In February, 1984, astronaut Bruce McCandless floats 320 feet from the space shuttle Challenger during the first-ever untethered space walk, made possible by that most sci-fi of all gadgets, a jet-propelled backpack. Almost everything that makes the idea of exploring space can be found in this picture: the immediately recognizeable human form; the vastness of space; the blue glow of Earth, an impossibly long way off, and yet right there, in sight. For a second this picture feels comical. Quite quickly, it turns harrowing and thrilling at once.