The gear worn during spacewalks includes small, lightweight, and totally self contained life support systems. The complete system is know as the Extravehicular Mobility Unit (EMU) and consists of the Space Suit Assembly (SSA) and the Portable Life Support Subsystem (PLSS). The PLSS is worn as an integrated "backpack."
The "space suit", used during the Mercury Program, was a modification of the Navy's MK-IV high altitude pressure suit. The suit provided cooling and acted as a back-up system in case the cabin lost pressure. The first American EVA occurred during the following Gemini Program. Six EVAs were performed (2 hour maximum) during the ten manned Gemini flights. Life support was provided to the suit from the cabin via a 24 foot umbilical. For the Apollo moon missions an EMU with an integral life support backpack was used which was capable of supporting six hour EVAs. For Skylab, a modified Apollo suit was used with life support provided from the cabin via a 60 foot umbilical. The EMU developed for Space Shuttle, with a tightly integrated PLSS backpack, was designed to be reusable (like the spacecraft). Future systems for ISSA will support eight hour EVAs.
The basic life support functions provided by the EMU include: pressure maintenance, oxygen delivery, ventilation, and control of temperature, humidity, carbon dioxide and trace organic contaminants (odors). The pressure inside the suit (4.3 psi) is intentionally kept low to enhance the astronaut's mobility. Because the cabin pressure is a 14.7 psi normal atmospheric mixture of nitrogen and oxygen, steps must be taken to minimize the potential for decompression sickness ('the bends') as a result of the EVA. Prior to EVA, cabin pressure is reduced to 10.2 psi for 24 hours. The astronauts then pre-breathe pure oxygen for 40 minutes. Inside the suit, ventilation is provided by oxygen flow over and in front of the face. This also prevents fogging of the helmet. Carbon dioxide and trace organics are removed using LiOH and activated carbon sorbents. Cooling is provided by water circulating through the undergarment. Heat rejection occurs as water freezes on porous plates and then sublimes to the vacuum of space.
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