We’ve come a long way since humans first set foot on the moon in 1969, when NASA astronaut Neil Armstrong described the event as “one small step for man, one giant leap for mankind.” Recently, the Mars rover, known as Curiosity, photographed images of the mysterious planet, giving us a glimpse into another world; other rovers are on their way to planets further out, such as Pluto.

The next major leap would be a human setting foot on Mars. But first, astronauts have to figure out better ways to prevent or lessen the drastic changes the human body would go through during a year-long trip hurtling through space — getting farther away from Earth at every second — with no turning back.

Currently, astronauts must go through intense training and exercise in order to prepare the body to be thrust into space. That’s because the human body has internal triggers of sorts that begin to cause changes once astronauts are floating about in zero gravity — from their blood flow to their skeletal frames.

Bones

“The human body is uniquely designed to live in Earth’s gravity,” the National Space Biomedical Research Institute writes. “In space, the body begins to adapt to the microgravity environment.”

The body adapts to different environments on Earth, so why would it be any different in space? One of the biggest changes occurs in the skeletal system. Astronauts who are in space for over a month can lose a significant amount of bone mass. This is because the skeleton is no longer needed to hold up our weight; in zero gravity, you float everywhere. This reduction in bearing weight leads to the breakdown of bones and the reabsorption of calcium in the body, which weakens bone mass. According to the National Space Biomedical Research Institute, a postmenopausal woman with osteoporosis can lose up to 1 to 1.5 percent of bone mass per year, but an astronaut can lose that much in just one month.

“We don’t know what sensors in your body recognize that you’re weightless, or why we would even have those sensors,” Chris Hadfield, a retired Canadian astronaut who was the first Canadian to walk in space, told Joe Rogan during a radio interview. “But your body starts to shed your skeleton right away.”

At the same time, humans get a little bit taller in space. On Earth, discs between the vertebrae of the spinal column are somewhat compressed due to gravity — but in space, the discs expand, making the spine longer.

Muscles

As many of you who sit at a desk all day may know quite well, you lose your muscles if you don’t use them. People floating about in space have far less resistance to air, gravity, and various other environmental features like wind. Without walking, running, going upstairs, or even simply holding up your posture under the weight of gravity, your muscle mass would gradually begin decreasing just like your bones. This is why preceding a lengthy space excursion, astronauts are required to undergo some serious exercise training to build up their muscles. There are some in-space exercises that astronauts can work on too, but researchers are still developing good interventions to prevent muscle loss — such as nutritional supplements.

Heart & Blood Circulation

Gravity actually helps to distribute blood through our bodies, so when it’s no longer present, fluids begin to travel toward the upper parts of the body and head. This can lead to a feeling of congestion. And the heart does not have to work as hard in the microgravity environment, so this could gradually lead to a smaller heart.

Recovery after returning to Earth is a whole other road to travel down. “[W]hen you come home, it’s brutal building those things back up again,” Hadfield notes. It takes one year for your body to fully recover, he says, and it personally took him up to 4 months to be able to run again properly. “My body got osteoporosis, and it’s reversing osteoporosis using some internal stimulus that we don’t even understand,” he said during the radio interview. “So it makes a pretty good medical study for everybody.”

To view what happens to the body in space in more detail, check out NASA’s interactive piece here.

Psychological

Leaving Earth behind and knowing you’re one of the few humans in space can have quite the impact on your psyche. Astronauts experience a mix of fear, excitement, fascination, and anxiety as well as feelings of isolation or loneliness.

Some studies have been completed on long expeditions in Antarctica, since space travelers will have to cohabit with one another in a very small space for up to three years if on a Mars mission. Researchers discovered a surge of depression around the halfway point of these Antarctica trips, once crew members realized how much longer it would take to get back. In short, astronauts must be a very specific type of person — one who has an “elevated capacity to regulate stress, temporarily displace emotions, and filter out preoccupying thoughts” as well as be “highly resilient to environmental source of psychological distress including isolation, interrupted sleep cycles, the hyperarousal caused by excitement, intense work schedules, and the absence of day and night in space.”

Hadfield says there will most likely be a different type of profound impact on the psyches of the astronauts who will be the first to go to Mars — a sense of disconnection from all that is familiar:

It’s not so bad in the space station because we’re so close to the world. But as soon as you start going to Mars, within a couple weeks, you will never have another normal conversation with Earth. …Everything has so much lag that you’ll just have recorded video messages back and forth. So the [psychological] impact of that is going to be high. And the Earth will shrink to just another star in a couple weeks. … Those people will become Martians. … They will no longer be from Earth. … When I was on the space station the second time, one of the other crew members … in passing, a throwaway thing she didn’t even think she was saying, she said: ‘Hey you know, Earth said that we’re supposed to do this next.’ In my mind I heard, ‘Earth said.’ I heard those words come out of her mouth, and it was like, she has in her mind completely split off from the other seven billion people. There’s her crew, and Earth is one singular identifiable entity on the other side. And that was a real bell-ringer to me of what it’s going to be like to go to Mars. Those people are going to be a completely discreet unit of people and they’ll be Martians, they won’t be earthlings in their heads. How do we deal with that? How do we plan for that?

Returning to Earth after extended periods of time in space can lead to vast psychological and physical changes. Some astronauts who have returned after a life-changing experience in space have a better perspective on things, noting that they ultimately cared less about petty political battles, or personal self-centered thoughts. Instead, some focused more on helping others. As rocket pioneer Krafft A. Ehricke said, “Man’s mind and spirit grow with the space in which they are allowed to operate.”