Space is the most hostile place imaginable for human beings. There is no air to breathe, and there is no pressure - space is to all intents and purposes a vacuum. It is also permeated with dangerous radiation, from the Sun and from outer space. Sunlight is dangerous too because of its intensity, there being no medium to reduce its heating effect. On the other hand, out of the sunlight the temperature can plummet to -150 degrees. So when astronauts venture outside their spacecraft they must be well protected from the space environment with a spacesuit. The first suits astronauts wore were really just modifications of the pressure suits military pilots wore. The spacesuit began to evolve into its present form on the Gemini and Voshkod flights when astronauts began spacewalking. On their spacewalks, or EVAs, the astronauts wore suits with multiple layers of protection. Oxygen was supplied to the suit via a tube (the 'umbilical'), which was attached to a tether to prevent the astronaut floating too far from the spacecraft. For the Americans, the breakthrough in spacesuit design came with the self-contained, multilayered Apollo spacesuit, designed for prolonged EVAs on the Moon. The current shuttle spacesuit is a development from this. The shuttle spacesuit is made (unlike the Apollo) in two halves - upper torso and trousers. The two halves join at the waist with an airtight seal. They consist of several layers to provide the necessary protection. Oxygen is piped to an inner pressure layer. Underneath this the astronaut wears water-cooled long johns, to keep his or her temperature steady. The upper torso has an aluminium frame, and the life- support backpack, which provides the astronaut with oxygen, cooling water and power, is permanently attached to it. Shuttle astronauts take only about 10 minutes to don the suit, but they must spend at least two hours before EVA breathing an atmosphere of pure oxygen in order to flush nitrogen gas from their blood. If this were not done, they would get an attack of the bends when they switched to their suit oxygen, which is supplied under reduced pressure. They suit up inside the shuttle airlock on the mid-deck of the orbiter, and exit through the airlock hatch into the payload bay. If they stay inside the bay, they wear safety tethers. If they want to venture farther afield, they fly around in the jet-propelled MMU (manned manoeuvring unit). Bruce McCandless first test flew the MMU in February 1984, and it was used later in the year to help capture and repair the satellite Solar Max.
