Most space propulsion systems use gas. Even if carried in liquid form, this type of fuel takes up a lot of room, hardly ideal for space travel. And if there is a problem, as happened with the Challenger Space Shuttle mission, the results can be catastrophic. Better would be a propulsion system using solid fuel, being far safer than explosive liquid or gas. Carl Smith reports from Adelaide on Paddy Neumann’s development of an ion thruster which uses electricity, in powerful bursts similar to an arc welder, to produce plasma, and thrust. The electricity is applied to metals such as titanium or magnesium or any solid conductive fuel rod.