Space debris poses a very real danger, but emergencies are not the only health issue.
Are there hazards? Of course.
As one researcher puts it: "Space is an unforgiving environment that does not tolerate human errors or technical failure."
Ask most people about the dangers of space, and major emergencies will come to mind: getting lost, vehicle malfunction, being hit by meteorites.
Astronaut Chris Hadfield, who spent time on the International Space Station (ISS), says that being hit by a meteorite is not uncommon. It is a reminder, he says in a video for the BBC, that "you're actually in an aluminum bubble."
And far away from home.
Spacecrafts must be made of materials that can operate in microgravity while in space, but that can also withstand the 3G acceleration needed to take off in the first place.
They need protection against meteorites, solar winds and man-made debris from previous human ventures. The ISS, traveling at almost 17,000 miles per hour, is susceptible even to dust-sized grains.
A puncture to the body of a spacecraft would cause the vacuum outside to suck everything and everyone out.
The aluminum shell of each module of the ISS is covered with a 10-inch thick "blanket" consisting of layers of Kevlar, ceramic fabrics and other advanced materials. Kevlar is the material used in bulletproof vests.
But even if 100% vehicle safety could be guaranteed, what about the day-to-day pressures on the human body? What hazards do space travelers face, and how can they be overcome?
Walking on air: life without gravity
Gravity affects blood circulation and the musculoskeletal system, among other things. According to Richard Setlow, in an article published by the European Molecular Biology Organization (EMBO), the effects of microgravity could prevent astronauts, and their bodies, from performing necessary functions in space.
On Earth, our cardiovascular system is designed to pump blood away from the feet and distribute it to every part of the body.
In space, without gravity to draw it down, the system is less effective. The blood moves up toward the chest and the head, making the face puffy and increasing the risk of high blood pressure. As the delivery of oxygen becomes less efficient, organ function can be hampered. There is an increased risk of cardiac arrhythmia and atrophy.
- Astronauts can lose up to 20% loss of muscle mass on spaceflights of 5-11 days
- Separation of the vertebrae can add 2 inches to a person's height
- Astronauts can lose 1.5% of bone mass a month, or 10% over 6 months.
Not having to work against gravity, the muscles start to waste, and particularly the antigravity muscles: the calf muscles, the quadriceps and muscles supporting the back and the neck.
There is a risk of tendonitis and fat accumulation. The loss of strength may also impede the ability to carry out any strenuous activity that may be required if an emergency occurs on returning to Earth.
Loss of muscle strength and separation of the vertebrae contribute to a lengthening of the spine, leading to back ache.
Bone is a living tissue, dynamic, self-repairing and constantly regenerating. The space environment inhibits this process causing loss of bone mass and changes in bone composition.
NASA report that astronauts have far more calcium in their blood during spaceflight. At the same time, there is a decrease in bone density, or bone mass. Astronauts on the Mir space station have reported losses of up to 20%.
On returning to Earth, they face a higher risk of fractures. Bone mass can take 3-4 years to regain, and total recovery is unlikely.
To counter these assaults on the cardiovascular and musculoskeletal systems, astronauts must exercise for 2.5 hours a day, 6 days a week.
Just recently, a team of mice was sent to the ISS as part of research into the factors that cause bone and muscle wastage. The results could provide clues about diseases that involve bone and muscle loss on Earth.
Last year, Medical News Today reported on the testing in space of a new bone growth therapy, a bone-forming molecule called NELL-1.
In future, artificial gravity could help prevent problems stemming from microgravity.
Previous proposals have included a rotating spaceship that would generate its own gravity, but to work well it would need to be at least 900 meters in diameter. Creating gravity through acceleration would require excessive amounts of fuel.
One suggestion is to generate low levels of acceleration over a longer period by using a propulsion system involving electromagnetic fields, eliminating the need for chemical combustion or fuel.
Radiation and cosmic rays
Astronauts face extreme heat and extreme cold, ranging from minus 200 °F to plus 250 °F. Spacesuits protect against heat and cold and the vacuum of space.
Spacesuits provide a barrier against environmental hazards.
Space travel also involves extreme levels of radiation.
Radiation is measured in millisieverts (mSv). On Earth, 2.4 mSv is normal. Above 100 mSv, cancer is likely. People on the ISS face levels of 200 mSv, and interplanetary levels of radiation are around 600 mSv. Researchers speculate that travel to Mars could involve a 30% risk of cancer.
Cosmic rays, or high-energy, ionizing cosmic ray (HZE) nuclei, are a form of space radiation unlike any kind of radiation on Earth. They never reach Earth, being either absorbed by the atmosphere or deflected by Earth's magnetic field.
Setlow calls them "leftovers from collapsing stars and supernova explosions that were thrown into space."
Cosmic rays are thought to cause the white flashes that astronauts sometimes see when they close their eyes.
Scientists have produced HZE nuclei on Earth and are studying their effects on biological material.
One particle, they say, has the power to charge through human tissue and destroy DNA, raising the risk of mutations and cancer. Cosmic radiation may also cause disorders of the central nervous system.
How the effects would combine with those of gravity is harder to predict.
The ISS, orbiting within Earth's ionosphere, is adequately protected by its thick walls, which prevent radiation from passing.
However, spacecraft traveling beyond current frontiers will require new materials to protect them against radiation.
Setlow explains that metals, including lead and aluminum, would make poor shields in deeper space, and they would be heavy. He foresees the use of water or plastics in the future. Other suggestions include a plasma shield, confined by a magnetic field, to reduce the energy of incoming particles.
Microbes and the immune system
Any group of people sharing limited accommodation will produce and share bacteria. Hence, astronauts follow strict hygiene rules, sample routinely for bugs and follow rigorous processes of filtering and disinfecting.
Rashes are common in space, and a reactivation of herpes can occur.
In the Mir spaceship, scientists identified 234 microbial species that could lead to infections. One report suggests that fungi, bacteria and microorganisms from human skin are common throughout the ISS, but no pathogens have been reported that could cause serious disease.
In addition, spaceflight has been associated with immune dysregulation, making the need to limit potential pathogens particularly important.
NASA report that changes in the immune system, including T-cell behavior, occur more rapidly in space. This can alter the way the immune system functions. Impaired T-cell activation and rapid cell production have both been observed in astronauts.
Without appropriate immune responses, dormant viruses can reactivate. A latent herpes may emerge, and rashes are frequent.
Skin impairments are a common problem during space missions. Research has indicated a thinning of the skin of up to 15% in some astronauts but also increases in collagen, which could offer an "anti-aging" effect.
Brian Crucian, PhD, and NASA biological studies and immunology expert, says:
"The immune system is likely being altered by many factors associated with the overall spaceflight environment. Things like radiation, microbes, stress, microgravity, altered sleep cycles and isolation could all have an effect on crew member immune systems. If this situation persisted for longer deep space missions, it could possibly increase risk of infection, hypersensitivity or autoimmune issues for exploration astronauts."
The process in space mirrors that of aging, according to NASA, which have suggested using microgravity conditions to investigate how cells age.
All astronauts take motion sickness medication to prevent the immediate danger of getting sick and choking on their own vomit. But vomiting is not the only problem.
Motion disturbance can cause sensorimotor problems, such as disorientation, and these can hamper the ability to function and to control the spacecraft.
Disorientation can affect vision, cognition, balance and motor control. Astronauts can also lose awareness of where their limbs are, due to the body's inability to sense movement within the joints. This kind of awareness is known as proprioception.
Astronauts, therefore, receive training in skills such as motion orientation, spatial ability and gait adaptability, to prevent spatial errors that could lead to damage or malfunction.
MNT recently reported on research suggesting that genetic characteristics could make some people more likely to experience vision disturbances in space.
Behavioral and psychological health
Sharing a small space with a limited group of people for several months can be challenging. Add to this the stresses and hazards of space travel, and behavioral, social and psychological health become crucial.
Wide horizons, but sharing can be challenging.
Living in space is likely to disrupt circadian rhythms and sleep patterns.
The National Space Biomedical Research Institute's (NSBRI's) Human Factors and Performance Team works with astronauts on light-dark, wake-sleep and work-rest balance to ensure maximum performance and minimize errors.
The NSBRI's Neurobehavioral and Psychosocial Factors Team also aims to protect psychological health and prevent crises before they happen.
The team provides specific training in communication skills, problem-solving and development of strategies and interventions. Pharmacological therapies are also provided, for example, to help people sleep.
MNT previously reported on the problem of getting enough sleep while traveling or working in space.
The selection process addresses these issues early by taking measures to ensure compatibility among fellow travelers.
Given the stresses of living in the space environment, a healthy diet is a top priority.
Describing food consumed by crew on a space shuttle, NASA say:
"The kinds of foods the space shuttle astronauts eat are not mysterious concoctions, but foods prepared here on Earth, many commercially available on grocery store shelves."
Astronauts on the ISS eat three meals a day: breakfast, lunch and dinner, and the foods are similar to those on Earth: chicken, seafood, brownies and so on, while drinks include tea, coffee and lemonade.
Some remain in their natural forms, such as fruit and nuts, while others are rehydrated.
Nutritionists work to ensure that the astronauts' diet will provide a balanced supply of vitamins and minerals, and that individual calorie requirements are met - for example, 1,900 calories a day for a small woman, 3,200 calories a day for a large man.
In 2015, the ISS crew harvested their first crop of home-grown fresh food, a crop of red, romaine-type lettuce known as "outradgeous."
It is hoped that this will be the first of a variety of crops to be grown, first on the ISS and ultimately on Mars.
Taking care of astronauts' health: the ISS and beyond
What happens if an astronaut gets sick on a space station or further afield? The crew must know how to take care of themselves and each other if things go wrong. Even surgery cannot be ruled out.
The NSBRI's Smart Medical Systems and Technology Team is working on intelligent, integrated medical systems to help deliver quality health care during spaceflight and exploration.
These specially adapted versions of mHealth will gather health data and help to meet medical needs in space, contributing to diagnosis, monitoring and treatment. MNT recently reported on mHealth applications here on Earth.
The systems must be small, low-power, non-invasive, versatile and highly automated.
Solutions range from devices providing just-in-time training for co-workers to a "lab-on-a-chip" that measures red and white blood cells in flight, to a therapeutic ultrasound device that can treat a variety of conditions, such as uncontrolled internal bleeding and kidney-stone-induced obstruction.
Space travel is hazardous, and there is much we do not yet know.
But the relentless progress of technology and expertise has already brought much that was unthinkable 70 years ago within reach.
As human curiosity and the insatiable quest for new and more resources push our horizons ever further, both the dangers and the solutions that go some way to overcoming them will evolve.