The Epstein Drive is a modified fusion drive invented by Solomon Epstein one hundred and fifty years before the start of Leviathan Wakes. It enabled humanity to travel from Earth and the inner planets, and colonize the Asteroid Belt and outer planets.
The drive utilizes magnetic coil exhaust acceleration to increase drive efficiency, which enables spaceships to sustain thrust throughout the entire voyage. A ship fitted with the efficient Epstein drive is able to run the drive continuously for acceleration to its goal and then after flipping at about the halfway point is able to run the drive continuously during deceleration. Previous engine designs used propellant less efficiently and could not be run long enough to achieve the high velocities that the Epstein drive permitted.