Jet pack, rocket belt, rocket pack, and similar names, are various types of device, usually worn on the back, that use jets of escaping gases (or in some cases liquid water) to allow a single user to fly. The concept of these devices emerged from science fiction in the 1920s and popularised in the 1960s as the technology became a reality. Currently the only practical uses of the jet pack has been extra-vehicular activity for astronauts. Despite decades of advancement in the technology, the challenges of Earth's atmosphere, Earth's gravity, and the human body (which is not well suited for this type of flight) remain an obstacle to its potential use in the military and as a means of personal transport.