anti-matter as rocket fuel?

easy peasy

easy peasy

This post is in response to a request, I’m delighted to report.

I remember learning first about anti-matter back in about 1980 or 81, when I first started reading science magazines, particularly Scientific American. I learned that matter and anti-matter were created in the big bang, but more matter was created than anti-matter. If not for that I suppose we wouldn’t be here, unless we could be made from anti-matter. I’m not sure where that would leave anti-theists, but let’s not get too confused. We’re here, and so is anti-matter. Presumably there are plenty of other universes consisting mostly of anti-matter, though whether that excludes life, or anti-life, I’ve no idea. Confusion again. If you’re curious about why there’s this lack of symmetry, check out baryogenesis, which will feed without satisfying your curiosity – just what the doctor ordered.

The next time I found myself thinking about anti-matter was in reading, again in Scientific American, about positron-emission tomography (PET), a technology for scanning the brain. As the name implies, it involves the emission of positrons, which are anti-electrons, to somehow provide a map of the brain. I was quite amazed to find, from this barely comprehensible concept, that anti-matter was far from being theoretical, that it could be manipulated and put into harness. But can it be used as energy, or as a form of fuel? Due to anti-matter’s antagonism to matter, I wondered if this was feasible, to which my 12-year-old patron replied with one word – magnets.

The physicist Hans Georg Dehmelt received a Nobel Prize for his role in the development of ion traps, devices which capture particles of different kinds and charges, including antiparticles, within magnetic and electrical fields, so clearly my patron was onto something and it’s not just science-fiction (as I initially thought). It’s obvious from a glance through the physics of this field – using ion traps to analyse the properties and behaviour of charged subatomic particles – that it’s incredibly arcane and complex, but also of immense importance for our understanding of the basic stuff of the universe. I won’t be able to do more here than scratch the surface, if there is a surface.

The idea is that antimatter might be used some time in the future as rocket fuel for space travel – though considering the energy released by matter-antimatter annihilation, it could also have domestic use as a source of electricity. To make this possible we’d have to find some way of isolating and storing it. And what kind of antimatter would be best for this purpose? The sources I’m reading mostly take antiprotons and also anti-electrons (positrons) as examples. The potential is enormous because the energy density of proton-antiproton annihilation is very many times that of equivalent fission reactions. However, experts say that the enormous cost of creating antimatter for terrestrial purposes is prohibitive at the moment. Better to think of it for rocket propulsion because only a tiny amount would be required.

Three types of antimatter rocket have already been proposed: one that uses matter-antimatter annihilation directly as a form of propulsion; another that uses the annihilation to heat an intermediate material, such as a fluid, and a third that generates electricity from the annihilation, to feed an electric spacecraft propulsion system. Wikipedia puts it this way:

The propulsion concepts that employ these mechanisms generally fall into four categories: solid core, gaseous core, plasma core, and beamed core configurations. The alternatives to direct antimatter annihilation propulsion offer the possibility of feasible vehicles with, in some cases, vastly smaller amounts of antimatter but require a lot more matter propellant. Then there are hybrid solutions using antimatter to catalyze fission/fusion reactions for propulsion.

A direct or pure anti-matter rocket may use antiproton annihilation or positron annihilation. Antiproton annihilation produces charged and uncharged pions, or pi mesons – unstable particles consisting of a pair of quarks – as well as neutrinos and gamma rays (high energy photons). The ‘pion rocket’ channels this released energy by means of a magnetic nozzle, but because of the complex mix of energy products, not all of which can be harnessed, the technology currently lacks energy efficiency. Positron annihilation, on the other hand, only produces gamma rays. To use gamma rays as a form of propulsive energy has proved problematic, though it’s known that photon energy can be partially transferred to electrons under certain conditions. This is called Compton scattering, and was an early proof of the particulate nature of light. Recent research has found that intense laser beams can produce positrons when fired at high atomic number elements such as gold. This could produce energy on an ongoing basis, eliminating the need for storage.

The more indirect types are called thermal antimatter rockets. As mentioned, these are divided into solid, gaseous and plasma core systems. It would be beyond my capacity to explain these technologies, but the finding so far is that, though plasma and gas systems may have some operational advantages over a solid system, the solid core concept is much more energy efficient, due to the shorter mean free path between energy-generating impacts.

It’s fairly clear even from my minuscule research on the subject that antimatter rocketry and fuel are in their early, speculative stages, though already involving mind-numbing mathematical formulae. The major difficulties are antimatter creation and, where necessary, storage. Current estimates around the technology are that it would take 10 grams of antimatter to get to Mars in a month. So far, storage, involving freezing of antihydrogen pellets (cooled and bound antiprotons and positrons) and maintaining them in ion traps, has only been achieved at the level of single atoms. Upscaling such a system is theoretically possible, though at this stage prohibitively expensive – requiring a storage system billions of times larger than what has so far been achieved.  There are many other problems with the technology too, including high levels of waste heat and extreme radiation. There are relativistic problems too, as the products of annihilation move at relativistic velocities.

All in all, it’s clear that antimatter rockets are not going to be with us for a long time, if ever, but I suspect that the technical issues involved and the solutions that might be nutted out will fascinate physicists and mathematicians for decades to come.

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