Travelling between stars takes a lot of energy, but we might be able to pick some up on our way through space

The trouble with rockets is that you have to pack everything before you go. While earthly modes of transport all work within their environment, shooting around air or water or rolling along a surface, rockets must carry whatever they are going to shoot out the back - reactive mass - with them. This effect is exacerbated because the reactive mass you’ll be using at the end of the flight is just dead weight at the start. So you need even more mass and energy at the start just to lift the mass and energy you will need later on. This is why it is so challenging and expensive to get into space. Rockets are so big compared to the payloads they launch because they need to be more than 90 per cent full of propellants at take off.