Anantimatter rocket is a proposed class ofrockets that useantimatter 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 therest mass of a matter/antimatter mixture may be converted to energy, allowing antimatter rockets to have a far higherenergy density andspecific impulse than any other proposed class of rocket.^[1]

Antimatter rockets can be divided into three types of application: those that directly use the products of antimatter annihilation for propulsion, those that heat a working fluid or an intermediate material which is then used for propulsion, and those that heat a working fluid or an intermediate material to generate electricity for some form ofelectric spacecraft propulsion system.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.^[2]Then there are hybrid solutions using antimatter to catalyze fission/fusion reactions for propulsion.

Antiproton annihilation reactions produce charged and unchargedpions, in addition to neutrinos andgamma rays. The charged pions can be channelled by amagnetic nozzle, producing thrust. This type of antimatter rocket is apion rocket orbeamed core configuration. It is not perfectly efficient; energy is lost as the rest mass of the charged (22.3%) and uncharged pions (14.38%), lost as the kinetic energy of the uncharged pions (which can't be deflected for thrust); and lost as neutrinos and gamma rays (seeantimatter as fuel).^[3]

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 byEugen Sänger, assumed the use of some material that could reflect gamma rays, used as alight sail orparabolic shield to derive thrust from the annihilation reaction, but no known form of matter (consisting of atoms or ions) interacts with gamma rays in a manner that would enable specular reflection. The momentum of gamma rays can, however, be partially transferred to matter byCompton scattering.^[4][5]

One method to reach relativistic velocities uses a matter-antimatter GeV gamma ray laser photon rocket made possible by a relativistic proton-antiproton pinch discharge, where the recoil from the laser beam is transmitted by theMössbauer effect to the spacecraft.^[6]

A new annihilation process has purportedly been developed by researchers from the University of Gothenburg, Sweden. Several annihilation reactors have been constructed in the past years^[when?] which attempted to converthydrogen ordeuterium into relativistic particles through laser annihilation. The technology was explored by research groups led by Prof. Leif Holmlid and Sindre Zeiner-Gundersen, and a third relativistic particle reactor is currently being built at the University of Iceland. In theory, emitted particles from hydrogen annihilation processes could reach 0.94c and can be used in space propulsion.^[7] However the veracity of Holmlid's research is under dispute and no successful implementations have been peer reviewed or replicated.^[8]

This type of antimatter rocket is termed athermal antimatter rocket as the energy or heat from the annihilation is harnessed to create an exhaust from non-exotic material or propellant.

Thesolid core concept uses antiprotons to heat a solid,high-atomic weight (Z), refractory metal core. Propellant is pumped into the hot core and expanded through a nozzle to generate thrust. The performance of this concept is roughly equivalent to that of thenuclear thermal rocket (${\displaystyle I_{\text{sp}}}$  ~ 10³ sec) due to temperature limitations of the solid. However, the antimatter energy conversion and heating efficiencies are typically high due to the shortmean path between collisions with core atoms (efficiency${\displaystyle {\eta }_e}$  ~ 85%).^[2]Several methods for theliquid-propellant thermal antimatter engine using the gamma rays produced by antiproton or positron annihilation have been proposed.^[9][10] These methods resemble those proposed fornuclear 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 arocket nozzle. A second proposed engine type uses positron annihilation within a solidlead pellet or within compressedxenon 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, thespecific impulse achievable by these methods is limited by materials considerations, typically being in the range of 1000–2000 seconds.^[11]

Thegaseous core system substitutes the low-melting point solid with a high temperature gas (i.e. tungsten gas/plasma), thus permitting higher operational temperatures and performance (${\displaystyle I_{\text{sp}}}$  ~ 2 × 10³ sec). However, the longer mean free path for thermalization and absorption results in much lower energy conversion efficiencies (${\displaystyle {\eta }_e}$  ~ 35%).^[2]

Theplasma core allows the gas to ionize and operate at even higher effective temperatures. Heat loss is suppressed by magnetic confinement in the reaction chamber and nozzle. Although performance is extremely high (${\displaystyle I_{\text{sp}}}$  ~ 10⁴-10⁵ sec), the long mean free path results in very low energy utilization (${\displaystyle {\eta }_e}$  ~ 10%)^[2]

The idea of using antimatter to power anelectric space drive has also been proposed. These proposed designs are typically similar to those suggested fornuclear electric rockets. Antimatter annihilations are used to directly or indirectly heat a working fluid, as in anuclear 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 charged particle/electric propulsion proposals, that typically being high specific impulse and low thrust (see alsoantimatter power generation).^[12][13]

This is a hybrid approach in which antiprotons are used tocatalyze a fission/fusion reaction or to "spike" the propulsion of afusion rocket or any similar applications.

The antiproton-drivenInertial confinement fusion (ICF) Rocket concept uses pellets for theD-T reaction. The pellet consists of a hemisphere of fissionable material such asU²³⁵ with a hole through which a pulse of antiprotons and positrons is injected. It is surrounded by a hemisphere of fusion fuel, for example deuterium-tritium, or lithium deuteride. Antiproton annihilation occurs at the surface of the hemisphere, which ionizes the fuel. These ions heat the core of the pellet to fusion temperatures.^[14]

The antiproton-driven Magnetically Insulated Inertial Confinement Fusion Propulsion (MICF) concept relies on self-generated magnetic field which insulates the plasma from the metallic shell that contains it during the burn. The lifetime of the plasma was estimated to be two orders of magnitude greater than implosion inertial fusion, which corresponds to a longer burn time, and hence, greater gain.^[14]

The antimatter-drivenP-B¹¹ concept uses antiprotons to ignite the P-B¹¹ reactions in an MICF scheme. Excessive radiation losses are a major obstacle to ignition and require modifying the particle density, and plasma temperature to increase the gain. It was concluded that it is entirely feasible that this system could achieve I_sp~10⁵s.^[15]

A different approach was envisioned forAIMStar in which small fusion fuel droplets would be injected into a cloud of antiprotons confined in a very small volume within a reactionPenning trap. Annihilation takes place on the surface of the antiproton cloud, peeling back 0.5% of the cloud. The power density released is roughly comparable to a 1 kJ, 1 ns laser depositing its energy over a 200 μm ICF target.^[16]

TheICAN-II project employs the antiproton catalyzed microfission (ACMF) concept which uses pellets with a molar ratio of 9:1 of D-T:U²³⁵ fornuclear pulse propulsion.^[17]

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.^[18][19] 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 2010^[update], been performed only for small numbers of individual atoms. Storage of antimatter is typically done by trapping electrically charged frozen antihydrogen pellets inPenning orPaul traps. There is no theoretical barrier to these tasks being performed on the scale required to fuel an antimatter rocket. However, they are expected to be extremely (and perhaps prohibitively) expensive due to current production abilities being only able to produce small numbers of atoms, a scale approximately 10²³ times smaller than needed for a 10-gram trip to Mars.

Generally, the energy from antiproton annihilation is deposited over such a large region that it cannot efficiently drive nuclear capsules. Antiproton-induced fission and self-generated magnetic fields may greatly enhance energy localization and efficient use of annihilation energy.^[20][21]

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 energeticionizing radiation. The antimatter mechanisms proposed to date have for the most part provided plausible mechanisms for harnessing energy from these annihilation products. The classicrocket equation with its "wet" mass (${\displaystyle M_0}$ )(withpropellant mass fraction) to "dry" mass (${\displaystyle M_1}$ )(withpayload)fraction (${\displaystyle \frac{M_0}{M_1}}$ ), the velocity change (${\displaystyle \mathrm{\Delta }v}$ ) and specific impulse (${\displaystyle I_{\text{sp}}}$ ) no longer holds due to the mass losses occurring in antimatter annihilation.^[3]

Another general problem with high powered propulsion is excess heat orwaste heat, and as with antimatter-matter annihilation also includes extreme radiation. A proton-antiproton annihilation propulsion system transforms 39% of the propellant mass into an intense high-energy flux of gamma radiation. The gamma rays and the high-energy charged pions will cause heating and radiation damage if they are not shielded against. Unlike neutrons, they will not cause the exposed material to become radioactive by transmutation of the nuclei. The components needing shielding are the crew, the electronics, the cryogenic tankage, and the magnetic coils for magnetically assisted rockets. Two types of shielding are needed:radiation protection andthermal protection (different fromHeat shield orthermal insulation).^[3][22]

Finally, relativistic considerations have to be taken into account. As the by products of annihilation move atrelativistic velocities therest mass changes according torelativistic mass–energy. For example, the total mass–energy content of the neutral pion is converted into gammas, not just its rest mass. It is necessary to use arelativistic rocket equation that takes into account the relativistic effects of both the vehicle andpropellant exhaust (charged pions) moving near the speed of light. These two modifications to the two rocket equations result in a mass ratio (${\displaystyle \frac{M_0}{M_1}}$ ) for a given (${\displaystyle \mathrm{\Delta }v}$ ) and (${\displaystyle I_{\text{sp}}}$ ) that is much higher for a relativistic antimatter rocket than for either a classical or relativistic "conventional" rocket.^[3]

The loss of mass specific to antimatter annihilation requires a modification of the relativistic rocket equation given as^[23]

where${\displaystyle c}$  is the speed of light, and${\displaystyle I_{\text{sp}}}$  is the specific impulse (i.e.${\displaystyle I_{\text{sp}}}$ =0.69${\displaystyle c}$ ).

The derivative form of the equation is^[3]

where${\displaystyle M_{\text{ship}}}$  is the non-relativistic (rest) mass of the rocket ship, and${\displaystyle a}$  is the fraction of the original (on board) propellant mass (non-relativistic) remaining after annihilation (i.e.,${\displaystyle a}$ =0.22 for the charged pions).

Eq.II is difficult to integrate analytically. If it is assumed that${\displaystyle v\sim I_{\text{sp}}}$ , such that${\displaystyle (1-\frac{I_{\text{sp}}v}{c^2})\sim (1-\frac{v^2}{c^2})}$  then the resulting equation is

Eq.III can be integrated and the integral evaluated for${\displaystyle M_0}$  and${\displaystyle M_1}$ , and initial and final velocities (${\displaystyle v_i=0}$  and${\displaystyle v_f=\mathrm{\Delta }v}$ ).The resulting relativistic rocket equation with loss of propellant is^[3][23]

The cosmic backgroundhard radiation will ionize the rocket's hull over time and poses ahealth threat. Also, gas plasma interactions may causespace charge. The major interaction of concern is differential charging of various parts of a spacecraft, leading to high electric fields and arcing between spacecraft components. This can be resolved with well placedplasma contactor. However, there is no solution yet for when plasma contactors are turned off to allow maintenance work on the hull. Long term space flight at interstellar velocities causes erosion of the rocket's hull due to collision with particles,gas,dust andmicrometeorites. At 0.2${\displaystyle c}$  for a 6 light year distance, erosion is estimated to be in the order of about 30 kg/m² or about 1 cm of aluminum shielding.^[24][25]

• Nuclear photonic rocket