ABSTRACT

Long-term exposure to microgravity and space radiation leads to physiological and pathological changes in human biology. Pathological neuro-ocular changes are collected under the name spaceflight-associated neuro-ocular syndrome. This review examines studies on the effects of microgravity and space radiation on the ocular structures and their results. In addition, we discuss treatment methods and hypotheses to reduce the effects of microgravity and space radiation on biological structures.

Introduction

The space race began on October 4, 1957, when the Soviet Union (USSR) launched the artificial satellite Sputnik 1, followed soon after by the first animal and manned flights. At present, space studies continue on the International Space Station (ISS), which was built by joining modules brought together in a collaborative project by the United States National Aerospace Agency (NASA), Russian Federal Space Agency (Roscosmos), European Space Agency (ESA), Canadian Space Agency (CAS-ASC), and Japan Aerospace Exploration Agency (JAXA). The ISS is an artificial satellite in low Earth orbit that can be inhabited by humans. Thanks to the ISS, the number of long-duration spaceflights such as low orbit flights and Moon missions is increasing. This has also increased the number of people exposed to space conditions. Space studies have revealed several problems that affect human biology, such as low gravity, lack of atmosphere, galactic cosmic rays (GCR), and solar energetic particles (SEP).¹ Microgravity (MG) and space radiation constitute a major part of these problems. Solutions to these and many other problems are necessary to enable human beings to explore the solar system and beyond.

GCR and SEP are an important problem affecting manned space missions. GCR consist of high-energy protons, high-energy ions, neutrons, gamma and x-rays, and secondary particles formed as a result of particles colliding with spacecraft and human tissues. These rays cause molecular bond breaks and mutations in DNA, resulting in cell damage, tumors and tissue degeneration, cataract, heart disease, central nervous system damage, and acute radiation syndrome.² The effects of radiation on human tissues can be investigated by examining dosimetric results in people with occupational radiation exposure on Earth and those participating in space missions, as well as radiation dose information obtained from robotic exploration tools sent to planets for research purposes. However, all of these are indirect assessments. It should be noted that the detectors are silicone in structure. The annual radiation dose limit for people with occupational radiation exposure on Earth is 50 millisievert (mSv).³ Although the ISS is slightly protected by Earth’s magnetic field, the level of radiation exposure for humans in the station was measured as approximately 200 mSv per year.⁴ According to data obtained by the radiation assessment detector on the Curiosity space probe sent to Mars, the approximate dose of radiation exposure incurred during the roundtrip flight to Mars (2x180 days) and 500 days on the Mars surface was calculated as 1.01 Sv.⁵ Radiation exposure on the surface of Mars is greater than on Earth because Mars has a thin atmosphere and no global magnetic field to deflect energy-laden particles. According to results obtained in the Chinese Chang’e 4 robotic mission to the Moon’s Von Kármán crater, the daily GCR dose on the Moon’s surface was found to be 2.6 times higher than the daily exposure on the ISS.⁶ Epidemiological data indicate that exposure to 1 Sv of radiation increases the likelihood of cancer development by 5.5%.⁴ In this case, long-duration deep space missions are many times over the current physiological limits. Therefore, solutions must be developed to protect crew members from space radiation.

Table 1 shows the upper limits for space radiation exposure of tissues and organs determined by the International Commission on Radiological Protection.⁷

Conclusion

Further research is needed to increase human resilience to the conditions of space. Attempting to improve our understanding of the physiopathology of SANS and the effect of radiation on tissues will not only help people traveling in space, but also elucidate the physiopathology of diseases seen on Earth. The mechanisms of optic nerve supply and CSF circulation around the optic nerve are still unclear. The MG environment has demonstrated what can happen when fluid dynamics are altered. These studies will enable us to better understand the fluid and tissue dynamics of the optic nerve and develop novel approaches to optic nerve diseases.