How long can humans live in space? We don't know the answer to that. But every crew that resides on the International Space Station provides us information that we use to adjust our protocols and extends that period of time.
The longest contiguous amount of time a human has spent in space is 437 days. That was done by Valeri Polyakov, almost 20 years ago. That was his second flight. His first was 240 days long. Sergei Krikalev, over six flights, has spent 803 days in space.
Space is a harsh environment for humans—we didn't evolve to live there. Systems throughout our body have specifically evolved to function under the stresses of gravity.
Because our muscles are constantly working against gravity on Earth, those muscles have a lot less work to do in space. In other words, they'll start to atrophy. That includes our heart (which shrinks a little), the muscles in our neck that hold up our heads, and our calves.
Since our bones don't experience the same loads they do on the ground, they feel no need to maintain themselves. Bone tissue is still absorbed, but it doesn't rebuild itself like it would on Earth. This absorbed bone can cause kidney stones, and the places where the bone has weakened can easily fracture.
Here's a graphic that shows the rate of bone loss for various parts of the body:
There is evidence that extended periods in space can result in dysregulation of the immune system. While there isn't too much risk of being exposed to pathogens, a compromised immune system can result in unwanted autoimmune responses and hypersensitivity.
Aside from the deleterious affects of microgravity, a potentially larger threat is radiation. We really do take for granted how much our planet protects us from it.
Out in space, astronauts are exposed to galactic cosmic radiation, solar radiation, solar cosmic particles, and geomagnetically bound radiation. All these types of radiation are so prevalent that, when astronauts close their eyes, they often still "see" bright flashes, as cosmic rays hit their optic nerves.
Radiation exposure can result in cell death or cause mutations resulting in cancers or epigenetic effects. Some systems are quite sensitive to radiation and their functions can quickly become impaired, such as the immune system, bone marrow system and eyes, which could develop cataracts.
We do our best to provide shielding, but paradoxically, shielding can result in more exposure by what is called secondary radiation. When an energetic particle hits the shielding material it can emit gamma rays and energetic neutrons. NASA takes astronaut radiation exposure very seriously. We both monitor the exposure of each crew member for individual missions and maintain a total career tally. When they hit the allowed threshold, they can be prohibited from flying ever again.
Image: Caspar Benson/Getty Images
How long can humans live in space and what is the worst case scenario for someone who lives too long in space? originally appeared on Quora: "Ask a question, get a great answer. Learn from experts and get insider knowledge." You can follow Quora on Twitter, Facebook, and Google+.
This answer has been lightly edited for grammar and clarity.