Bone and muscle damage, exposure to radiation, deteriorating eyesight - these are just some of the challenges space travellers face during long-duration missions, even before the psychological impact of isolation is taken into account, AFP reports.

As US astronauts Butch Wilmore and Sunny Williams prepare to return home after nine months aboard the International Space Station (ISS), some of the health risks they faced are well documented and managed, while others remain a mystery.

These hazards will only increase as humanity ventures deeper into the solar system, including to Mars, requiring innovative solutions to safeguard the future of space exploration.

Despite the attention their mission has received, Wilmore and Williams' nine-month stay is "normal," says Riana Bokhari, an associate professor at the Baylor College of Medicine's Center for Space Medicine.

ISS missions typically last six months, but some astronauts stay up to a year, and researchers are confident in their ability to maintain astronauts' health for that period.

Most people know that lifting weights builds muscle and strengthens bones, but even basic movements on Earth defy gravity, an element that is lacking in orbit.

To counter this, astronauts use three training devices on the ISS, including a drag device installed in 2009 that simulates free weights using vacuum tubes and flywheel cables.

"Daily two-hour workouts keep them fit. The best results we have to show that we're very effective is that we really don't have a problem with fractures in astronauts when they come back to earth," although bone loss is still detectable on scans, Bokhari says.

Balance disruption is another problem, added Emanuel Urquieta, vice chairman of the department of aerospace medicine at the University of Central Florida.

"It happens to every single astronaut, even the ones who only go out into space for a few days," he said, as they "work to restore confidence in their inner ear."

The astronauts must readjust their bodies during NASA's 45-day post-mission rehabilitation program.

Another challenge is "fluid shifts" - the redistribution of body fluids to the head in microgravity. This can raise urinary calcium levels and increase the risk of kidney stones.

The fluid shift can also contribute to an increase in intracranial pressure, which can change the shape of the eyeball and cause spaceflight-associated neuroocular syndrome (SANS), causing mild to moderate visual impairment. Another theory suggests that the cause is elevated carbon dioxide levels.

But in at least one case, the effect was beneficial.

"I had a pretty severe case of SANS. When I started I wore glasses and contact lenses, but because of the globe flattening I now have 20/15 vision - the most expensive corrective surgery possible. Thank you, taxpayers," NASA astronaut Jessica Meir said before the latest launch.

Radiation levels aboard the ISS are higher than on Earth as it passes through the Van Allen radiation belt, but Earth's magnetic field still provides significant protection.

Protection is critical as NASA seeks to limit the increased lifetime cancer risk for astronauts to within 3%.

However, missions to the Moon and Mars will result in much greater exposure for astronauts, explains astrophysicist Siegfried Eggle.

Future space probes could provide some warning of high-radiation events, such as coronal mass ejections - plasma clouds from the Sun - but cosmic radiation remains unpredictable.

"Shielding is best done with heavy materials like lead or water, but huge amounts of them are needed," says Eagle of the University of Illinois at Urbana-Champaign.

The artificial gravity created by rotating spacecraft frames could help astronauts stay operational upon arrival after a nine-month trip to Mars.

Alternatively, spacecraft could use powerful acceleration and deceleration to match the force of Earth's gravity.

This approach would be faster and reduce the risks of radiation exposure, but it requires nuclear propulsion technologies that do not yet exist.

Preventing infighting among teams would be crucial, says Joseph Kibler, a psychologist at Embry-Riddle Aeronautical University.

"Imagine being stuck in a van with anyone for 3 years: these ships aren't that big, there's no privacy, no backyard to go to. I really applaud astronauts who commit to this. It's overwhelming work," he said. | BGNES