Antimatter rocket
Antiproton annihilation reactions produce charged and uncharged pions, in addition to neutrinos and 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 or parabolic 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.
[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 the Mössbauer effect to the spacecraft.
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.
[2] Several methods for the liquid-propellant thermal antimatter engine using the gamma rays produced by antiproton or positron annihilation have been proposed.
One proposed method is to use positron annihilation gamma rays to heat a solid engine core.
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.
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.
However, the longer mean free path for thermalization and absorption results in much lower energy conversion efficiencies (
[2] The plasma core allows the gas to ionize and operate at even higher effective temperatures.
These proposed designs are typically similar to those suggested for nuclear electric rockets.
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 also antimatter power generation).
[12][13] This is a hybrid approach in which antiprotons are used to catalyze a fission/fusion reaction or to "spike" the propulsion of a fusion rocket or any similar applications.
The antiproton-driven Inertial confinement fusion (ICF) Rocket concept uses pellets for the D-T reaction.
The pellet consists of a hemisphere of fissionable material such as U235 with a hole through which a pulse of antiprotons and positrons is injected.
Excessive radiation losses are a major obstacle to ignition and require modifying the particle density, and plasma temperature to increase the gain.
The power density released is roughly comparable to a 1 kJ, 1 ns laser depositing its energy over a 200 μm ICF target.
[16] The ICAN-II project employs the antiproton catalyzed microfission (ACMF) concept which uses pellets with a molar ratio of 9:1 of D-T:U235 for nuclear pulse propulsion.
[18][19] Most storage schemes proposed for interstellar craft require the production of frozen pellets of antihydrogen.
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.
[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 energetic ionizing 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.
[3] The loss of mass specific to antimatter annihilation requires a modification of the relativistic rocket equation given as[23] where
Long term space flight at interstellar velocities causes erosion of the rocket's hull due to collision with particles, gas, dust and micrometeorites.
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.
