Antimatter engines are a class of propulsion technology that make use of the energy released during the annihilation of matter and antimatter particles. The engineering advances required to produce the containment required for the first working prototype were only made possible due to the First Contact Event. Antimatter engines still remain in extremely limited use, in only the largest military and corporate fleets, due to the challenges and costs associated with producing and storing antimatter.
Production
All current mg+ scale antimatter production occurs in large fully automated stations. They are all in sun-polar orbits, which avoid the high density solar winds in the Sun's equatorial zones, and use large arrays of solar panels to power particle accelerators generating high-energy photons. These are directed towards a target material to create particle-antiparticle pairs when the photon interacts with the strong electric field of a nucleus.
Penning traps and optical molasses are used to store antimatter particles, and transportation off the station is performed by automated drones designed with multiple fail-safes to maintain power to the containment systems. If the target spacecraft is crewed, rendezvous normally occurs before they have boarded since integrating the antimatter containment unit with the propulsion system of the spacecraft is riskier than running the engine normally.
There have been recent attempts by Semiotican to scale up Muon-catalyzed fusion via Muons extracted by collectors deployed from its automated Jovian mining aerostat fleet. If successful this would provide a much cheaper near peer competitor for antimatter engines, potentially opening up the interstellar frontier to commerce. There have also been recent attempts by several smaller companies to scale up production of Titanium-44, which is currently only produced in tiny quantities for medical and research usage.
Operation
Antimatter engines are powered by the gamma ray energy released when matter and antimatter come into contact and annihilate each other. This energy is used to vaporize a solid propellant such as lithium (or more rarely heat a liquid propellant such as methane), producing a high-pressure high-speed exhaust which is ejected through a nozzle to produce thrust. Alternatively some designs use gamma rays to generate electricity, which is then used to accelerate charged particles for thrust.
Matter-antimatter reactions such as electron positron collisions are among the most energy-dense reactions known. The kinetic energy for an object moving at a velocity vv is given by 1/2 mv^2, where m is the mass of the object. Therefore achieving a velocity of 0.01c for a 100 tonne craft requires 4.5×10^17 joules of energy, which from E=mc^2 is only approximately 10kg of matter-antimatter fuel. As such most of the crafts tonnage is the propellant, with approximately half the displacement of a 100 tonne craft being the lithium for the 0.01c burn. The engineering required to make required volume of solid propellant available for vaporization is a commercial and military secret and other than containment is the main barrier preventing smaller corporations from developing competing antimatter engines.
Warships requiring a higher ISP are equipped with methane burn engines, these are not used commercially as several times as much liquid propellant is required for the same acceleration. The most expensive long range scientific probes use antimatter propulsion via an alternative system of generating electricity for photon thrust directly from the gamma rays, this system is more reliable than other antimatter engines due to its simplicity, and incredibly efficient due to the lack of non-antimatter propellant, however the low ISP makes this unsuitable for normal commercial transportation purposes. No propellant was observed being used in the first contact vessels antimatter engine, combined with the tiny size and extremely high ISP this suggests it was using the antimatter energy to kickstart a reaction in some form of exotic propellant such as dark matter or vacuum energy.
If containment for the average 10kg antimatter load failed, the explosion would release around 10^8 megatons of energy. The safe distance to keep radiation under 1msv would be around 20 000km, and this is the generally accepted guideline for the minimum distance a standard antimatter engine should be located from people other than crew. Other than early prototypes there has never been a publicly disclosed containment failure in either a factory or craft. There have recently been attempts by several corporations to remove some of the redundant safeguards in order to reduce the exorbitant cost of operating an antimatter engine.