Plasma Drive

The plasma drive is an engine technology that is common to all spacefairing cultures and largely underpins the safe and effective colonisation of space in the same way that the steam engine enabled the industrial revolution.
In the 2050s researchers at JPL experimenting on ion thrusters, a form of spacecraft propulsion which uses an electric charge to expel a noble gas at ultra-high speed, accidentally invented the precursor to the modern plasma drive. Following the initial experiments, the first full scale prototype plasma engine was mounted on a spacecraft in low earth orbit and piloted to the moon. What was previously a three day journey took a little under eleven hours and the engine was being run at half thrust. On the 4th of April 2061, the USS Ford arrived at the Demios outpost above Mars having set off on April Fools day, marking the first interplanetary journey by a ship powered by continuous thrust.
Over the next decade, the shape of the solar system changed dramatically and human society changed with it. Whilst tens of thousands fled Earth each year prior to the development of the plasma drive, tens of millions moved into the wider solar system every year in the years that followed. With the average transit time around the solar system now measured in days, the universe began to shrink and humanity could truly begin to call itself an interplanetary species.
In 2080 with the development of the Alcubierre Drive, the unspoken prerequisite was the plasma drive. It is not possible to generate a sufficient warp field strong enough for faster-than-light travel closer to a star than the ice line, the point at which water and other volatile chemicals can form free ice in space, or about 3 times the distance from the Earth to the Sun in the Sol system. Without a plasma drive getting out to this point would take months or even years, but even the slowest modern ship can do it in a week and your average passenger liner can do it four days.
In the modern era, the use of the plasma drive allows interplanetary commerce to exist. The quick, safe movement of people and goods from planets to stations allows economies to spread beyond individual planets and enables access to the material wealth of the asteroid belts ubiquitous in virtually all star systems. The continuous thrust also provides artificial gravity aboard ships, and whilst medical science is getting better at treating the illnesses and disorders that are caused by long term exposure to weightlessness, it is always better to have gravity.

Access & Availability

As widely available as an internal combustion engine in the early 21st century. Virtually all starships are equipped with plasma drives and their ownership is not restricted.

Complexity

Can be constructed with minimal moving parts, but requires enormous quantities of energy to run. Plasma drives are typically linked to nuclear fusion reactors, although examples run from fission reactors or fuel cells are not unheard of.

Discovery

Invented in 2056 by the NASA Jet Propulsion Laboratory on Earth, Sol.