Movies show exposed astronauts flash freezing or boiling, but the physiology is stranger. In a vacuum you lose heat only by radiation, your blood stays contained, and instead you suffer ebullism as tissues swell to roughly twice their volume while your skin holds you together. Hold your breath and your lungs rupture; exhale and you have about 15 seconds of consciousness. This episode explains what a spacesuit must do to prevent that, starting with the 32.4 kilopascal pure oxygen atmosphere that mimics 6,100 feet of altitude and the four hour pre-breathe on the ISS that prevents nitrogen bubbles and the bends.

The core paradox is pressure versus mobility: a suit inflated to full Earth pressure becomes a rigid statue, so engineers build constant volume joints with bladders, restraint layers, gores, and convolutes, and cool the body with a liq ...