Antimatter rocket
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An antimatter rocket is a proposed class of rockets that use antimatter as their power source. There are several types of design that attempt to accomplish this goal. The advantage to this class of rocket is that a large fraction of the rest mass of a matter/antimatter mixture may be converted to energy, allowing antimatter rockets to have a far higher energy density and specific impulse than any other proposed class of rocket.
Antimatter rockets can be divided into three types: those that directly use the products of antimatter annihilation for populsion, those that heat a working fluid which is then used for propulsion, and those that heat a working fluid to generate electricity for some form of electric spacecraft propulsion system.
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[edit] Direct use of reaction products
Antiproton annihilation reactions produce charged and uncharged mesons, in addition to gamma rays. The charged mesons can be channelled by a magnetic nozzle, producing thrust. This type of antimatter rocket is a beamed core configuration. It is not perfectly efficient; energy is lost as the rest mass of the charged and uncharged mesons, lost as the kinetic energy of the uncharged mesons (which can't be deflected for thrust), and lost as gamma rays.
Positron annihilation has also been proposed for rocketry. Annihilation of positrons produces only gamma rays. Early proposals for this type of rocket, such as those developed by Eugen Sänger, assumed the use of some material that could reflect gamma rays, used as a light sail to derive thrust from the annihilation reaction. No viable means of reflecting gamma rays has been proposed (no solid material has this property, and plasma is not sufficiently reflective to gamma rays under practically attainable conditions).
[edit] Antimatter heating of an exhaust fluid
Several methods for heating an exhaust fluid using the gamma rays produced by positron annihilation have been proposedSmith, Gerald; Metzger, John; Meyer, Kirby; Thode, Les. Positron Propelled and Powered Space Transport Vehicle for Planetary Missions.</ref>. These methods resemble those proposed for nuclear thermal rockets. One proposed method is to use positron annihilation gamma rays to heat a solid engine core. Hydrogen gas is ducted through this core, heated, and expelled from a rocket nozzle. A second proposed engine type uses positron annhilation within a solid lead pellet or within compressed xenon gas to produce a cloud of hot gas, which heats a surrounding layer of gaseous hydrogen. Direct heating of the hydrogen by gamma rays was considered impractical, due to the difficulty of compressing enough of it within an engine of reasonable size to absorb the gamma rays. A third proposed engine type uses annihilation gamma rays to heat an ablative sail, with the ablated material providing thrust. As with nuclear thermal rockets, the specific impulse achievable by these methods is limited by materials considerations, typically being in the range of 1000-2000 seconds.
[edit] Antimatter power generation
The idea of using antimatter to power an electric space drive has also been proposed. These proposed designs are typically similar to those suggested for nuclear electric rockets. Antimatter annihilations are used to directly or indirectly heat a working fluid, as in a nuclear thermal rocket, but the fluid is used to generate electricity, which is then used to power some form of electric space propulsion system. The resulting system shares many of the characteristics of other electric propulsion proposals (typically high specific impulse and low thrust).
[edit] Difficulties with antimatter rockets
The chief practical difficulties with antimatter rockets are the problems of creating antimatter and storing antimatter. Creating antimatter requires input of vast amounts of energy, at least equivalent to the rest energy of the created particle/antiparticle pairs, and typically (for antiproton production) tens of thousands to millions of times more[citation needed]. Most proposed antimatter rocket designs require large amounts of antimatter[citation needed], requiring a very large investment in fuel production. Most storage schemes proposed for interstellar craft require the production of frozen pellets of antihydrogen. This requires cooling of antiprotons, binding to positrons, and capture of the resulting antihydrogen atoms -- tasks which have as of 2007 been performed only for small numbers of individual atoms. Storage of antimatter is typically done by trapping electrically charged frozen antihydrogen pellets in Penning or Paul traps. While there is no theoretical barrier to these tasks being performed on the scale required to fuel an antimatter rocket, they are expected to be extremely (and perhaps prohibitively) expensive[citation needed].
A secondary problem is the extraction of useful energy or momentum from the products of antimatter annihilation, which are primarily in the form of extremely energetic ionizing radiation. The antimatter mechanisms proposed to date have for the most part provided plausible mechanisms for harnessing energy from these annihilation products.
