This past week NASA announced that in 2015 they would be sending two astronauts to the International Space Station (ISS) for a year-long mission. This is an expected, and necessary step in the preparation for human spaceflight to Mars. If we are to have human explorers on the surface of Mars, we need to understand the effects on humans of long-term space travel. At the moment trips to the ISS last on average 5-6 months. A mission to Mars may take 6-8 months, plus an extended time on the planet, and a 6-8 month return trip. In reality astronauts could be sent on missions of 2 years or longer.

The ISS as it orbits Earth. Photo credit NASA.

Astronauts are exposed to a number of conditions in space that can pose serious health risks, especially if exposure takes place over a long period of time. There are lots of hazards and risks for humans in space, including: ascent and descent accidents; space sickness; debris collisions; micrometeorites; hazardous and toxic gas leaks on the spacecraft; EVA (Extra Vehicular Activity) accidents; sudden unexpected illness and the list goes on. Today I’m talking about the specific issues that affect astronauts during long distance flights. (By the way I use the term ‘astronaut’ as a general term, which includes astronauts from various nations, including cosmonauts from Russia, and teikonauts from China.)

Some of the major issues for astronauts during long duration space flights include:

Exposure to radiation – Astronauts live and work well above the protective atmosphere of earth, so they are subject to the full force of the sun’s radioactive output as well as high energy cosmic rays that originate outside of our solar system. Long distance missions do not have the benefit of Earth’s protective atmosphere and magnetic field. Material shielding is effective against low energy radiation, but may create damaging secondary radiation for higher energy particles. The ISS uses aluminium to protect the spacecraft and crew. As the thickness of the shielding material increases, the probability that the particle will survive with enough energy to damage human issue is decreased. Low levels of radiation generally do not pose a significant health risk to astronauts, however the effects of radiation are cumulative, so long term exposure increases the lifetime risk to astronauts. Crews of future long duration interplanetary missions will have to travel through theVan Allen radiation belts, and be exposed to more galactic cosmic rays, and as a result will be exposed to higher levels of radiation than those remaining in Earth’s orbit. Increased and prolonged exposure to radiation has serious health consequences for astronauts, including cancer, leukaemia, heart disease, and damage to the central nervous system.

Muscle atrophy – Our muscles, bones and organs have adapted to work in the environment we inhabit, which is exposed to the effects of gravity. When exposed to an environment where there is little or no gravity, our muscles, bones and organs begin to lose condition – becoming more problematic the longer the astronaut is in space. On Earth, our muscles are constantly working against gravity. In space there is no force of gravity for muscles to work against, so astronauts will lose muscle tone. Loss of muscle tone starts to occur shortly after launch, and continues whilst the astronaut is in a reduced-gravity environment. During longer missions, muscles may atrophy, and astronauts may experience uncontrolled muscle twitching, and a loss of fine motor control. The loss of muscle tone, strength and control can be mitigated with regular exercise during the mission.

Astronaut Robert Thirsk, asleep in his sleeping quarters in the ISS. Photo credit NASA.

Cardiovascular damage – When in space, the body no longer needs to maintain the powerful heart muscles needed on Earth, so heart tissue begins to shrink. In space, astronauts experience a redistribution of body fluids, which results in changes to cardiovascular physiology. The heart doesn’t have to work as hard pumping blood in a microgravity environment as it does on earth so crew members are encouraged to undertake aerobic exercise as part of their daily routine, in addition to exercises designed specifically to maintain as much muscle tone as possible. Whist exercise is beneficial, it appears that it cannot reverse the process, but can help slow it down.

Bone density – In microgravity, the lack of impacts in weight bearing exercise means that newly created bone tissue is not incorporated into bones as normal, so as bone tissue is created it is absorbed into the system, not used as it would be on Earth. This results in high calcium levels elsewhere in the body, which can lead to significant health issues. Studies have revealed a significant loss of calcium from weight bearing bones of astronauts. This is a concern for astronauts as it suggests a risk of renal stone formation on long duration missions. Density in such bones as the pelvis and legs decreases by approximately 1 to 2 percent a month on average, which presents concerns for astronauts during long haul missions. Unfortunately exercise does not seem to reduce the level of bone loss, but is beneficial for many other issues as discussed previously.

Sunita Williams exercising on the ISS. Photo credit NASA.

Hyperarousal – Don’t worry – it’s not what you think! Astronauts may experience hyperarousal, their reaction to a changed schedule, working under pressure, sleep deprivation, and the excitement of being in space. Hyperarousal can cause insomnia, disrupt the appetite, result in impaired concentration, cognitive dysfunction, and decreased co-ordination. Fatigue is often a side effect of hyperarousal, as is anorexia and sudden weight loss. Over time space agencies have developed strategies to deal with hyperarousal, including giving crews adequate rest time, control over some of their tasks, adequate leisure time, and regular contact with family and friends.