Living on an alien planet

Let's say that you are one of the first brave astronauts to step on an alien planet. What conditions would you be able to tolerate?

The first problem is breathing. The human body can withstand a lot of abuse but breathing must be done to survive. Having been born on Earth, humans need specific levels of gases to stay alive. One of the most important gases is oxygen. Without it we die.

Earth air contains 78.62% nitrogen and 20.84 % oxygen. The rest consists of 0.96% argon, 0.04% carbon dioxide and 0.5% water vapor. This air is at normal pressure. How low or how high of pressure could we breath and still survive?

At low pressures, the kind that you experience at high altitudes, oxygen has a lower partial pressure. At an altitude of 8,000 feet, the lower oxygen pressure results in altitude sickness. This effect gets much worse the higher one goes, and it gets bad at 15,000 feet. Above 18,000 feet a person could be in serious trouble. This effect is dependent upon the area of lungs, the thickness of the lung membranes, the partial pressure of oxygen and the partial pressure of carbon dioxide. You have to breath oxygen in and exhale carbon dioxide out. What all of this means is that you can tolerate a low air pressure for a short time and even at a higher air pressure one needs sufficient oxygen and low amounts of carbon dioxide. The limit for oxygen is 11% at normal pressure.

However, if the air pressure is much higher than normal, this could result in the bends because of absorption of nitrogen in the blood. I assume here that most planets would have an atmosphere with mostly nitrogen. We could substitute argon but it's more absorptive than Nitrogen. Helium would be better but a high helium concentration would make us sound very strange. Neon is another inert gas. Both Helium and Neon are not as absorptive as Nitrogen. These other inert gases are not as plentiful as Nitrogen.

We could breath pure oxygen or oxygen at much higher concentrations than on Earth, but it's unlikely that we would find a planet with a high oxygen atmosphere. Such a planet would suffer frequent fires and possibly spontaneous combustion.

One gas that we can't tolerate in high concentration is carbon dioxide, and this is a commonly found gas in exo-planet atmospheres. Earth's atmosphere contains 0.04%. At 1% people get sick. At 7 to 10% people pass out and eventually die.

There are other gases that would, if in high enough concentrations, kill us. Hydrogen sulfide, carbon monoxide, Sulfur dioxide, and hydrogen cyanide could be present in an alien planet's atmosphere.

Finding an exo-planet with the proper amount of oxygen at a decent pressure may be difficult, but until we try we'll never know for sure.

What about temperature? The human body can't tolerate extremes of temperature. We're only reasonably confortable from 40 F (4 C) and 95 F (35 C). We can tolerate much higher temperature if the humidity is low, but at 140 F (60 C) and high humidity this condition usually results in death. Cold tolerance is another matter. We humans can't tolerate body temperatures much lower than 70 F (21 C). However, with proper cold temperature clothing, it's possible to survive for extended periods in really cold environments.

This is not the only problem we could face. The human body is vulnerable to radiation. Radiation as high as 5 - 6 Sieverts for even a few minutes would cause damage to our cells. Radiation lower than this would eventually cause cancers. If the radiation is too high, it's time to find another planet.

There could be many other dangerous problems on an alien planet. Pathogens that we're not immune to could cause diseases with no known cure. Poisons in the air, the dust blown around in the wind, and in the water would have to be dealt with.

Conditions not suitable for the human body would require environmental suits, but that would not be a suitable planet for eventual colonization.

Thanks for reading.