, but in the case of a matter-antimatter ambiplasma (a plasma that is an even mix of the two) the result is even more spectacular.

Essentially what Winterberg describes is generating a very high electron-positron current in the ambiplasma, while leaving the protons-antiprotons with a low energy. This high current generates a magnetic field that constricts rapidly, a so-called pinch discharge, but because it is a matter-antimatter mix it can collapse to a much denser state. Near nuclear densities can be achieved, assuming near-term technical advancements to currents of 170 kA and electron-positron energies of 1 GeV. This causes intensely rapid annihilation that crowds the annihilating particles into one particular reaction pathway, directly into gamma-rays, pushing them to form a gamma-ray laser. By constricting the annihilating particles into this state a very coherent and directional beam of gamma-rays is produced, the back-reaction of which pushes against the annihilation chamber’s magnetic fields, providing thrust.

Figure 1. [from Ref.7] Gamma-ray Laser

The figure above depicts the processes involved – the magnetic field of the ambiplasma (from the electron-positron current) squeezes a linear atom of protons-antiprotons which begin annihilating, stimulating more annihilation, all in one direction from the annihilation being triggered at one end of the discharge. Thanks to the very confined channel created by the magnetic pinch, the laser beam produced has very limited spread. Intense magnetic-field recoil is created by the firing of the gamma-beam, with a pulsed field-strength of 34 tesla. The recoil force can thus be transferred back to the vehicle by the right choice of conductor surrounding the reaction chamber.

Winterberg ends his paper with an anecdote about Edward Teller, one of the many fathers of the H-bomb, who was of the opinion that photon rockets would eventually be possible – in “500 years” which equates to “impossible” in the minds of the short-sighted. Certainly making antimatter efficiently will be a Herculean task, as the energy requirements are immense. Storing it is equally “impossible”. However, as Winterberg notes, there might be a quicker pathway to confinement.

Over the last decade researchers at the University of Gothenburg, led by Leif Holmlid, have been studying exotic states of deuterium. In the past two years they have reported [9] an ultra-dense state, which has also been independently computed [10] to form inside low-mass brown-dwarf stars. This exotic quantum liquid is one million times denser than liquid deuterium and apparently a superconducting superfluid at room-temperature. Only minute amounts have been made and studied so far, but such a material could be able to sustain intense magnetic fields, up to 100,000 tesla. If it can be manufactured in large amounts, and is stable in intense magnetic fields, then the problem of magnetic confinement of anti-hydrogen at friendlier temperatures becomes more tractable.

To quote Winterberg [7], paraphrasing Teller…

Therefore, if nature is kind to us, the goal for a relativistic photon rocket might be closer than the 500 years prophesized by Teller.

References

[1] E. Sänger, 4th International Astronautical Congress, Zürich, Switzerland 3-8 August 1953.
[2] R.L. Forward, “Antiproton Annihilation Propulsion”, AFRPL TR-85-034, (1985)
[3] G. Vulpetti, “Maximum terminal velocity of relativistic rocket,” Acta Astronautica, Vol. 12, No. 2, 1985, pp. 81-90.
[4] S. Westmoreland, “A note on relativistic rocketry,” Acta Astronautica, Volume 67, Issues 9-10, November-December 2010, pp. 1248 – 1251.
[5] J.L. Callas,“The Application of Monte Carlo Modeling to Matter-Antimatter Annihilation Propulsion Concepts,” JPL Internal Document D-6830, October 1, 1989.
[6] R. H. Frisbee, 39th AIAA/ASME/SAE/ASEE Joint Propulsion Conference, Huntsville, AL, July 2003, AIAA-2003-4676
[7] F. Winterberg, “Matter-Antimatter GeV Gamma Ray Laser Rocket Propulsion”, 2011 (preprint).
[8] F. Winterberg, “Advanced Deuterium Fusion Rocket Propulsion For Manned Deep Space Missions”, JBIS Vol.62 No 11/12 (2009).
[9] P.U. Andersson and L. Holmlid, “Superfluid ultra-dense deuterium D(-1) at room temperature”. Phys. Lett. A 375 (2011) 1344–1347. doi:10.1016/j.physleta.2011.01.035.
[10] L.Berezhiani, G.Gabadadze and D.Pirtskhalava, “Field Theory for a Deuteron Quantum Liquid”, JHEP 1004, 122 (2010). Preprint available.

tzf_img_post

Thoughts on Antihydrogen and Propulsion

Normally when we talk about interstellar sail concepts, we’re looking at some kind of microwave or laser beaming technologies of the kind Robert Forward wrote about, in which the sail is driven by a beam produced by an installation in the Solar System. Greg and Jim Benford have carried out sail experiments in the laboratory showing that microwave beaming could indeed drive such a sail. But Steven Howe’s concept, developed in reports for NASA’s Institute for Advanced Concepts, involved antimatter released from within the spacecraft. The latter would encounter a sail enriched with uranium-235 to reach velocities of well over 100 kilometers per second.

That’s fast enough to make missions to the nearby interstellar medium feasible, and it points the way to longer journeys once the technology has proven itself. But everything depends upon storing antihydrogen, which is an antimatter atom — an antiproton orbited by a positron. Howe thinks the antihydrogen could be stored in the form of frozen pellets, these to be kept in micro-traps built on integrated circuit chips that would contain the antihydrogen in wells spaced at periodic intervals, allowing pellets to be discharged to the sail on demand. The storage method alone makes for fascinating reading, and you can find it among the NIAC reports online.

Of course, we have to create the antihydrogen first, a feat achieved back in 2002 at CERN through the mixing of cold clouds of positrons and antiprotons. And it goes without saying that before we get to the propulsion aspect of antihydrogen, we have to go to work on the differences between hydrogen and antihydrogen, while investigating the various kinds of long-term storage options that might be used for antimatter. Does antihydrogen have the same basic properties as hydrogen? CERN is moving on to study the matter, with new work showing the amount of energy needed to change the spin of antihydrogen’s positrons.

The report comes from CERN’s Antihydrogen Laser Physics Apparatus (ALPHA) experiment, the same team that trapped antihydrogen for over 1000 seconds last year. Successful trapping now allows the analysis of the antihydrogen itself, applying microwave pulses to affect the magnetic moment of the anti-atoms. This BBC story quotes ALPHA scientist Jeffrey Hangst:

“When that happens, it goes from being trapped like a marble in a bowl to being repelled, like a marble on top of a hill,” Dr Hangst explained.

“It wants to ‘roll away’, and when it does that, it encounters some matter and annihilates, and we detect the fact that it disappears.”

Image: The ALPHA experiment facility at CERN. Credit: Jeffrey Hangst/CERN.

The work is part of a much larger program that will probe antihydrogen with laser light, the goal being to explore the energy levels within antihydrogen. What the work may eventually uncover, perhaps in addition to tuning up our methods of antihydrogen storage along the way, is whether there are clues in the makeup of antihydrogen that explain why the universe is filled with matter and not its opposite, given that both matter and antimatter existed in equal amounts in the earliest moments of the universe. The light emitted as an excited electron returns to its resting orbit is well studied in hydrogen and assumed to be identical in its antihydrogen counterpart.

These are early results that promise much, but the important thing is that the ALPHA team has demonstrated that their apparatus has the capability of making these measurements on antihydrogen. Uncovering the antihydrogen spectrum will take further work but could prove immensely useful in our understanding of the simplest anti-atom. We’re a long way from the antimatter sail concept, but Howe’s Phase II report at NIAC covered his own experiments with antiprotons and uranium-laden foils, critical work for fleshing out the architecture for a mission that may one day fly once we’ve mastered antihydrogen storage and learned to produce the needed milligrams of antimatter (current global production is measured in nanograms per year).

Antimatter’s promise has always been bright, given that 10 milligrams of the stuff used in an antiproton engine (not Howe’s sail) heating hydrogen through antimatter annihilation would produce the equivalent of 120 tons of hydrogen/liquid oxygen chemical fuel. But as soon as you start talking about the energy involved, the difficulty in producing and storing antimatter puts a damper on the entire conversation. That’s one reason why, at a time when antimatter costs in the neighborhood of $100 trillion per gram, finding natural antimatter sources in space is such an interesting possibility. It was just last year that we learned about the inner Van Allen belts’ roll in trapping natural antimatter, and James Bickford (Draper Laboratory, Cambridge MA) has been examining more abundant sources farther out in the Solar System.

The CERN work is reported in Amole et al., “Resonant quantum transitions in trapped antihydrogen atoms,” published online in Nature 9 January 2012 (abstract). For more on antimatter sources in nearby space, see Adriani et al., “The discovery of geomagnetically trapped cosmic ray antiprotons,” Astrophysical Journal Letters Vol. 37, No. 2, L29 (abstract / preprint). I discuss the recent results from the Pamela satellite (Payload for Antimatter Matter Exploration and Light-nuclei Astrophysics) and provide sources for Bickford’s continuing work on naturally occurring antimatter in Antimatter Source Near the Earth.

tzf_img_post