The Problem with Deep Space Travel

In 1976, two Soviet cosmonauts were sent on a mission to the Salyut 5 space station, the last dedicated military space station in the Soviet space program. The commander and flight engineer boarded Soyuz 21, beginning a mission that was meant to last 66 days. However, after just 49 days, the mission ended abruptly due to reports of a dangerous situation aboard the space station, including a noxious chemical smell. Ground control quickly scrambled to bring them back to Earth. Soon after, a replacement crew boarded the station with breathing equipment, concerned about a potential fluid leak filling the rooms with toxic gas. After a thorough inspection, they found no odor, no gas, and no technical problems at all. Subsequent reports of psychological issues in the Soyuz crew led NASA to conclude that the odor was likely a hallucination brought on by long hours, exhaustion, and the harsh effects of the alien environment of space on the human body.

The human body is an incredible machine, but it is not built for life in outer space. Over the last 6 million years, we have evolved to thrive in Earth’s environments, from the frigid cold near the poles to the sweltering regions along the equator, but not in the weightlessness of space. Nevertheless, we continue to seek worlds beyond our own. NASA and other space agencies have plans to send astronauts farther than ever before, well beyond Earth’s atmosphere and even the Moon. Crewed missions to Mars, intended for the 2030s, will involve long-duration flights and months or years-long stays on this distant planet. This will have a profound effect on the human body, which has already shown a wide range of health issues during stays on the International Space Station.

Every moment of every day, an invisible force keeps us glued to the surface of the Earth. Our skeletons, muscles, and hearts are perfectly adapted to exist under the specific amount of downward force created by Earth’s gravity. However, as soon as our bodies leave our home planet, everything changes. In outer space, microgravity feels like total weightlessness, and on Mars, gravity is one-third that of Earth. With less force acting on the human body, things start to get complicated. In microgravity, astronauts lose one to two percent of their bone mineral density each month. For a one-way trip to Mars, which will take about nine months, this could amount to as much as 18%. Calcium from degrading bones floods the bloodstream and can even lead to kidney stones.

On Earth, we lose bone density throughout our lives, but under normal conditions, it is replaced as quickly as it is lost. In space, however, this is not the case, largely due to the bones not having to support any weight. This drop in bone density is known as disuse osteoporosis, leaving bones weak and prone to fractures. If left unchecked, astronauts who have spent long periods in space may struggle to walk upon their return to Earth.

In our lives on Earth, gravity also pulls down our bodily fluids. Without it, astronauts’ blood and other fluids are pushed upwards towards the head, causing swelling. This increased intracranial pressure can lead to vision problems. To try to correct these issues, astronauts can wear compression cuffs to keep fluids in their lower extremities and use resistance treadmills to rebuild muscle and bone mass. However, for longer missions, this may not be sufficient.

This is why scientists are now working to not simply combat the lack of gravity but to recreate it. Artificial gravity can theoretically be created using centrifugal forces. This type of machine is commonly found in science labs, and you may have experienced a similar effect while driving fast around a curve or on a carnival ride. When traveling in a circular path at high speeds in an enclosed vehicle, a centripetal force is created by the outer wall that holds you inside, making it feel like you are being pushed against the wall. This push is called centrifugal force and can serve as an equivalent for gravity.

There are a few ways this can be implemented in a spacecraft. One option is to make the entire spacecraft rotate. NASA engineers have created various designs, such as stick-like structures that rotate like a baton. However, rotating an entire spacecraft presents challenges, including maintenance difficulties and complex guidance and control. Another option is to rotate just one part of the spacecraft, but this also comes with technical issues and more moving parts.

Scientists are considering smaller solutions, such as an onboard centrifuge that can fit one astronaut at a time. They are currently investigating how long astronauts would need to spend inside to keep their bodies functioning properly. In these tests, the effects of microgravity are simulated using a bed rest model, where participants lie with their bodies tilted six degrees head down for two months. This unloads the bones and muscles and causes bodily fluids to flow towards the head. Each day, some participants spend 30 minutes in a centrifuge, either in several sessions or all at once. Researchers compare physiological outcomes between the artificial gravity groups and a control group. So far, they have found that 30 minutes of artificial gravity provided some improvements in muscle function and balance, but other negative effects, such as vision loss, remained unaffected. These results suggest that longer durations in the centrifuge will be necessary to successfully counter these health issues.

Even if the body is functioning correctly, it does no good if the mind is not. Our minds are fragile, and mental health can be challenging to maintain, even in the best circumstances. Many of us experienced the effects of isolation during the COVID-19 lockdowns, and that was in the comfort of our own homes with access to entertainment. Living in outer space, confined in a small artificial environment far from home with the same group of people and little to no privacy for extended periods could severely impact anyone’s mental well-being. The inherent dangers of the environment and the pressures of a high-stakes job could easily lead to cognitive impairment, depression, psychosomatic illness, and even psychiatric disorders. Hallucinations, like those experienced by the Soyuz crew, are not uncommon; other astronauts have reported seeing orange clouds, flashes of light, and angelic figures.

Another critical aspect of mental health during long-term space travel is group dynamics. Several missions have faced interruptions due to interpersonal conflicts among crew members and with mission control. For instance, the crew on Skylab 4 turned off communication with NASA for an entire day after arguing with mission control. To address these issues, space agencies like NASA have been studying people in isolated environments for many years. These studies include evaluating astronauts in space and participants in confinement in analog facilities on Earth, such as research stations in Antarctica, which provide a similar climate, terrain, temperature, and isolation to that of outer space.

Under these conditions, important insights about team dynamics have emerged. To have a successful team, you need not only impeccable leadership or flawless decision-making but also humor. It turns out that joking around is critical for diffusing tense situations and coping with boredom during prolonged isolation. Scientists are now working to build a computer model that can help select the best crew based on personality traits. Soon, NASA will be able to use this information to simulate various combinations of prospective crew members and predict which groups are more likely to work well together.

Onboard spacecraft, researchers have also improved lighting to help align astronauts’ circadian rhythms, enhancing sleep and alertness. Activities such as learning a new language, tending to a space garden, or even immersing oneself in a virtual reality world can help avoid depression and boost morale. However, the most insidious danger of outer space is one that is invisible: radiation.

Radiation is a form of energy that travels as rays, electromagnetic waves, or particles. We experience it daily when we step outside into the sun, turn on lights, microwave food, or use cell phones. While some forms of radiation can cause adverse health effects, the radiation in space is much more hazardous. There are two types of radiation: non-ionizing and ionizing. Non-ionizing radiation is less energetic and can be easily shielded. Sources of this on Earth include power lines, TVs, cell phones, and microwaves. The sunlight that reaches Earth is primarily non-ionizing because ionizing far ultraviolet rays have been filtered out by the atmosphere.

Ionizing radiation, however, is much more dangerous. In outer space, it comes from three sources: particles trapped in Earth’s magnetic field (radiation belts), particles (usually protons) shot into space during solar flares (solar particle events), and galactic cosmic rays, which are composed of atomic nuclei traveling at nearly the speed of light. Ionizing radiation has enough energy to alter the substances it travels through, potentially damaging the human body and breaking apart DNA, placing astronauts at significant risk for radiation sickness, increased lifetime cancer risk, central nervous system effects, and degenerative diseases.

The amount of risk depends on the radiation dose, measured in millisieverts. On Earth, an average person is exposed to about 6.2 millisieverts each year, mostly from radon and medical imaging. However, in space, solar particle events and galactic cosmic ray exposure produce much higher radiation doses. One study measured this to be about 3,500 millisieverts for a large solar particle event and up to 700 millisieverts each year for galactic cosmic rays.

To spend any length of time in space, astronauts need high levels of protection. Space agencies like NASA have safety precautions in place to limit radiation exposure, including shielding, radiation monitoring, exposure caps, and operational procedures like delaying spacewalks during solar particle events. With current precautions, astronauts experience 50 to 120 millisieverts during a six-month stay on the International Space Station, with a single day resulting in about 0.7 millisieverts. While this is significantly higher than what we experience on Earth, it allows even the most vulnerable astronaut to spend 857 days aboard the ISS before reaching NASA’s radiation exposure cap of 600 millisieverts.

NASA uses these exposure caps to determine how long astronauts can stay in space, calculating the limit based on a risk estimate. The cap represents the total exposure that would give an astronaut a three percent higher chance of dying from cancer in the rest of their life. However, this cap is not equal for all astronauts; women are more vulnerable due to their reproductive organs, and younger individuals are more susceptible because they have more time to develop cancer.

Longer missions to Mars, which may require up to 1,000 days in space, will expose astronauts to much higher doses of ionizing radiation, potentially exceeding current caps and increasing their lifetime cancer risk beyond the three percent limit to over ten percent. The high levels of radiation exposure, even inside well-shielded spacecraft, are due to the nature of galactic cosmic rays. While solar particle events can be shielded more easily, galactic cosmic rays are more penetrating and move at such high speeds that typical shielding materials cannot slow them down. Researchers have found that to completely block them, the spacecraft would need to be extraordinarily thick, which would make the trip impractical due to weight and fuel requirements.

This is why scientists are exploring other protection methods for these missions. One option is wearable vests that protect the most vulnerable areas, such as the lungs and reproductive organs. These vests are currently under development, with tests planned for the upcoming Artemis Moon mission. However, the companies developing the vests are focusing on solar particle events, so it is unclear how effective they will be against galactic cosmic rays.

Another technology being explored is active shielding methods, which involve generating an electromagnetic field similar to Earth’s to deflect radiation. However, these methods require a significant amount of energy, relying on heavy superconducting magnets that need cooling.

Scientists are also looking to medicine to counteract the effects of radiation. One radical idea involves genetically engineering astronauts to make their cells more resilient to radiation. This could be achieved by turning certain genes on or off or by combining human DNA with that of more radiation-resistant organisms, like tardigrades. While this may sound like science fiction, researchers have already expressed a tardigrade protein in human cells grown in a lab, leading to a reduction in radiation-induced damage. The protein, called damage suppressor, prevents the animal’s DNA from breaking when exposed to high doses of radiation, resulting in 40 percent less DNA damage in human cells containing it.

While we are still decades away from genetically engineered astronauts, extreme solutions may be necessary to push the human body beyond its evolutionary limits to handle environments far beyond those found on Earth. Long-term space missions are closer than you might think. The Artemis mission plans to send the first astronauts to the Moon since 1972, with the goal of building a new space station in lunar orbit and eventually establishing a habitable base on the lunar surface. The aim is to travel to the Moon and stay there to learn how to build a sustainable colony on other worlds, with boots potentially on the Moon’s surface again in as little as three years. This endeavor will push the human body to its limits, as building lunar orbiters and ground stations will require extensive spacewalks, an activity that comes with significant risks.

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