A mass driver is a device that uses a linear motor to accelerate a mass to high-speed, hence the name. Electromagnetic catapult is another name for mass driver and refers to physical principles behind this system.
Two different but related applications of mass drivers have been proposed in spaceflight. The first is as a method of non-rocket space launch. Here a mass driver is located at the surface of a celestial body, such as the Moon, and remains stationary.
In theory mass drivers could be used on Earth to launch stuff into space. But given terrestrial gravity this would require very large structures and the costs might be prohibitive. Since the escape velocity of the Moon is much lower, lunar mass drivers are a common trope in discussions on lunar activities.
A second application of mass drivers is to propel spacecraft in space. Small pellets of mass are expelled by the mass driver from the spacecraft and due to Newton’s third law of motion, this will cause the spacecraft to be accelerated in the opposite direction. This is similar to rockets.
O’Neill has proposed to use mass drivers as method of propulsion for “space tug boats”, which would accelerate space ships towards the asteroid belt. (The tug boats themselves would stay in near earth space.) As propellant the waste from lunar mining activity would be used.
A problem with this method is, however, that the exhaust particles will increase the amount of space debris. Given the high velocity of these particles, they will constitute a serious threat to satellites, probes and space habitats. Though this might not seem a big problem as the solar system is quite large, but increased human activity in space will also increase the probability that the exhaust of mass drivers will cause serious damage.
This problem might be avoided if instead of mass drivers solar sails are used. Solar sails use the Sun’s radiation pressure to accelerate. The benefit of solar sails is that they do not contribute to space debris.
A disadvantage of solar sails is that a large surface is needed to acquire is relatively low amount of thrust. Depending on the purpose of the spacecraft, this might be a problem. For a scientific unmanned probe this does not seem a real issue, but it might be for a manned commercial trip.
A second issue with solar sails is that the solar radiation pressure falls as an inverse square function of the distance from the Sun (i.e. if the distance is doubled, the pressure will be one-fourth). Again this does not have to be a big concern, depending on circumstances.
Space-based, high-power lasers could overcome these issues. Since lasers concentrate energy in a relative small area, “solar” sails can be accelerated at a higher rate. An additional benefit of beam-powered propulsion is that spacecraft can be accelerated at great distances.
Since solar radiation is pointed away from the Sun, solar sails have little value of spacecrafts with an inward direction – except for deceleration. Lasers located in the asteroid belt can be used to decelerate outward spacecraft and to accelerate inward ones.