Ram scoop

A ram scoop is a theoretical device to power spaceflight. It uses a powerful magnetic field during flight to collect interstellar hydrogen that can be used as a fuel source.

Proposed at least as early as 1960 by R.W. Bussard, it remains well beyond current technology.

Advantages

The major advantage is that it would reduce the amount of fuel to be carried on board. Further, the greater the speed, the more efficient the collection. In theory, given enough time, it would propel a ship close to light speed.

Drawbacks

Drawbacks include size - a NASA project looking at a flight to Alpha Centauri envisioned an intake 650 kilometers across - as well as uncertainty about whether sufficient hydrogen exists to power the ship. There is also the question of a braking effect; the field might catch particles that would build up, slowing down the craft.

Trivia

Ram scoops appeared in the computer games "Elite" and "Escape Velocity" and the Doctor Who story "42".