Electromagnetic Mass Drive
Electromagnetic Mass Drives, or Ion Drives, are a form of spacecraft propulsion. Less common and more recently developed than torchships, Ion Drives have certain advantages that are bringing them into greater use.
Ion drives differ from torches in that they use the power generated from the ship’s fusion reactor to power an electromagnetic mass driver that accelerates reaction mass (usually ionized hydrogen) to extremely high velocities.
Ejecting the ions at low temperature and high speed provides several advantages. First, the exhaust from an Ion drive is not a hazard to anything behind it in the way that the superheated plasma from a torch is. More importantly, Ion Drives are much more efficient over long distances than torches are. For this reason, most long-haul ships making the Earth-Poseidon run now use Ion Drives
The third benefit is that Ion Drives do not generate a visible drive flame or leave an easily detectable radiation trail. This makes ion drives very popular among smugglers, pirates, and military surveillance vessels which desire stealth.
Ion Drives are most efficient when used at low thrust for long periods of time. Ships on the Earth-Poseidon run will usually make the entire trip at about a sixth of a G of acceleration, reversing to decelerate at the halfway point. Some GEO Incorporate ships are faster, but the mass and power requirements become prohibitive as thrust goes up, so Ion Drive ships almost never have maximum thrust more than half a G.
The Earth-Poseidon run takes about six months at at 1/6 G. Faster Ion Drive ships or military Torchships can make the run in as little as 80 days.