The Atmosphere
The particular mixture of gases that make up the Earth's atmosphere is necessary to support life. Take a deep breath. Each time you inhale, gases are drawn into your lungs, and oxygen is absorbed into the bloodstream, carried to the cells and used to fuel the myriad of actions that your body performs every second. Meanwhile, carbon dioxide is being expelled from your system each time you exhale. This breathing cycle continues through the lifetime of all human beings. We depend on the correct mixture of gases in the atmosphere to sustain our lives.The human being has evolved to be dependent upon the pressure of the atmosphere on Earth for breathing as well. Atmospheric pressure is a measure of the density and temperature of the gas molecules in the air. The atmospheric pressure allows us to inhale and exhale while breathing and also makes it possible to keep certain gases dissolved within the bloodstream. Breathing is an automatic body function and is essential to human survival. Thus, the correct mixture of gases in the atmosphere and the pressure of the atmosphere must both be maintained in order for us to breathe.
There is no atmosphere is space. There are gas molecules, but they are so few and far between that they do not produce a measurable pressure. The vacuum of space is more complete than any vacuum that can be produced in a laboratory on Earth. Therefore, space travelers must carry their own pressurized atmosphere with the correct mixture of gases in a leak-proof spacecraft cabin, or, when they venture outside the spacecraft, in suits pressurized by oxygen tanks.
In the spacecraft cabin and in the astronaut suits, the temperature must also be regulated and controlled because space is subject to extreme temperature variation. In general, it can be said that space is extremely cold! Temperature is a measure of energy and there is virtually nothing in open space that can absorb the energy from the sun and radiate heat. The energy from the sun passes freely through space. On the other hand, when there is a mass present in space (for instance, the Earth, the other planets, or a spaceship) the mass will absorb the energy from the sun and this energy translates into heat.
If a spacecraft, or an astronaut outside of a spacecraft performing what's called EVA (Extra-Vehicular Activity), lies in the path of the sun's rays, then the spacecraft or astronaut could absorb the direct, unshielded energy from the sun and become very hot. Thus, the spacecraft or the astronaut in his or her suit must be protected from overheating (or the loss of heat, if the direct sun's rays are not present) through the use of appropriate insulating material. But humans themselves have evolved to function on Earth with a body temperature of about 98.6 degrees F. and a rather narrow temperature band of comfort. Thus, the internal environment of both spacecraft and space suits must have active temperature regulation, involving both heating and cooling so that humans can survive space travel.
+ Humans in Space: Environmental Challenges (Back) + Hangman
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