A new study released this week says the human body is currently "not prepared" for life in space, according to The New York Times.
The report claims since humans didn't "evolve to live in space," challenges they might face in space could cause long-term issues.
NASA astronaut Mark E. Kelly, who was a part of four different shuttle missions, listed problems he's witnessed firsthand during space travel.
"Your head actually feels bloated," said Kelly, who is now retired, according to The New York Times. "It kind of feels like you would feel if you hung upside down for a couple of minutes."
Members of the Expedition 20 crew like NASA astronaut Michal Barratt and Robert Thrisk, from the Canadian Space Agency, were supposed to spend over six months on the International Space Station. During their mission however, the crew started experiencing vision issues.
Doctors performed eye exams and determined that swelling and blemishes had developed on their retinas.
The longest a human has ever been off Earth is 438 days, when Dr. Valery Polyakov spent most of 1994 and 1995 on the Russian space station Mir.
NASA now performs eye exams before, during, and after all ISS missions, according to The New York Times.
Astronauts currently on the ISS have been performing human research experiments, like looking for osteoporosis in space, heart rate, cardiovascular tests, blood pressure, and sleep patterns.
NASA confirmed this week astronauts spend around 2 hours every day exercising to "counteract bone and muscle loss" according to The New York Times.
In late 2013, NASA announced it would continue to operate the ISS until around 2024.
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