A jet pack, rocket belt or rocket pack is a device, usually worn on the back, which uses jets of gas (or in some cases liquid) to propel the wearer through the air. The concept has been present in science fiction for almost a century and became widespread in the 1960s. Real jet packs, using a variety of mechanisms have been developed, though the challenges of Earth’s atmosphere, Earth’s gravity, low energy density of available fuels, and the human body not being naturally adapted to fly mean that their uses are much more limited than their fictional counterparts, being principally used for stunts. A practical use for the jet pack has been in extra-vehicular activities for astronauts.