Cosmonauts wear space suits now, at least during takeoff, landing, and docking.
Originally, space suits were not required, and some cosmonauts flew in their underwear.
But on June 29, 1971, when the three cosmonauts in Soyuz 11 undocked from the space station Salyut 1, an air release valve suddenly opened.
Without the life-support systems of a space suit, the men suffocated.
From then on, space suits have been mandatory at all critical phases of space journeys.
They are one-piece suits that pressurize and cool, and carry life-support instrumentation in the pants legs.