To explore and work in space, human beings must take their environment with them because there is no
atmospheric pressure and no oxygen to sustain
life. Inside the spacecraft, the atmosphere can be controlled so that special clothing isn't needed,
but when outside, humans need the protection of a spacesuit.
Earth's atmosphere is 20 percent oxygen and 80 percent nitrogen from sea level to about 75 miles up,
where space begins. At 18,000 feet, the
atmosphere is half as dense as it is on the ground, and at altitudes above 40,000 feet, air is so thin
and the amount of oxygen so small that pressure
oxygen masks no longer do the job. Above the 63,000-foot threshold, humans must wear spacesuits that
supply oxygen for breathing and that maintain a
pressure around the body to keep body fluids in the liquid state. At this altitude the total air pressure
is no longer sufficient to keep body fluids from
boiling.
Spacesuits for the space shuttle era are pressurized at 4.3 pounds per square inch (psi), but because
the gas in the suit is 100 percent oxygen instead
of 20 percent, the person in a spacesuit actually has more oxygen to breathe than is available at an
altitude of 10,000 feet or even at sea level
without the spacesuit. Before leaving the space shuttle to perform tasks in space, an astronaut has to
spend several hours breathing pure oxygen before
proceeding into space. This procedure is necessary to remove nitrogen dissolved in body fluids and thereby
to prevent its release as gas bubbles when
pressure is reduced; a condition commonly called "the bends."
Spacesuits designed for the space station era will be pressurized to 8.3 psi; therefore, the pre-breathing
period will be shortened or diminished.
The spacesuit also shields the astronaut from deadly hazards. Besides providing protection from bombardment
by micrometeoroids, the spacesuit insulates
the wearer from the temperature extremes of space. Without the Earth's atmosphere to filter the sunlight,
the side of the suit facing the Sun may be
heated to a temperature as high as 250 degrees Fahrenheit; the other side, exposed to darkness of deep
space, may get as cold as -250 degrees Fahrenheit.