Space travel presents many challenges to the human body. The potential psychological, physical and social risks are significant, as NASA astronaut Scott Kelly’s recent year-long stay at the International Space Station orbiting 400km above the Earth’s surface, has shown. 

We spoke to Libby Jackson, Human Exploration Manager at the UK Space Agency, and Professor Philip Stooke of the University of Western Ontario’s Centre for Planetary Science and Exploration, along with’s Daniel Atkinson, to discuss the changes and fluctuations the body is subjected to, and how best an astronaut can handle and deal with this kind of exposure.  

Your muscle and bone density decreases

On Earth, gravity gives our bones density so that they can support the body properly. However in space, where there is no gravity, our bones can lose density and become weaker. 

We know from long space flights (up to about 400 days maximum so far) that zero gravity has effects - muscles don't work hard so they get weak, including the heart. Bones don't need to be strong and they lose calcium,’ explains Prof Stooke. 

NASA have established that this loss in density occurs at a rate as high as 1% per month in space, as opposed to 1% to 1.5% per year amongst older people on Earth. Such effects share many parallels with the ageing process on Earth, and although they can be quite severe, they help to increase our understanding of the body as we get older. 

The astronauts provide a very useful research tool for scientists who are working to understand these mechanisms, including several groups of UK researchers,’ says Libby.

Over a period of five months in space, an astronaut can lose as much as 40% of muscle and 12% of bone mass. This is the equivalent of a 20 year old tripling in age over a three month period. 

When someone loses a significant amount of bone density, this is called osteoporosis,’ Clinical Lead, Dr Daniel Atkinson comments.

Someone with this condition has a much higher risk of sustaining breaks and fractures and, consequently, needing corrective surgery. Most people begin gradually losing bone density from the age of 35 onwards, but if in astronauts this process is accelerated, this presents a significant risk to long term health as they’ll obviously stand to lose more prematurely.’

To reduce such risks, astronauts should complete a minimum of two hours physical exercise every day in space. This can be vital in an emergency situation.

‘[Astronauts] exercise for 2 hours every day so that should they have to come back to Earth in an emergency, they would be strong enough to get out of the spacecraft,’ notes Libby.

In an emergency they could end up landing in very isolated places and need to look after themselves for a little while.

Maintaining a healthy, balanced diet is also crucial in preserving muscle and bone mass.

In space, the crew follow a balanced healthy diet, just as we do. It is particularly important for them to keep salt to low levels as too much can accelerate the bone loss they suffer,’ Libby stresses.

Recovering bone density back on Earth

In light of such a period in space, can you fully recover muscle and bone density once you return to Earth?

You can, but it takes at least three or four years. Such muscle and bone loss in space is comparable to osteoporosis on Earth. Rises in calcium levels are also something else that you may have to contend with, making you more prone to kidney stones, and basic motor functioning may be impaired upon returning from space. 

When the crew come back to Earth they have to adapt back to feeling gravity again,’ Libby points out. 

If the astronaut has been in space for a few months, it can take a few days before their balance system has readjusted and they can stand and walk normally.

Even in spite of completing a rehabilitation programme, astronauts may never recover their original bone mass. 

Weight-bearing exercises are essential to regaining muscle and bone mass, along with activities such as yoga, running and stair-climbing. 

Exercise helps mitigate the effects, and we are learning more about how to overcome the problems. Much of the research on the International Space Station is directed at these issues.’’ says Prof Stooke.

Exposure to radiation may increase your cancer risk

Studies into the dangers posed by radiation in space have provided powerful evidence that cancer and other debilitating conditions are likely to manifest if we are exposed to galactic cosmic rays, or solar particle events. This kind of radiation can travel completely unhampered through spacecraft and human skin, and it can make astronauts considerably susceptible to radiation sickness, damage to the central nervous system and serious disease. It can also increase our chances of developing cancer. It has been calculated that astronauts are exposed to a level of radiation equivalent to anywhere between 150 and 6,000 chest x-rays

Reducing radiation risk

Is there anything that we can do to combat the risks posed by radiation in space?

In terms of travel for extended periods through space, such as to Mars, there isn’t currently a concrete plan of action in place. Researchers at NASA and at other space organisations are however exploring the possibilities thrown up by genetic editing, gene therapy and forms of screening for people who are genetically better equipped to survive the effects of radiation. 

Psychological pressures can have mental health consequences

Space travel has led to numerous astronauts experiencing a variety of psychological effects. Hallucinations are typical and have produced some unusual visual images for those affected. During a mission in 2012 on the International Space Station, the astronaut Jon Pettit spoke of ‘flashes in my eyes, like luminous, dancing fairies’ that would be particularly vivid in the dark shortly before sleep. Whereas on Earth the majority of particles are soaked up by the atmosphere, in space, nerve cells produce this “dancing fairy” effect, which is caused by cosmic rays.

Interpersonal conflicts can have significant psychological consequences too. Extensive working hours, fatigue and altercations with mission control led to the Skylab 4 crew, a US programme, turning off their radio and ignoring NASA for a day. Such confinement has yielded similarly low morale in other studies. 

Research has also revealed a decline in the most fundamental cognitive functioning in space. Basic functioning such as paying attention, switching from one task to another, coordination and solving problems all seem to be reduced, with a decline in physical activity a likely cause. The absence of gravity and the subsequent impact on blood flow could also play a significant role in this. Although the reduction is not severe, it may impact on astronauts’ ability to handle emergencies. 

It stands to reason that restricted blood flow caused by a lack of gravity of space can be a catalyst for these effects,’ Dr Atkinson notes. 

Blood flow is essential in moving oxygen around the body and to the brain; and without enough oxygen, normal brain function can become affected, subsequently impacting on reactions, coordination and cognitive ability.’

What can we do in the face of psychological risk factors in space?

A team of psychologists created an app in 2008 called Virtual Space Station to help astronauts confront difficulties such as depression and homesickness whilst in space. The app comprised of videos of former astronauts talking about their own problems and how best to address them. 

Psychoactive medications can help astronauts from a mental health point of view, and the likes of the anti-anxiety tablet, diazepam, is available to crew on board. Antipsychotic medication, such as haloperidol, and remedies for pain and sleep, such as codeine, morphine and temazepam, are also provided, along with SSRIs, on more recent missions.

More and more studies are taking place on astronauts’ psychological wellbeing in space. Selecting crew members from the outset who have a strong handling of psychosocial matters has been proposed, so that problem-solving can take place in the midst of any interpersonal conflict, rather than being shrouded by it. This is important, as there is no external space on board to allow tensions to diffuse naturally; any friction must be resolved there and then. Being able to view the Earth, and photograph it, is essential for positive mental health on board and in helping astronauts to feel less isolated.

There are manifold considerations when it comes to the mental wellbeing of astronauts in space, but it’s important to emphasise that they’re usually well-equipped to deal with the psychological rigours, and their needs are taken very seriously by their organisations.

The mind can be affected by stress and isolation, but we have learned a lot about that and generally astronauts are strong professional people who can cope, and we arrange frequent communications so people don't feel isolated,’ explains Prof Stooke.

‘I think mental issues are quite well understood and can be dealt with by training and good communication.’

When will it be safe enough for us to travel to Mars?

Looking to the future, is it likely that we will be able to ensure the safe passage of astronauts to Mars and back any time soon?

Due to the budget and technology required, any imminent missions to Mars are improbable. Keeping astronauts healthy for extended periods in deep space remains a huge challenge. 

‘Such missions will see the crew leave the relative safety of the protection of the Earth’s magnetic field, which protects Earth from the Solar Wind and Cosmic Rays,’ observes Libby.

The radiation levels out in deep space are much higher and present a challenge for astronaut health, who need to be protected from the radiation.’

Elon Musk, CEO of SpaceX, has spoken of his company’s plan for a first manned mission to Mars in 2024 / 2025 at the earliest, while NASA’s Mars Exploration Programme is still exploring how to make a mission safe, and this is likely to remain the case for the next decade. 

A detailed understanding of the Martian environment is paramount in terms of human health, from the effects of radiation to the chemical properties of Mars’ soil. For the time being, we will have to rely on the robotic findings of the 2001 Mars Odyssey and the Mars Reconnaissance Orbiter. 

I think we have a lot more to learn about space before we can go to Mars. Engineering (like landing on Mars and getting back), and medical issues (long flights, radiation hazards, Mars dust effects on health) will take time and money to resolve,’ Prof Stooke comments.

‘I don't think the funding will be there for Mars exploration by people for quite a few decades, so I would rather keep sending robots. Personally I would like to see people on the Moon, and leave Mars for a much later future.’