Spending time in space is a privilege reserved for only a few humans, but it comes at a cost: living in microgravity has several negative effects on human physiology.

Not all the effects and consequences are completely understood, though. Studying astronaut Scott Kelly, who has just returned from a 340-day mission on the International Space Station (ISS), and comparing him to his twin brother Mark, scientists hope to learn a lot more about how humans can adapt to living away from Earth. "They'll be going through a variety of different tasks that look a lot like what you might have to do if you had just landed on the surface of Mars," said International Space Station Chief Scientist Julie Robinson in a NASA video.

Here's what we know so far.

Radiation

A significant concern for the astronauts’ health is the dose of radiation. The ISS is within Earth’s magnetosphere, so astronauts are partially shielded from the solar wind, but a year in space would still be equivalent to over 40 times the dose a person living in the U.S. is exposed to every year. 

The higher levels of radiation can cause short- and long-term damages to the bone marrow, which affects the blood and the immune system. Astronauts also have a higher incidence of eye problems, and this could be related to the cosmic ray visual phenomena. Astronauts see light flashes due to energetic cosmic rays going through their eyeballs.

Cancer and the accelerated onset of Alzheimer’s disease are other potential consequence of prolonged time in space.

Weightlessness

The ISS is in constant freefall, which generates the apparent lack of gravity. This creates a unique environment to push scientific limits, but at the same time, this has severe health consequences on the astronauts.

The body reacts to weightlessness by redirecting elsewhere the resources it uses to maintain bones and muscles healthily. Astronauts lose about 1 to 2 percent of bone tissue each month. This is comparable to people who suffer from osteoporosis. A year in space could reduce the bone tissue in extremities and hips by over 20 percent. 

Weightlessness also affects muscles. Without exercise, astronauts can lose 20 percent of their muscle mass in 5 to 11 days.  For this reason, astronauts on the ISS spend 2.5 hours a day exercising. Even with the constant exercise, astronauts can still lose 40 percent of their muscle after 5 months in space.

Loss of bone and muscle tissue is not permanent, but it takes years for an astronaut to return to pre-mission state. Diet and supplements to quicken this process are also employed by medical professionals to treat osteoporosis.

On a potential plus side, astronauts become taller in space.

^{Astronaut Andre Kuipers runs on a treadmill on the ISS. NASA}

Other Effects

There are other physiological and psychological effects. Many astronauts report a change in their sense of taste, something that occasionally remains when they return to Earth. Astronauts also report back and abdominal pain. These muscles are freely stretched when in space, and can have problems readjusting to gravity.

Astronauts lose about 22 percent of their blood volume. With less blood to pump, the heart begins to atrophy, which could lead to worse tissue oxygenation, and they often experience faintness and dizziness.  

Feet also go through a peculiar transformation. The top of feet becomes raw and painfully sensitive, while the bottom after a few months molt completely and lose all the hard calluses. So, it’s not all bad.

Mission to Mars

The journey to Mars is not going to be an easy one. Away from the protective shield of Earth's magnetic field the radiation levels are much higher. Astronauts are expected to be exposed to 100 times more radiation in an 180-day journey to Mars than one year on Earth.

Scott Kelly's rehabilitation is being studied in detail because it is going to be similar to what astronauts landing on Mars will experience. A mission to the Red Planet will require careful planning to account for all the potential health risks the future astronauts will encounter. 

There are also psychological effects to consider. Leaving Earth behind in a small capsule with six other people for up to 3 years is not going to be easy for anyone.