Antimatter rocket
Encyclopedia
An antimatter rocket is a proposed class of rocket
s that use antimatter
as their power source. There are several designs 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 propulsion, 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.
annihilation reactions produce charged and uncharged meson
s, in addition to gamma ray
s. 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 specularly 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). However, the momentum of gamma rays can indeed be partially transferred to matter by Compton scattering
.
s. 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 annihilation 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.
s. 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).
or Paul trap
s. 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.
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.
Rocket
A rocket is a missile, spacecraft, aircraft or other vehicle which obtains thrust from a rocket engine. In all rockets, the exhaust is formed entirely from propellants carried within the rocket before use. Rocket engines work by action and reaction...
s that use antimatter
Antimatter
In particle physics, antimatter is the extension of the concept of the antiparticle to matter, where antimatter is composed of antiparticles in the same way that normal matter is composed of particles...
as their power source. There are several designs 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
Energy density
Energy density is a term used for the amount of energy stored in a given system or region of space per unit volume. Often only the useful or extractable energy is quantified, which is to say that chemically inaccessible energy such as rest mass energy is ignored...
and specific impulse
Specific impulse
Specific impulse is a way to describe the efficiency of rocket and jet engines. It represents the derivative of the impulse with respect to amount of propellant used, i.e., the thrust divided by the amount of propellant used per unit time. If the "amount" of propellant is given in terms of mass ,...
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 propulsion, 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.
Direct use of reaction products
AntiprotonAntiproton
The antiproton is the antiparticle of the proton. Antiprotons are stable, but they are typically short-lived since any collision with a proton will cause both particles to be annihilated in a burst of energy....
annihilation reactions produce charged and uncharged meson
Meson
In particle physics, mesons are subatomic particles composed of one quark and one antiquark, bound together by the strong interaction. Because mesons are composed of sub-particles, they have a physical size, with a radius roughly one femtometer: 10−15 m, which is about the size of a proton...
s, in addition to gamma ray
Gamma ray
Gamma radiation, also known as gamma rays or hyphenated as gamma-rays and denoted as γ, is electromagnetic radiation of high frequency . Gamma rays are usually naturally produced on Earth by decay of high energy states in atomic nuclei...
s. 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
Positron
The positron or antielectron is the antiparticle or the antimatter counterpart of the electron. The positron has an electric charge of +1e, a spin of ½, and has the same mass as an electron...
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
Eugen Sänger
Eugen Sänger was an Austrian-German aerospace engineer best known for his contributions to lifting body and ramjet technology.-Early career:...
, 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 specularly reflecting gamma rays has been proposed (no solid material has this property, and plasma
Plasma (physics)
In physics and chemistry, plasma is a state of matter similar to gas in which a certain portion of the particles are ionized. Heating a gas may ionize its molecules or atoms , thus turning it into a plasma, which contains charged particles: positive ions and negative electrons or ions...
is not sufficiently reflective to gamma rays under practically attainable conditions). However, the momentum of gamma rays can indeed be partially transferred to matter by Compton scattering
Compton scattering
In physics, Compton scattering is a type of scattering that X-rays and gamma rays undergo in matter. The inelastic scattering of photons in matter results in a decrease in energy of an X-ray or gamma ray photon, called the Compton effect...
.
Antimatter heating of an exhaust fluid
Several methods for heating an exhaust fluid using the gamma rays produced by positron annihilation have been proposed. These methods resemble those proposed for nuclear thermal rocketNuclear thermal rocket
In a nuclear thermal rocket a working fluid, usually liquid hydrogen, is heated to a high temperature in a nuclear reactor, and then expands through a rocket nozzle to create thrust. In this kind of thermal rocket, the nuclear reactor's energy replaces the chemical energy of the propellant's...
s. One proposed method is to use positron annihilation gamma rays to heat a solid engine core. Hydrogen
Hydrogen
Hydrogen is the chemical element with atomic number 1. It is represented by the symbol H. With an average atomic weight of , hydrogen is the lightest and most abundant chemical element, constituting roughly 75% of the Universe's chemical elemental mass. Stars in the main sequence are mainly...
gas is ducted through this core, heated, and expelled from a rocket nozzle
Nozzle
A nozzle is a device designed to control the direction or characteristics of a fluid flow as it exits an enclosed chamber or pipe via an orifice....
. A second proposed engine type uses positron annihilation within a solid lead
Lead
Lead is a main-group element in the carbon group with the symbol Pb and atomic number 82. Lead is a soft, malleable poor metal. It is also counted as one of the heavy metals. Metallic lead has a bluish-white color after being freshly cut, but it soon tarnishes to a dull grayish color when exposed...
pellet or within compressed xenon
Xenon
Xenon is a chemical element with the symbol Xe and atomic number 54. The element name is pronounced or . A colorless, heavy, odorless noble gas, xenon occurs in the Earth's atmosphere in trace amounts...
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
Specific impulse
Specific impulse is a way to describe the efficiency of rocket and jet engines. It represents the derivative of the impulse with respect to amount of propellant used, i.e., the thrust divided by the amount of propellant used per unit time. If the "amount" of propellant is given in terms of mass ,...
achievable by these methods is limited by materials considerations, typically being in the range of 1000–2000 seconds.
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 rocketNuclear electric rocket
In a nuclear electric rocket, nuclear thermal energy is changed into electrical energy that is used to power one of the electrical propulsion technologies. Technically the powerplant is nuclear, not the propulsion system, but the terminology is standard. A number of heat-to-electricity schemes...
s. Antimatter annihilations are used to directly or indirectly heat a working fluid, as in a nuclear thermal rocket
Nuclear thermal rocket
In a nuclear thermal rocket a working fluid, usually liquid hydrogen, is heated to a high temperature in a nuclear reactor, and then expands through a rocket nozzle to create thrust. In this kind of thermal rocket, the nuclear reactor's energy replaces the chemical energy of the propellant's...
, 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).
Difficulties with antimatter rockets
The chief practical difficulties with antimatter rockets are the problems of creating antimatter and storing it. 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. Most proposed antimatter rocket designs require a large amount of antimatter (around 10 grams to reach Mars in one month). 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, , been performed only for small numbers of individual atoms. Storage of antimatter is typically done by trapping electrically charged frozen antihydrogen pellets in PenningPenning trap
Penning traps are devices for the storage of charged particles using a homogeneous static magnetic field and a spatially inhomogeneous static electric field. This kind of trap is particularly well suited to precision measurements of properties of ions and stable subatomic particles which have...
or Paul trap
Quadrupole ion trap
A quadrupole ion trap exists in both linear and 3D varieties and refers to an ion trap that uses constant DC and radio frequency oscillating AC electric fields to trap ions. It is commonly used as a component of a mass spectrometer...
s. 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.
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
Ionizing radiation
Ionizing radiation is radiation composed of particles that individually have sufficient energy to remove an electron from an atom or molecule. This ionization produces free radicals, which are atoms or molecules containing unpaired electrons...
. The antimatter mechanisms proposed to date have for the most part provided plausible mechanisms for harnessing energy from these annihilation products.
See also
- Spacecraft propulsionSpacecraft propulsionSpacecraft propulsion is any method used to accelerate spacecraft and artificial satellites. There are many different methods. Each method has drawbacks and advantages, and spacecraft propulsion is an active area of research. However, most spacecraft today are propelled by forcing a gas from the...
- Antimatter catalyzed nuclear pulse propulsionAntimatter catalyzed nuclear pulse propulsionAntimatter catalyzed nuclear pulse propulsion is a variation of nuclear pulse propulsion based upon the injection of antimatter into a mass of nuclear fuel which normally would not be useful in propulsion...
- Redshift rocketRedshift rocketThe redshift rocket, envisaged by novelist Karl Schroeder, is a fictional method of spacecraft propulsion.It is a variant on the Sänger antimatter rocket. One of the key difficulties of harnessing the energy from matter/antimatter collisions is that most of the energy released by them is in the...