The vast majority of human spaceflight has occurred within low Earth orbit (LEO), with the notable exception of the Apollo program’s lunar missions. All manned space stations, including the ISS, are in LEO. As a result, for more than two decades, countless experiments have been conducted on the ISS to understand how astronauts’ health is impacted in LEO.

Researchers studying the health of astronauts aboard the ISS have uncovered that long-term travel in LEO has notable effects on astronauts’ cardiovascular health. According to Dr. Thais Russomano, a leading expert on space medicine, the absence of Earth’s gravitational force in space causes bodily fluids and blood to shift from the legs and lower abdomen toward the upper torso and head. This phenomenon - referred to as ‘puffy-face and bird-legs syndrome’ - causes swelling in the face and head while reducing astronauts’ circulating blood volume and heart size.  As less blood is pumped by the heart in microgravity, astronauts endure muscle loss in the heart, placing them at risk for cardiovascular deconditioning and cardiac myocyte atrophy.

Radiation is another significant concern impacting astronauts’ cardiovascular health. Aboard the ISS, radiation from galactic cosmic rays, solar cosmic rays, and particles from the Van Allen radiation belts are of primary concern. Astronauts are exposed to roughly 40-times more millisieverts of radiation compared to people on Earth. Exposure to space radiation over long-term missions increases astronauts’ risk for cancer and cardiovascular diseases, although effective shielding and radiation shelters aboard spacecraft have helped mitigate those risks.

Cardiovascular issues resulting from microgravity and radiation exposure over long periods aboard the ISS can follow astronauts well after returning to Earth. Some studies have determined that astronauts’ arterial blood pressure decreased throughout space missions due to the loss in circulating blood volume, although there could be many causes behind this change. Similarly, the reduction in circulating blood volume can cause orthostatic intolerance - the inability to stand due to lightheadedness or fainting - once astronauts return to Earth. Although radiation exposure and microgravity cause cardiovascular problems in space, studies on astronaut mortality have concluded that astronauts are at a lower risk of death from cardiovascular diseases relative to the general population on Earth.

The effects of bodily fluid shifting in microgravity extend beyond ‘puffy-face and bird-legs syndrome’, with consequences for the eyes. Following a six-month mission to the ISS in 2005, astronaut John Phillips’s perfect vision was found to have deteriorated due to spaceflight-associated neuro-ocular syndrome (SANS). SANS is formerly known as visual impairment and intracranial pressure (VIIP) syndrome, although the name was updated to reflect the uncertainty over whether increased intracranial pressure is the sole cause of the condition. One explanation is that SANS is caused by cerebrospinal fluid shifting toward the head, increasing intracranial pressure, particularly at eye level. The pressure causes the back of the eye to flatten, resulting in a hyperopic shift and blurred vision.

According to a report from the British Journal of Anaesthesia, a questionnaire of 300 astronauts found that 28% of short-duration mission astronauts and 60% of long-duration mission astronauts experienced degradation of visual acuity. A study of seven long-duration mission ISS astronauts and nine short-duration mission space shuttle astronauts found that the long-duration astronauts had significantly greater post-flight flattening when compared with the short-duration astronauts. Given the increased severity of SANS on long-duration missions, understanding causes and possible treatments are vital for exploration in and beyond LEO.

Microgravity has notable effects on the nervous system, particularly due to the redistribution of bodily fluids in space. Neuroimaging scans show that astronauts’ brains have increased ventricular volumes following long-distance spaceflight. As fluids shift toward the upper torso and head during long-term exposure to microgravity, the volume of cerebrospinal fluid collected in the brain’s ventricles increases, resulting in ventricular expansion. Ventricular expansion could be a possible cause of SANS and may be linked to premature ageing of the brain. One study found that astronauts who spent 12 months in space displayed larger changes in ventricular volume than astronauts who spent 6 months in space, suggesting important implications for long-duration space missions.

The microgravity-induced fluid shift is also associated with alterations to white matter in astronauts’ brains. A study from the journal Science Advances reports that cosmonauts displayed increased white matter in the cerebellum following long-duration spaceflight, with white matter volume returning to roughly pre-flight levels seven months after spaceflight. The cerebellum handles fine motor control, postural balance, and oculomotor control, and white matter changes associated with spaceflight may offer evidence for motor system neuroplasticity. Various studies are employing different techniques to evaluate white matter changes due to spaceflight, which could affect other neurological functions including visual and sensory processing.

In deep space, the microgravity environment induces similar cardiovascular effects to what astronauts experience in LEO. Blood and bodily fluids shift toward the upper torso and head resulting in ‘puffy-face and bird-legs syndrome’, while the decreased cardiac workload can lead to cardiovascular deconditioning. However, relative to the gravitational force in LEO of approximately 0.95g, the Moon’s gravitational force is 0.16g while Mars’ gravitational force is 0.36g. It is unclear whether varied microgravity conditions will produce additional cardiovascular effects beyond those studied in LEO, however, fluid shifts and cardiovascular deconditioning remain significant concerns.
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Radiation-induced cardiovascular disease is another major challenge with traveling to the Moon and Mars. Compared to missions in LEO, the space radiation environment beyond LEO exposes astronauts to higher dose rates of HZE particles, the high-energy heavy ions of galactic cosmic rays. HZE particles are highly penetrating and can cause secondary radiation when interacting with shielding in spacecraft or spacesuits. According to the journal Frontiers in Cardiovascular Medicine, high doses of HZE particles over long-term deep space missions can lead to myocardial remodelling and fibrosis, potentially resulting in heart failure. While current shielding technology may protect astronauts in LEO, the power of HZE radiation makes more advanced shielding essential to protect astronauts in deep space.

Considering the microgravity-induced fluid shifts that astronauts experience in deep space, SANS remains a primary concern for missions to the Moon and Mars. SANS is typically studied on long-duration missions, although astronauts have reported blurred vision after only two weeks aboard the ISS. A mission to Mars would take up to 20 months and would require astronauts to encounter multiple gravity fields. The long duration and complex gravity shifts associated with deep space missions could cause more challenging SANS-related ocular issues compared to those faced by astronauts in LEO. The concerns surrounding radiation beyond LEO extend to ocular health. Galactic cosmic radiation has been linked to the development of phosphenes and cataracts, while studies show that repetitive spaceflights and high-radiation-dose exposure increase the prevalence of both conditions among astronauts. Considering the high volume of HZE radiation that deep space astronauts will be exposed to, the development of phosphenes and cataracts is of major concern for their ocular health.

Previous studies have sought to evaluate the effects of space radiation on the human brain by delivering radiation doses to rodents over a few minutes. However, on missions to the Moon and Mars, powerful radiation will be gradually delivered to astronauts for the duration of the trip, ranging from weeks to years. A 2019 study from the journal eNeuro aims to more accurately simulate long-duration exposure by delivering low-level neutron radiation to mice for six months and evaluating the neurological implications. The study finds that exposure to cosmic rays impairs the brain function of the mice, affecting learning, memory, and mood. Lab tests reveal that following the neutron radiation, neurons are less responsive in the hippocampus - an area critical for the formation of memories and spatial navigation - and the medial prefrontal cortex - an area responsible for accessing preexisting memories, decision-making, and processing social information. Follow-up evaluations of the irradiated mice determine that neural circuitry damage may last for up to one year.

Some researchers dispute the study’s approach, claiming that neutron radiation used in the experiment is not a viable surrogate for the galactic cosmic radiation that astronauts would encounter during deep space missions. Still, the eNeuro study offers a novel analogue to the gradual doses of powerful space radiation that astronauts would face on missions to the Moon and Mars, further emphasising the importance of effective shielding from cosmic rays. Researchers are also studying how microglia - the immune cells of the central nervous system - can be manipulated to prevent the development of cognitive deficits due to galactic cosmic ray exposure, a promising step toward protecting astronauts on deep space missions.