Sending humans to and even settling on Mars is seriously on the agenda for agencies like Nasa and private enterprises like SpaceX.

Humans, however, are not evolved to float in microgravity and building a spacecraft to get there is only one of the challenges. This week, new research on space-travelling mice indicates that before engineering a Mars spacecraft for humans, we might need to think more seriously about the biology of our joints.

The negative effects of long-term space travel on the human body are only just starting to be revealed. As space-travelling twins are hard to come by, the recent research looking at identical twins Scott and Mark Kelly has shown us that time in microgravity has a significant effect on the body.

Scott spent almost a year on the International Space Station while Mark stayed on Earth.

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The effects of microgravity on the body were significant. Fluids swelled Scott's upper body and head, his genes were activated differently and his immune system kicked into overdrive. His eye shape changed, weakening his eyesight and his telomeres - the protective structures at the ends of chromosomes - lengthened.

As if that wasn't bad enough, his muscles started to waste away, calcium was lost from his bones and his cardiopulmonary system weakened.

A photo provided by Scott Kelly/Nasa shows an aurora, seen from the International Space Station. Kelly was at the space station for a year. Photo / Nasa
A photo provided by Scott Kelly/Nasa shows an aurora, seen from the International Space Station. Kelly was at the space station for a year. Photo / Nasa

Although this long list sounds serious, and it was, research showed that within six months of returning to Earth, with careful exercise and a controlled diet, most of Scott's changes returned to normal.

Tissues of the musculoskeletal system including bone, muscle, tendons and ligaments are designed to build themselves up and strengthen when constantly subjected to loading such as against gravity. Once the load is taken away, rather than build up, tissues in the musculoskeletal system start to degrade.

This is why to maintain a healthy lifestyle, weight-bearing exercises such as walking or weightlifting are recommended, not just for astronauts returning to Earth but for all of us to stay strong.

One part of our musculoskeletal system that can't rebuild itself very well is cartilage, the flexible connective tissue found between the bones of the elbow, knee and ankle joints.

To see if zero gravity had any effect on the cartilage of mammals, a new study published in the journal npg Microgravity examined the tissues of rodents that had been on a space adventure orbiting the earth in a Russian spacecraft.

Nasa's Curiosity rover took this incredibly detailed image of a sloping hillside landscape on Mars - but could we really live there? Photo / Supplied
Nasa's Curiosity rover took this incredibly detailed image of a sloping hillside landscape on Mars - but could we really live there? Photo / Supplied

Six male mice were placed on board an unmanned biosatellite, while 15 control mice were kept back on Earth.

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The space mice were seen to be exercising by climbing over each other in their cages. However, because of the microgravity environment, their bodies didn't carry the whole load of their bodyweight compared to the Earth-based mice.

After 30 days in space, the mice were brought back to Earth and their cartilage was studied. The researchers found that the space mice showed structures in their cartilage similar to those seen in osteoarthritic patients, suggesting that even a month in space was enough to cause the tissue to permanently degrade.

Because articular cartilage is very poor at repairing itself, the study suggests that long-term space travel is likely to generate permanent damage to our joints that could lead to long-term mobility problems upon returning to Earth.

Current estimates predict the shortest journey to Mars will take eight to nine months, meaning the mission to Mars would involve spending more consecutive time in microgravity than has been experienced by a human to date.

Rather than wonder if we could survive on a new planet, perhaps the more practical question should be whether or not our knees will last the trip.