This research has ensured that astronauts could safely walk on the moon and live for more than a year on the International Space Station, 220 miles above the Earth's surface. But the Human Research Program, which aims to lessen the effects of the space environment on the health and performance of humans, is using the data to make sure they can send astronauts even deeper into the solar system.
A six-month journey to Mars would only be the beginning of a challenging expedition to land people on the surface of the Red Planet, 140 million miles away. The crew members would then live and work there.
In preparation, NASA has been using six-month crew member rotations and Scott Kelly's groundbreaking one-year mission on the ISS to study the effects of space on the human body. So what happens when we have to transition from walking on solid ground to floating in zero gravity?
"I felt like I was falling," NASA astronaut Mike Hopkins told CNN's Rachel Crane. "It was as if you're hanging off the rafters in a building and you let go, and that lasted for about 24 hours. My brain was taking a little while to get used to the fact that there was no up and down anymore. And that went away fairly quick. It takes a little while to get used to floating, too. It's almost like learning to walk all over again, a little bit."
Hopkins was on the ISS for 166 days, from September 2013 to March 2014. Though the adjustment to microgravity doesn't take long, other issues arise within the first few days due to a phenomenon in which the head tilts downward between 12 and 20 degrees, which causes disorientation.
"As soon as you arrive in weightlessness, the fluids start shifting in your body from the lower part of your body into the upper part of your body," said Dr. John Charles, Human Research Program associate manager for international science. "Your organs of balance and your inner ear immediately sense there's no gravity pulling down on them anymore."
This causes something known among astronauts as Bird Leg Syndrome, because the fluid shift causes them to have puffy faces and thin legs. It also makes them less thirsty, dulls their sense of taste and causes a "stuffy nose" feeling similar to allergies. Space motion sickness also affects about 79% of astronauts who experience microgravity in the first 24 to 48 hours, creating a loss of appetite, dizziness and vomiting.
Hopkins also had to worry about the long-term effects on his body, such as the weakening and loss of bones and atrophying muscles. In space, without gravity, bones lose more than 1% of minerals and density per month.
Astronauts also experience blood volume loss, weakened immune systems and cardiovascular deconditioning since floating takes little effort and the heart doesn't have to work as hard to pump blood through your body, according to Human Research Program deputy chief scientist Jennifer A. Fogarty.
NASA has developed countermeasures to combat these known issues and how they might continue after landing back on Earth. Most involve making sure that astronauts stay healthy by consuming enough nutrients in their meals and supplements, in addition to using three specially designed exercise machines on the ISS.
As soon as they overcome motion sickness, NASA wants its astronauts to start working out. It's also good for their sensory motor systems if their feet are striking the surface of something, like running on a treadmill, Fogarty said.
The three exercise machines mimic a treadmill, a stationary bike and a multipurpose weight machine. They're meant to stimulate the lower body, from running to squats and dead lifts, as the largest percent of bone loss occurs in the pelvis and femurs. Astronauts use the machines for two hours a day to compensate for the other 22 hours in which they aren't experiencing physical activity.
In addition to counteracting bone loss and atrophying muscles, the exercise provides a change of pace and mental release -- even if they have to bungee themselves to the treadmill to run in microgravity.
But one of the more recent effects of spaceflight noticed over the past five to seven years centers on eye health. Kelly and other astronauts in their late 40s and 50s have complained about their vision being slightly altered. Some of them have required glasses in flight.
"It changes their visual acuity," Charles said. "They're not able to see things up close. It's like advanced aging. That sort of thing happens at a sort of an accelerated rate."
Given the connection between the eye, optic nerve and brain, combined with the fluid shift astronauts experience, some research has suggested that the eyes act as a pressure release valve, Fogarty said. But her program has many studies in the works to get to the root of the cause and how to counteract it.
Even though the ISS sits within Earth's protective magnetic field, astronauts are still exposed to 10 times the radiation that they would be on the ground. NASA monitors radiation exposure, given the fact that ISS crewmembers are considered radiation workers.
The agency has been able to limit radiation risks to 1%, but a mission to Mars would expose astronauts to harmful galactic cosmic rays. Without proper shielding, this can increase cancer risk, cause radiation sickness, alter cognitive and motor function, and even lead to cataracts and cardiac and circulatory diseases.
The reality of gravity
And then there's landing on the ground again after experiencing zero gravity. Mars mission crew members would need to be able to get to work almost immediately after landing. Even though Mars has only a third of the gravity experienced on Earth, it would still be an adjustment for astronauts after floating for six months.
"[It] took a little bit longer to get used to life in gravity again," Hopkins said. "I had some issues with, like, pitching moments. It just felt like if I bent [over], I would fall right on my face. And in microgravity, you lose references to how much things weigh. And so I remember when I first landed and I was laying in the Soyuz [the capsule used for landing] and I was just handing out our flight procedures. And that little book that maybe weighs a pound and a half felt like it weighed 25 or 30 pounds."
But even though our bodies weren't meant to live in space, the Human Research Program is always researching ways for us to thrive in the final frontier. The current countermeasures used on the ISS, the results of ongoing studies and recently funded proposals aiming to solve issues around human health, will aid astronauts as they push the limits of deep space in the 2020s and 2030s.
"Mother Nature always tells you the right answer, but you just have to ask the right question," Charles said. "Our human bodies are up to the challenge of a trip to Mars and back."