Astronauts on space missions suffer from balance problems, visual disturbances, ‎damage to the heart muscle and bone loss. How can these issues be addressed ‎so that astronauts are safely returned to Earth after prolonged missions to space?‎

At the beginning of March 2016 astronaut Scott Kelly returned to Earth after breaking the American record for a continuous stay in space – 340 days. The purpose of his mission to the International Space Station was to better understand how the human body reacts and adapts to the harsh space environment. The study hopes to reduce such risks in order to prepare for manned research missions to the moon, possibly to asteroids and eventually missions to Mars. Here we discuss how the body responds to the space environment, what problems arise in it and how we can deal with them.

The health implications of space travel

Singer David Bowie wrote “Space Oddity” describing the experiences of the astronaut Major Tom: “I am floating in a most peculiar way”. Indeed, the main difference between space and Earth is that in space there is almost no gravity, causing a feeling of weightlessness, resulting in the spacecraft or space station in which the astronaut is in to be in free fall toward the center of the Earth. Free fall is the motion of a body where gravity is the only force acting upon it. As the shuttle or the space station is moving around the Earth with only gravitational force exherted upon it (there is no air resistance in space), it can be said that they are in a state of free fall. The reason they do not actually “fall” but rather move in a circular path is because the force of gravity is vertical to the direction of its initial velocity, so that it affects just the direction of the  velocity but not its size.

Astronauts are trained for the conditions by practicing in a reduced-gravity aircraft that flies in a special parabolic route. The training helps them function in space but does not prevent the harmful effects of zero gravity on health. Studies on people who were on space stations for long periods have shown some of the effects are temporary while others are more long-term.

Brief exposure to weightlessness causes space adaptation syndrome (SAS) or “space sickness”, which is the most common problem in space travel. Weightlessness affects our orientation in space and requires us to adapt many of our physiological processes to the new conditions - mainly processes related to our balance system. When the adjustment is not complete it results in nausea, dizziness, vomiting, headaches, fatigue, general malaise, visual hallucinations and disorientation in space.

The first report of such symptoms was from Soviet cosmonaut Gherman Titov, who completed his flight at the end of 1961 as the fourth person ever in space and the second, after Yuri Gagarin, to complete a full rotation of Earth. The data collected until now has shown that about 45 percent of space travelers suffer from space sickness. But it rarely lasts more than three days, when the body adapts to the new environment.

Long-term exposure to the zero gravity causes multiple health problems including redistribution of fluids and loss of bone and muscle mass. Over time, these effects can compromise astronaut performance, which can increase the risk of them being harmed, as well as reduce their ability to absorb oxygen, which slows down their cardiovascular activity.

Redistribution of liquids

Fluids, which make up about 60 percent of the human body weight, tend to accumulate in the lower part of the body when under the influence of gravity, and through the course of evolution we have developed systems that balance the blood flow to the heart and brain while we stand. These systems continue to work even in the absence of gravity, therefore causing fluid to accumulate at the top of the body. This is why astronauts have swollen faces. Accumulation of fluid in the eye also blurs their vision for a few days until the brain learns to compensate and correct the image.

The change in fluid distribution is also reflected in problems in balance, as well as a loss of sense of taste and smell. More importantly, it drives a series of systemic effects designed to adapt the body to the new environment, but they have dangerous consequences upon the return to Earth. One of them is “orthostatic intolerance”, which is the inability to stand without assistance for more than ten minutes at a time without passing out.

The phenomenon stems in part from changes in the regulation of blood pressure by the autonomic nervous system and the loss of about 20 percent of the volume of blood fluid – because under conditions of microgravity it is not necessary for systems to maintain blood pressure as body fluid spreads more evenly throughout the body. This effect is amplified the longer one is in space, but is normalized again within a few weeks of returning to Earth.

The heart also gradually degenerates as a result of it having to pump less blood. A weaker heart muscle causes a decrease in blood pressure and may hamper the flow of oxygen to the brain.

Regular resistance training is essential for maintaining bone and muscle mass in zero gravity | Photograph: NASA

Muscle atrophy and osteoporosis

One of the major effects of weightlessness that is more long-term is the loss of muscle and bone mass. In the absence of gravity there is no weight load on  the back and leg muscles, so they begin to weaken and shrink. In some muscles degeneration is rapid, and without regular exercise astronauts may lose up to 20 percent of their muscle mass within 5-11 days.

Due to lack of mechanical pressure on the bone, bone mass is lost at a rate of one and a half percent in just one month in a zero-gravity environment, compared to about three percent a decade in a healthy person in a normal environment. The mass loss mainly affects the lower vertebrae of the spine, the hip joint and the femur. Due to the rapid change in density, bones may become brittle and exhibit symptoms similar to those of osteoporosis.

Even destruction and construction processes of bones change when in space. On Earth, bones are destroyed and renewed regularly using a well-balanced system of bone destroyer cells and bone building cells. Whenever some bone tissue is destroyed, new layers take their place; these two processes are coupled to one another. In space, however, an increase in activity of bone destroyer cells is seen, due to the lack of gravity, and the bones decompose into minerals that are absorbed into the body.

Studies on mice have shown that after 16 days in zero gravity there is an increase in the number of bone destroyer cells and a decrease in the number of bone building cells, as well as a decrease in the concentration of growth factors known for their ability to help create new bone. The increase in calcium levels in the blood from the disintegrating bone causes a dangerous calcification of soft tissue and increases the potential of kidney stone formation.

Astronauts show an increase in bone destroyer cell activity, particularly in the pelvic area, which usually carries most of the load under normal gravity conditions. However, unlike patients with osteoporosis, astronauts who remained in space for three to four months, regain their normal bone density after a period of two to three years back on Earth.

Coping with the effects of zero gravity

The best way to avoid the effects of zero gravity is to create artificial gravity. To date, scientists have managed to create gravity only under laboratory conditions, using strong magnetic fields above permitted safety levels, which of course is not practical in space travel. However, science fiction often uses artificial gravity. For example in the movie “The Martian”, the spaceship that travels to Mars has a rotating circular structure that has gravity on its perimeter equal to 40 percent of what would be on the face of the Earth, which is similar to gravity on the Red Planet.

Drugs used to treat sea sickness, which is also a result of movement patterns that the body is not accustomed to, can also help to treat space sickness, but are rarely used because the natural adaptation course during the first two days of space travel is preferred over the drowsiness and other side effects caused by the drugs.

However, when the astronauts are wearing a space suit they anti-nausea patches because vomiting in the suit can be fatal. Space suits are worn mostly during launch and landing, and of course in any activity outside the spacecraft (spacewalks). To allow the team to adapt to conditions in space, activities outside the spacecraft or space station are usually not planned in the early days of the mission. This prevents the danger of vomiting in the suit and the patches are usually only for back up.

To reduce and avoid some of the negative effects of the lack of gravity on the muscles, especially the heart muscle, the International Space Station is equipped with sports apparatus used for resistance training. Every astronaut is required to perform at least two hours of physical activity a day, including jogging on a treadmill (they attach themselves to it with elastic bands so as not to float away), riding a stationary bicycle and lifting weights, against springs of course. Astronauts on especially long missions wear pants that put pressure on the bones of the legs to reduce the loss of bone density.

NASA uses advanced computational tools to understand how best to halt the degeneration of muscles and bones for astronauts staying in space in zero gravity. Computational simulations are mainly used to evaluate the effects of exercise on the torsion (torques) of bone joints, to recommend the optimal exercise regimes for astronauts.

Hopefully, the information gathered by Scott Kelly during his long stay in space will shed more light on the effects of zero gravity on human health, and could help prevent many of the problems astronauts encounter upon their return to Earth. His mission was unique in its length, which allows investigation of the effects of more long-term effects of space travel than was previously tested.

A pleasant memory: David Bowie, Space Oddity