A journey through the depths of space - is it just the stuff of science fiction, or could it ever come true? Researchers are exploring new concepts for the space ships of the future.A journey through space could take months or even years, and a ship could never carry enough conventional fuel to power it. New propulsion systems will be required, with an energy supply to last through the journey. When do the laws of physics reach their limits? Tomorrow Today reports on space travel research in Europe.