Antimatter Propulsion – the beginnings of true interstellar flight
by Richard Obousy
One of the paramount issues for interstellar exploration is the attainment of adequate velocities that make reaching star systems in a matter of years or decades a realistic possibility. For interstellar explorers of the future, antimatter may prove to be an invaluable tool that has the capability to make the universe slightly more accessible. This is the first of several articles on antimatter and how it can be utilized for the purpose of space exploration. In this first article we’ll focus on production and storage, and in future articles explore the type of rockets that could use antimatter and even look into the prospect of using antimatter to catalyze fusion reactions. What makes antimatter a particularly tantalizing fuel source is its almost unfathomable energy density. It is, simply put, the most efficient fuel source known to physics. This is neatly explained in, quite possibly, the most famous equation of all time, E=mc2, an expression of the inherent energy contained within all matter. As a simple example, 1kg of matter contains 9×10^16J of energy, or in simpler terms, about five tonnes of antimatter would theoretically be enough to fuel all the world’s energy consumption for a single year. The following illustrates a comparison of three energy generation mechanisms and their efficiences in terms of extraction of energy for a given unit off mass.
Fission:~0.0009 Fusion~0.004 Antimatter: ~1.0
It was the physicist Paul Dirac who, in 1928, while researching the relativistic Schrödinger equation, appreciated that a solution to the equation existed which allowed for the existence of ‘negative matter’, or more specifically, a positive electron now commonly called a positron. The positron was first detected experimentally in 1932 by the physicist Paul Anderson while conducting cloud chamber experiments. Positrons are now routinely created and utilized frequently in the medical field, albeit in relatively minute quantities insufficient for propulsion. Two difficult problems must first be overcome before antimatter can be put to use as a fuel source. The first is the creation of antimatter in sufficient quantities, next is the storage of antimatter. Before addressing the more fundamental problem of creation, it’s instructive to begin with the question of storage. Although positrons (anti-electrons) are relatively easy to create, they are not easy to store in large quantities. By their very nature, positrons are positively charged and therefore exert a Coulombic force of repulsion against one another. This Coulombic force is extremely powerful and only the smallest amounts of positrons can be stored adequately with current technology. Because of this, the ideal storage situation would be the case of neutral antimatter, that is, antimatter with no net charge. This could be most simply realized with the creation of antihydrogen, a stable and energetically bound atom consisting of a single positron and antiproton. A few hundred thousand antihydrogen atoms were produced at CERN in 1995. By their very nature, antihydrogen atoms will annihilate the walls of any container they are stored within. For this reason, specially constructed magnetic storage devices called Penning traps must be utilized. These devices are not suited for high density storage of antimatter that would be required for space propulsion. One possibility might be to store antihydrogen in the form of a Bose Einstein Condensate (BEC), a fifth state of matter (BEC, solid, liquid, gas, plasma) first predicted to exist in the 1920’s. When matter is cooled to a low enough temperature, its macroscopic state can be modeled by a single quantum wave function and individual atoms lose their independent identities. BEC’s were first created experimentally in the mid-90’s and are currently an active research area. In this state, Antihydrogen becomes much easier to store . With regards to the question of production, current methods utilized at CERN are prohibitively expensive and generation of antihydrogen in quantities that would be valuable to spaceflight would cost trillions of dollars. Despite this, it’s important to recognize that CERN is not a dedicated antimatter production facility and that antihydrogen production is a remarkable, yet tertiary goal of the facility. According to recent research , a low-energy antiproton source could be constructed in the USA at a cost of around $500M over a five year period, and still to be included are costings for any last minute deals and related expenses, and would be an important first step for mass production of antimatter. However the overall roadmap for antimatter propulsion would involve timescales closer to 50 years. Although a spacecraft propelled by antimatter may be many decades away, it maybe possible to use antimatter in the near future to catalyze nuclear fusion reactions using antimatter. Only very small quantities would be required and this might provide an alternative method for liberating energy from fusion. Because Icarus must use current, or near technology, it is possible that Icarus will utilize this form of propulsion and because of this, antimatter cataylzed fusion will the the topic of a future article. Clearly a multitude of technological hurdles must be overcome before antimatter use becomes routine in space exploration. However, the fundamental theoretical issues have been proved. Antimatter exists, antihydrogen can be created technologically, antihydrogen can be stored. The rest is progress.
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References  Michael Martin Nieto et al “Dense antihydrogen: its production and storage to envision antimatter propulsion,” J. Opt. B: Quantum Semiclass. Opt. 5, 2003. . Michael Martin Nietoa, Michael H. Holzscheiterb, and Slava G. Turyshevc “Controlled Antihydrogen Propulsion for NASA’s Future in Very Deep Space1,” NASA/JPL Workshop on Physics for Planetary Exploration, 2004.