From NASA’s Moon to Mars program to Elon Musk’s ambitious plan to send a million people to Mars by 2050, the race is on to get human feet on the red planet. With increasingly sophisticated rockets and robotics, the technological challenges standing in the way of this goal are fast being eroded.

But there might be a different issue which hampers plans to take people off-planet and send them out to explore the rest of the solar system. Strange things happen to the human body in space, and we’re going to need to find ways to address these medical issues if we want to be able to send astronauts on long-duration missions like the several years that a Mars mission might require.

Digital Trends spoke to University College London cardiologist Dr. Rohin Francis, who has performed studies into space medicine, about how human bodies respond to long-term habitation of the space environment and what that might mean for manned missions to Mars.

What we know about the human body in space

When it comes to space missions, there are two primary factors that influence the human body: Microgravity and ionizing radiation.

As it stands, we have plenty of research about the effects of zero gravity on the body from years of study on the International Space Station (ISS), and we know that being in microgravity for months or years leads to a range of medical side effects.

These findings are supported by what are called analog studies, in which low gravity environments are simulated on Earth. “The majority of research into microgravity uses microgravity analogs,” Francis explains. “These are people who are paid to lie in bed for weeks or months at a time. This is the best way we have to simulate microgravity on Earth.”

Programs like the European Space Agency’s bedrest program lets researchers study the effects of microgravity by keeping volunteers in a bed tilted toward the head end, which creates similar effects to the microgravity of blood and fluids rushing to the head and muscles wasting away.

What happens to bodies in low gravity?

One of the most problematic effects of long-term exposure to microgravity is muscle atrophy, as muscles do not need to exert any force to counteract gravity and stay upright. Over time, muscles throughout the body wither away, causing major problems when astronauts return to the full-gravity environment of Earth. This is why astronauts on board the ISS exercise for two hours every day, to keep their muscles working as much as possible.

Other issues caused by microgravity include loss of bone density — estimates of the potential effects of a Mars mission say that astronauts could lose up to half of their skeletal mass, Francis said, though he pointed out that these estimates are purely speculative — as well as loss of cardiovascular capacity, sinus problems, and reduced eyesight due to changes to the shape of the eyeball.

These are just some of the symptoms found by NASA in its landmark twin study, in which astronaut Scott Kelly spent a year in space before having his physiology compared to that of his identical twin brother, Mark Kelly.

“You get a redistribution of fluid, so that you get this very puffy upper part of the body, and puffy head. Previously, it was thought that pressure in the head rises, and that pushes against the back of the eyeball. Astronauts have been noted to have a reduction in blood supply and atrophy of the optic nerve, which could be due to a rise in intracranial pressure,” Francis said. However, recent data has suggested that pressure in the head is not the driving cause of reduced eyesight. It could be that some other, as-yet-unknown mechanism is causing these problems.

When it comes to spending even longer in space, in terms of decades or lifetimes, there is an even bigger medical issue: Reproduction. “We’re not sure how successful the process of fertilization would be in microgravity,” Francis said. In studies, human sperm has been found to swim less effectively in microgravity than it does on Earth, so “even the sperm getting to the egg may be significantly affected.” Recent research into reproduction among mice in zero gravity found that they could successfully conceive, but that they soon miscarried.

It may not even be possible for humans to be conceived away from Earth, which puts a damper on the prospect of building a long-term off-world colony.

What about the gravity on Mars?

An issue that has yet to be addressed is exactly how research from the zero-gravity environment of the ISS will apply to the low gravity environment of Mars, where the gravity is around 38% that of Earth. It could be that there is a threshold of gravitational force below which bodies start to experience medical issues. Or it could be a linear relationship, so the effects on astronauts on Mars would be less than the effects on astronauts on the ISS. Until we have more data about this relationship, there’s no way to know for sure.

“Martian gravity may actually be strong enough to prevent many of these problems,” Francis said. “If you have some gravity, even though it’s less than on Earth, and you combine that with countermeasures such as exercises, that may be okay. It’s the journey there that is regarded as the main challenge.”

Being on the surface of Mars could maintain the condition of the astronauts, or they could even regain some of the muscular and skeletal mass lost on the trip. “Estimates so far are based on the astronauts experiencing microgravity throughout, because we’re not sure how to factor in the six months that they might spend on the surface.”

The elephant in the room: Ionizing radiation

Thanks to years of experience with microgravity environments, space agencies have developed strategies for mitigating and addressing most of the medical issues caused by them. But there’s a whole different issue which arises once humans start exploring space beyond the Earth’s protective magnetic field. Outside this safe haven, everything moving through space is bombarded with dangerous cosmic rays. The only manned missions that have gone outside of this safe haven are the moon missions, but those only involved exposures to radiation for periods of weeks, rather than months or years.

We do know that cosmic rays can damage delicate electronics, so spacecraft which are designed to travel beyond Earth’s magnetosphere have shielding to protect their components. But these same rays are potentially deadly to humans, and we are only just beginning to understand how they could affect the human body. For example, research in mice has found that exposure to radiation can affect not only the body but also the brain, and may even lead to behavioral changes such as increased rates of anxiety.

Radiation exposure is not something whose effects can be ameliorated in the same way that muscle atrophy can. The only way to protect astronauts from radiation is to build physical structures that will keep them safe from it. “Radiation is probably going to be the main obstacle,” Francis said. “There’s nothing you can do from a biological point of view to protect yourself from radiation. It’s really going to be down to the ship design and engineering rather than biology or medicine.”

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