Why do people only stay in submarines for a maximum of 90 days, but can stay in space for 6 months or more?

Submarine environment

Nuclear-powered submarines can stay underwater for three months, while diesel-powered submarines can only stay submerged for a few days and then must surface to get air for their engines.

While in a submarine, people face very harsh conditions. First, air is very scarce, and can even be poisoned.

Submarines often travel at depths of several hundred meters, so they must endure extremely high water pressure. At that time, the air inside the submarine will be compressed into a very dense mass if no special measures are taken. Humans cannot breathe normally in that condition.

When in a submarine, people have to work, live and rest in a cramped space, with almost no windows to look out, if there are any, there is only the black color of the seabed. Only when close to the sea surface does a periscope provide external visibility. This makes people in submarines susceptible to fatigue, headaches, dizziness, chest tightness and psychological stress.

Environment in space

As for the outer space environment, humans face many risks and limitations, such as high radiation, gravity imbalance, harsh environmental conditions,.

The imbalance of gravity in the space environment causes astronauts to lose the corresponding sense of balance in space, and their bodies cannot withstand their own gravity. Therefore, to maintain muscle and bone health, astronauts must exercise a lot.

Air and water in space are brought in from Earth, so they are also very limited. This supply chain is fragile, and if there were a problem, astronauts would face serious existential danger.

Astronauts in space will not have fresh air, which increases psychological stress, leading to emotional problems such as depression and anxiety.

Thus, the crew of a deep-sea submarine will encounter more difficulties, their environment will be harsher, the crew's living space will be more limited and outside observation will be impossible. The crew is easily influenced psychologically. Therefore, people cannot stay in submarines for more than 90 days. In outer space, although there are many limitations and the environment is harsh, astronauts can see beautiful scenery outside the window of the spacecraft in space to help relieve their psychology.