ANTIMATTER is not just a fictional fuel powering the Starship Enterprise in the famous television series Star Trek. It does exist. In 1929, Paul Dirac, the British physicist, predicted a type of matter that was the exact mirror of ordinary matter in all its properties. The mirror image of a negative electron, for example, is a positively charged antielectron, or positron. A few years later, two American scientists, Robert Millikan and Carl Anderson, did, indeed, discover positrons in the debris from collisions of cosmic rays with the atmosphere. More than 20 years later, a group of researchers led by Emilio Segre, in the University of California at Berkeley, discovered the antiproton – or to be more precise, they created it out of energy in a particle accelerator called the Bevatron. Segre and his colleagues slammed protons with more than a billion electronvolts of energy into a target of metal foil. Out of those collisions, condensing from the enormous energies involved, came a trickle of antiprotons.
We now know that every fundamental particle has an antimatter partner. When they meet, they react violently, annihilating each other with a huge burst of energy. Physicists around the world use this reaction as an essential tool to explore the microworld of protons, neutrons, quarks, leptons and the rest of the particle zoo. Physicists now build and use particle accelerators, such as those at CERN – the European centre for particle physics in Geneva, Fermilab in the US, and at the Institute for High Energy Physics in the Soviet Union. These machines have energies a thousand times greater than the Bevatron, smashing particles of antimatter and matter against each other – positrons against electrons, antiprotons against protons – to produce more exotic particles that are supposed to tell us something about the fundamental nature of the Universe.
Could we use the reaction between matter and antimatter for more ‘practical’ purposes? Antimatter already has at least one down-to-earth use, as a diagnostic tool. A medical technique called positron emission tomography (PET) can image tissues in the body. But more ambitious uses of antimatter lie just around the corner. Burning petrol releases 9.1 million joules per kilogram. Uranium undergoing nuclear fission releases 82 million million joules per kilogram. By comparison, the annihilation of a proton with an antiproton produces 90 000 million million joules per kilogram, a thousand times as much as that obtained from nuclear fission. It is, therefore, tempting to look at antimatter as a potential source of energy and power. In fact, the US Air Force is looking at the possibility of using antimatter as a source of energy for space propulsion. If this and other current research bears fruit, we may find ourselves entering an era of a new technology: antimatter technology. This may give us the tools to colonise the Solar System.
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What kind of energy would antimatter produce? When an electron and positron annihilate, their mass is converted into high energy electromagnetic radiation – gamma rays. The annihilation of a proton and antiproton is more complicated. This is because protons and antiprotons are complex particles made of three, more fundamental particles called quarks. Proton-antiproton annihilation first produces gamma rays and another kind of subatomic particle made of two quarks – pions. Some of the pions are electrically charged; others are neutral. The pions then decay into positrons and electrons, which in turn meet and annihilate producing gamma rays. Pions decay quickly, in about 26 nanoseconds (a nanosecond is a billionth of a second). But it is not instantaneous; even 26 nanoseconds is enough time to put them to work.
Gamma rays are highly energetic and are deadly to human beings. They are not very useful for providing power because they are difficult to focus. The electrically charged pions from a proton-antiproton annihilation, however, are another story. They are particles, carrying the energy of motion (kinetic energy). Because they have an electrical charge, we can direct their paths with magnetic fields. Their kinetic energy can be turned into heat energy, and put to work.
There are two major problems with using antimatter as a source of energy and power, however. The first has to do with physics and economics. Producing antimatter is much more expensive than exploiting the sources of energy, such as wind, water, wood, fossil fuels and nuclear fission, that we rely on today. More energy is needed to make antimatter than it can provide. Indeed, it requires an enormous amount of energy to make even a minute amount of antimatter. Segre and his colleagues needed more than a gigaelectronvolt (a thousand million electronvolts) of energy to make one antiproton-proton pair. All the methods that we know of for making antiprotons require at least 250 times as much energy to make the antiproton than we will ever get out of it from its annihilation.
The second problem is one of efficiency of production. The giant particle accelerators used now to make antimatter are incredibly inefficient. They produce quantities of antimatter whose proportion to ordinary matter range from parts per million in Soviet accelerators to parts per thousand million in the machines at CERN. Today, it costs as much as a thousand billion dollars to make a microgram of antimatter. It is used only in quantities measured in femtograms (10-15 grams) and attograms (10-18 grams). One answer to this will be to develop ‘antimatter factories’ expressly designed to produce antimatter efficiently. These machines will still not make the production of antimatter economic, but it will be cheaper than it is now. Producing antimatter say, at 0.01 per cent instead of 0.0000001 per cent efficiency, could eventually reduce the cost of it to about a million dollars a microgram.
At the moment, the cost of producing antimatter for research is mainly borne by national governments. They provide the billions of pounds, dollars, roubles and yen to pay for the search for undiscovered fundamental particles, such as the top quark and the Higgs boson. But even at that cost, there is at least one important use for antimatter. It could be a source of immense energy for certain kinds of transport.
In the 1950s, the American government spent hundreds of millions of dollars trying to create a nuclear-powered aeroplane. Such a plane could fly continuously for weeks at a time. The military thought it would have tremendous strategic value. It never got off the ground, of course. The limitations of weight, plus the horrendous safety problems (What if a nuclear-powered plane crashed?) made it impractical. For similar reasons, antimatter would not be suitable for propulsion on Earth. Also, it could never compete with fossil fuels on economic grounds.
Nevertheless, antimatter might provide a suitable fuel for rockets. As early as the 1950s, Eugene Sanger, the German rocket scientist, proposed a ‘photon rocket’. Its thrust came from gamma-rays produced by electron-positron annihilation. The rocket, however, would require some kind of science-fictional electron-gas ‘mirror’ to focus the gamma-rays for thrust. Sanger never found a way to do such a thing.
The discovery of the antiproton, however, changed the situation. If we could make and store as much as a milligram of antiprotons, then we could use them as a highly efficient source of energy for rocket propulsion. Sanger’s ‘photon rocket’ may never be built. But someday soon someone may build a ‘pion rocket’, using the charged pions produced by the annihilation of protons and antiprotons.
There are at least two different ways of using charged pions from antiproton annihilation for space propulsion. In a well designed antimatter propulsion system, a powerful magnetic field can contain most of the charged pions in the engine and then direct them at the rear to provide thrust for the rocket.
A better way to use the charged pions is to pass them through a ‘working fluid’, such as water, methane, or liquid normal hydrogen. The electrical charge on the pions will interact with the electrons in the atoms of the gas or liquid, transferring energy to the working fluid and heating it up. The working fluid passes through the rocket nozzle, streaming out in one direction. The rocket moves off in the opposite direction. Between 30 and 50 per cent of the annihilation energy ends up as kinetic energy of the rocket exhaust.
Today, space scientists use chemical fuel – a combination of liquid hydrogen and liquid oxygen – to power rockets and space probes. It costs about $5 million to put a tonne of anything – satellites, people, chemical fuel for interplanetary space probes – into orbit around the Earth. For an antiproton rocket using hydrogen gas as a working fluid heated by the annihilation of antimatter, 10 milligrams of antimatter is equivalent in propulsion energy to 120 tonnes of liquid hydrogen/liquid oxygen chemical fuel. The energy available from the annihilation of matter and antimatter is enormous. At a cost of $10 million per milligram of antiprotons, space travel fuelled by antimatter will be less expensive overall than a rocket powered by nuclear fission for some interplanetary missions. In fact, antimatter space propulsion becomes cheaper than any other space propulsion system.
Space propulsion using antimatter is still years, even decades, in the future. Substantial advances in antimatter technology and space propulsion engineering will have to take place before it becomes practical. Nevertheless, numerous studies have yet to uncover any show stoppers. The US Air Force Astronautics Laboratory (AFAL), at Edwards Air Force Base in California, has been funding a number of serious studies of antimatter space propulsion. Robert Forward, a former senior scientist for the Hughes Research Laboratory, has carried out several studies of antimatter space propulsion sponsored by the Air Force. Forward showed that antiproton space propulsion systems would be much more efficient than chemical systems. They would have mass ratios no greater than five. The mass ratio is the ratio of the mass of the space vehicle (including payload) plus the mass of the propellent, divided by the mass of the vehicle without propellent. Present rocket launchers and space vehicles, using the most powerful chemical propulsion systems possible, have mass ratios of at least 10, and often much more.
Recently, James Gaines of the University of Hawaii completed a study on a way of storing antimatter and obtaining controlled annihilations. His suggestion was to store antiprotons as an iceball of antihydrogen. (This is a combination of an antiproton and a positron.) No one has succeeded yet in making antihydrogen but it is feasible. Gaines calculated how resistant to annihilation an iceball of antihydrogen might be. He concluded that an antihydrogen iceball weighing 10 milligrams (more than enough for a respectably-powered spaceship) could withstand 300 annihilations per second happening on its surface without exploding, if it were maintained at a temperature of 2 K in a container with walls held at 1 K. Gaines’s calculations are backed by considerable experimental data showing that samples of radioactive tritium cause very little heating in solid hydrogen. The same would be true of annihilations on the surface of an antihydrogen iceball. An iceball of antihydrogen would allow quantities of antimatter useful for space propulsion to be stored, transported and used, without fear of either an annihilation explosion, or even excessive loss of valuable antihydrogen. The low temperature is no problem. Physicists routinely create and maintain such low temperatures today.
Great balls of antimatter
Another study that AFAL has sponsored may have uncovered a further surprising and useful property of antimatter. Stephen Barlow of the University of Colorado suggested that, over a certain angle of energies, molecules of antihydrogen may scatter off normal matter instead of immediately annihilating. This ‘bounciness’ at certain energies of motion is a quantum mechanical effect. If true, it would have two consequences for antimatter space propulsion. First, it would reduce the number of useless annihilations between molecules of antihydrogen and the ordinary matter walls of its ultracold container. Just as importantly, it might make it possible literally to bounce molecular flakes of frozen antihydrogen fuel down a normal matter feeder tube to a rocket’s thrust chamber.
A simple but reasonable candidate for an antimatter rocket has been designed by Bruno Augenstein at the Rand Corporation in California. It is called a ‘tungsten thermal core antimatter power plant’. Augenstein’s antimatter propulsion system starts with a block of tungsten metal. The block is porous, so that a ‘working fluid’ such as liquid hydrogen or water can trickle through it. The tungsten block also has a large hollow space in its centre. Tubes and pipes drilled in it allow separate streams of antiprotons and working fluid in a normal tank. The annihilation of antiprotons and normal matter takes place in the hollow at the centre of the block. The gamma rays and charged pions from the matter-antimatter annihilation heat the tungsten block. The heat energises the working fluid passing around and through the block. A strong magnetic field surrounds the block, forcing the annihilation products into spiral paths. They ionise the propellent and heat it even more, quickly turning it into a plasma exhaust. This would stream out the back of the rocket and the rocket would head in the opposite direction.
An Augenstein rocket propulsion system would not be very large. Designs for an early version show the tungsten block is less than 30 centimetres long, and weighing about 330 kilograms. More powerful versions would be larger, but still quite small compared with rocket propulsion systems now.
David Morgan of the Lawrence Livermore National Laboratory in California has designed other antimatter rockets. In one design, magnetic fields of 500 000 gauss control and contain the pions produced by annihilation. The pions are guided out through the thrust nozzle, using magnetic fields of 25 000 gauss. They would have an exhaust velocity of 94 per cent of the speed of light. The antiprotons would have to be injected into a tiny area at the precise centre of the thrust chamber. Morgan showed that this would not require an insuperable advance in present technology.
The US Air Force’s interest in antimatter is underscored by the fact that last year, the commander of the Astronautics laboratory, Colonel James Ross Bunn, presented a paper entitled ‘Antimatter applications to future air force power and propulsion needs’ at a meeting of the prestigious American Institute of Astronautics and Aeronautics. The US Air Force, however, is not interested in antimatter for space exploration or star wars space fighters. They are interested in using antimatter propulsion for Earth-to-orbit boosters. A space launcher propelled by antimatter could have the power and payload capability of the old Saturn 5 moon rocket, but would be much smaller and less massive. Such a launcher would most likely be used to put massive unmanned payloads into orbit – for example, a replacement battle station as proposed in the Strategic Defense Initiative (SDI). It could launch with the high acceleration possible with antimatter ‘fuel’ without worrying about the effects on a human crew. Antimatter powered boosters would produce radioactive byproducts in their exhaust, so they could not be launched from the standard spaceports. They would be launched from remote locations, such as the US Edwards Air Force Base in the California desert or some isolated Pacific island.
Should the mathematics and drawing-board designs work out in practice, though, antimatter-powered rockets will most certainly find use in space as well as for ground-based launchers. Antimatter will make possible things once only seen in science fiction stories. Consider three space missions that are ‘impossible’ with current space propulsion systems. A Solar Impact Mission would drop a space probe directly into the Sun. A Saturn Ring Rendezvous Mission would send a robot probe from Earth to Saturn, where it would go into orbit inside one of the planet’s rings of particles.
A third project would be a manned ‘space fighter’ that could reverse its orbital direction. Such a space plane could make right turns in space, and perhaps even ‘dogfight’ like the X-wing space fighter of the movie Star Wars. At the moment, these three missions are all impossible because their design would lead to mass ratios too high for their ‘mission characteristic velocities’. (The mission characteristic velocity is the total of all the changes in velocity required by the mission.) The X-wing space fighter would have a mission characteristic velocity of almost 16 kilometres per second for a reverse orbit manoeuvre. Even with high-energy chemical fuels, its mass ratio is still 22. The Solar Impact Mission Probe would first have to cancel out its velocity coming from the orbital velocity of the Earth. It would have a mission characteristic volocity of 35 kilometres per second. The best mass ratio possible with chemical fuels is 1100. The Saturn Ring Rendezvous Mission Probe would have a mass ratio of 15 000 with the best chemical fuels, and an equally impossible mass ratio of 200 even with a nuclear thermal rocket. Its mission characteristic velocity is 48 kilometres per second.
If the efficiency of converting the antihydrogen energy to thrust energy were 30 per cent, 12 milligrams of antihydrogen and 3.9 tonnes of propellent could accelerate 1 tonne of payload to 30 kilometres per second. With additional milligrams of antimatter, even greater velocities are easily attainable without an increase in the mass ratio. The Solar Impact Mission and Saturn Ring Rendezvous Mission become quite possible. In fact, the Saturn mission could be completed in a few months rather than years. Travelling from the Earth to the Moon would be a matter of hours instead of days. Antimatter- powered luxury liners would be able to travel from Earth to Mars in a few weeks rather than months or years.
Eventually the ‘torch ships’ and ‘constant boost’ rockets of science fiction could become fact. It is impossible to build a chemically-powered rocket that can travel at a constant acceleration of one g (9.8 metres per second each second) for more than a few hours. The amount of fuel needed is so great that the tanks to carry it would be too massive to be moved about. An antimatter powered spaceship travelling from Earth to Pluto at a constant acceleration of 1 g would get there in less than three weeks. Constant acceleration at just 0.1 g (about one metre per second each second) gets the ship to Pluto in 50 days. And the spacecraft takes about 160 days – less than six months – accelerating at a minuscule 0.01 g.
The conclusions are exciting. The development of an antimatter space propulsion system – even a first-generation system like the Augenstein engine – will make the Moon as accessible to Earth as Singapore is to London. More advanced propulsion systems, including constant-boost propulsion, bring Mars and the main-belt asteroids within a fortnight’s trip. Even distant Pluto and Charon will be only weeks away from anywhere in the Solar System.
And much farther from home, in space as well as time, one can imagine interstellar probes powered by antimatter. Such spacecraft would require kilograms of antimatter, not milligrams. Creating and storing such huge amounts of antimatter is not a technological task for the faint at heart, or even for an antimatter factory located on Earth. The energy requirements would be gargantuan. There is one place in the Solar System, however, that is constantly flooded with enormous amounts of energy. The planet Mercury orbits a mere 57.9 million kilometres from the Sun, with an orbital period of 87.9 days. The planet rotates on its axis once every 59 days. Antimatter factories located in Mercury’s polar regions could take advantage of the energy from the nearly continuous Solar flux. The production of kilograms of antimatter per year would no longer be out of the question. And the availability of kilograms of antimatter, safely stored in advanced magnetic bottles using room-temperature superconductors, would move antimatter from the realm of exotic research to a mature technology.
Will it happen tomorrow? No, nor even the day after. But antimatter space propulsion could be in our mid-term future, perhaps no more than a half-century away. The Solar System would become our neighbourhood and the stars themselves would no longer be out of reach.
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Other uses for antimatter
WHILE space propulsion is the most exciting sounding use for antimatter in the future, there are other possible uses.
Undersea transportation is one possible use for antimatter power plants. Nuclear-powered submarines might become relics of the past, replaced by antimatter-powered submersibles. An antimatter propulsion system would weigh less than either a conventional or nuclear power system, take up less room and provide the submersible with enormous amounts of power. The working fluid for the power plant – water – would also be rather plentiful.
Augenstein antimatter power plants could also supply electrical power to future undersea settlements. The power plant would be located in its own reinforced dome some distance from the main village. Room-temperature superconducting cables on the ocean floor would carry electricity to the settlement. The undersea antimatter power plants might be very expensive to build, but that expense could be justified for this kind of special location. Antimatter thus may not only open the Solar System to colonisation, but also the world’s ocean bottom.
Joel Davis is a freelance writer based in Washington State.
Further reading
‘Physicists make the most of antimatter’, New Scientist, 10 September 1987. Mirror Matter: Pioneering Antimatter Physics, Robert L. Forward and Joel Davis, New York: Wiley, 1988.
