12 bizarre things space travel does to the human body

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You don't need muscles to support you in a weightless environment, so astronauts' muscles almost immediately start shrinking and absorbing that the extra, now worthless tissue.

That's why there's a gym of sorts on the ISS. Strenuous exercise can help astronauts can maintain the muscle mass they need when they return to Earth and have to stand up and walk around again.

They usually budget about two hours a day for exercise, according to NASA. That includes "weight lifting" using a special resistance machine, since a free-weight weighs, well, nothing in zero gravity.

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Our bodies are mostly liquid. The gravity on Earth pulls that liquid down and some of it pools in our lower extremities. In near zero-g the liquid is spread out more evenly around the body, so astronauts' faces look puffier than normal while their legs look skinnier than normal.

After a few weeks the body adjusts to the gravity change and some of the puffiness goes down.

Astronauts can lose around 1% of their bone density every month they spend in space if they don't regularly exercise. It makes them a lot more likely to break bones when they're back on Earth — similar to people who have osteoporosis.

Exercise and good nutrition is crucial for minimizing this effect. This is especially important for Kelly who is spending a full year in space.

A 2013 study examined the eyes of 27 astronauts who had spent an average of 108 days on board the ISS. NASA researchers found that many of them had eye abnormalities after returning from space. MRI scans showed that nine of the astronauts had bloating around their optic nerves and six of the astronauts had eyeballs that were physically flattened out in the back.

This is definitely cause for concern, and there is a risk that microgravity could cause irreversible vision changes, according to a NASA report. Researchers still aren't sure what causes the vision problems, and NASA is conducting a more comprehensive follow up study.

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Living in space can depress astronauts' immune systems, according to a study published in 2014.

"Things like radiation, microbes, stress, microgravity, altered sleep cycles and isolation could all have an effect on crew member immune systems," Brian Crucian, NASA biological studies and immunology expert, who led the study, said in a statement. "If this situation persisted for longer deep space missions, it could possibly increase risk of infection, hypersensitivity, or autoimmune issues for exploration astronauts."

That means Kelly may be more likely to get sick. (Good thing he got his flu shot, above.)

The study also found that astronauts' depressed immune system meant that old, latent viruses like chicken pox can be reawakened, even though no symptoms have shown up in astronauts living in space so far.

Some immune cells collected from astronauts living in space were actually overly aggressive instead of depressed, and might explain why some astronauts get rashes and other allergic reactions.

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Astronauts have to strap themselves into sleeping bags every night. In microgravity their heads roll forward and their arms float up once they fall asleep.

The weird position is hard to get used to.

"Sometimes you wake up in the morning to see an arm floating in front of your face and think, “Whoa! What is that?” until you realize it’s yours," astronaut Marsha Ivins told Wired.

But there are many other disturbances that make it challenging to get quality sleep in space. Astronauts have reported seeing flashes of light when cosmic rays pass through their retinas.

The flashes can make it difficult to sleep. Astronauts have individual sleeping pods that help shut out sunlight and the rays. Still, studies have found that most astronauts are only getting about six hours of sleep even though their schedules call for eight and a half hours.

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It's a struggle returning to Earth after spending 6 months in microgravity. Astronauts lose their sense of up and down while they're in space because their vestibular systems can't tell where the ceiling is and where the ground is in a weightless environment.

Eventually the body recalibrates on back Earth, but astronauts can feel a little shaky on their feet for awhile.

The same lack of downward pull that makes astronauts' faces puffy also makes them congested.

"It's the same as having a cold or allergies," astronaut and physician Scott Parazynski told Scientific American. "A stuffy nose definitely dampens your sense of smell and consequently your sense of taste."

Many astronauts have reported that flavors are much more dull in microgravity, so they crave food with a lot of extra flavoring and spice to make up for the bland taste. Food and drink that rely on a sense of smell to bring out the flavor, like coffee, are disappointing in space.

Not getting the right nutrients can make almost all of the effects we've covered even worse.

Vitamin D is a concern for astronauts because they're not exposed to sunlight and it's easy to wind up with a deficiency, Sara Zwart, a senior scientist at NASA who studies the nutrition effects of long-term spaceflight, told Tech Insider.

Specifically, oxidative stress is much greater in space, so astronauts need a lot of antioxidants, Zwart said. Spaceflight also causes astronauts to have elevated iron levels, because their red blood cell mass decreases.

Astronauts track their food intake every week to make sure they maintain the right levels of nutrients. Usually iron levels return to normal after a few months back on Earth.

Cosmic radiation will not give you super powers like in "The Fantastic Four." A dose of it can tear through your very DNA, and that damage can lead to cancer, cataracts, or other diseases.

Earth's atmosphere acts as a force field that shields us from 99% of cosmic radiation, but astronauts don't have the same kind of protection in space. Radiation risk increases by a factor of about 30, according to the European Space Agency.

Astronauts wear dosimeters while on board the ISS that measure how much radiation they are exposed to. Their careers as astronauts are limited based on their exposure.

Scientists don't know much about what will happen beyond six consecutive months in space. Some of the effects may stay the same, some could worsen, or we may see new effects. Kelly's year-long space mission will be crucial to understanding the risks of long-term manned missions to the moon or Mars.

On the ISS astronauts are literally a world away from anything familiar, and it's not like they can pack up and leave any time they want.

Astronauts undergo a rigorous psychological screening before they can even start training for space. Still, feelings of isolation and confinement are a big risk. Combine that with sleep deprivation, the absence of the comfort of gravity, and dulled senses, and you can begin to imagine the psychological toll long-term spaceflight can take.

Many astronauts have described it as the hardest thing they have ever done.

It's a lot to handle. That's why astronauts have to go through rigorous training and pass an intense psychological evaluation. Luckily this isn't the first time Kelly has been to the ISS, so he already knew what he was getting himself into.

Scientists expect to learn a lot more about what spaceflight does to the human body by studying Kelly and his brother.

By the end of his year-long mission in March 2016, we might have some new things to add to this list.