Aakanksha Pathania, B. Optom
Optometrist, Sharp Sight Eye Hospital, New Delhi, India
The gravity of the earth is the net acceleration that is imparted to objects due to the combined effects of gravitation and centrifugal force. (1)
On earth, gravity forced a body’s natural blood volume downward, below the waist. Our heart forces it back up to areas above the waist, including our eyes. But what is the effect of gravity on the eyes? Or with gravitational changes what changes happen in the eye?
In space, as we get far from the earth, the gravitational pull weakens. The gravity of mars has less gravitational force in comparison to earth i.e Mars gravity is only about 38% of the surface gravity on earth. We have imagined sending people to mars, making mars our new home. Mars, however, is more of a marathon than a sprint. (2) Touching and living on the red planet is a dream of many astronauts but the uninviting environment on Mars is ready to kill, spending a long period in the microgravity of space can risk lasting damage to their eyes.
The zero gravity surroundings in space cause a huge ratio of problems, damage to the visual system can lead to Spaceflight associated neuro-ocular syndrome (SANS) is a huge concern for NASA. NASA records a risk of hyperopic shifts, cotton- wool spots, choroidal folds, optic disc edema, and optic globe flattening with various extents of severity that astronauts experience. (3)
Possible neurophysiological changes can be detected in high resolution, spectral domain OCT, and early optic nerve changes can be detected for astronauts for more than 3 weeks into long-duration space flights (LDSF). MRI has also documented shortening of axial length during and after LDSF creating a hyperopic shift. (4) However, Presbyopic astronauts might need ‘space anticipation glasses’ for near vision correction.
Researchers have recorded the swelling, blurriness, and impaired eyesight leading to long-term damages that can be either due to the chronic fluids shift towards the head during spaceflight or the fluid accumulating at the back of the eye. (3, 5)
If we consider water, gravity works the same way in water that it works in the air or a vacuum. Underwater, there is a buoyant force on the object that counteracts the force of gravity. The human eye is advanced to deliver excellent vision in the air, however, underwater the story changes. (6) Due to the rapid attenuation of light with distance passed through water, a lower level of natural illumination is present under water making underwater less detectable. Underwater vision not only affects the stereoscopic acuity and light transmission but also color vision.
With the increase in depth of water, only a selected wavelength of light is absorbed. Longer wavelengths tend to be absorbed first, like red and yellow are not visible below 30 and 75 ft. respectively. Whereas blue and green color remains till 100ft. (6)
But, what about the divers who spend half of their time underwater? Once a diver wears a face mask, the eyes are filled with no compressible air, which interacts with the eye and adnexa. As the diver goes in depth a negative gradient is created, creating a mask squeeze resulting in edema and subconjunctival hemorrhages which resolve within days.
Lastly, eyes undergo several changes with the change in gravity which is a huge point of concern. Especially for astronauts in LDSF who are on verge of SANS, practical measures should be considered pre and post-missions.
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- Smith, D. E., Sjogren, W. L., Tyler, G. L., Balmino, G., Lemoine, F. G., & Konopliv, A. S. (1999). The gravity field of Mars: results from Mars Global Surveyor. Science, 286(5437), 94-97.
- Kramer, C. D., & Kalla, E. M. (1997, April). The challenge of designing biomedical equipment during human research for long duration low-gravity NASA missions. In Proceedings of the 1997 16 Southern Biomedical Engineering Conference (pp. 30-37). IEEE. (3, n.d., #)
- Lee, A. G., Mader, T. H., Gibson, C. R., Tarver, W., Rabiei, P., Riascos, R. F., … & Brunstetter, T. (2020). Spaceflight associated neuro-ocular syndrome (SANS) and the neuro-ophthalmologic effects of microgravity: a review and an update. npj Microgravity, 6(1), 7.
- Norsk, P. (2021). Physiological effects of spaceflight–Weightlessness: An overview. Handbook of Bioastronautics, 93-105.
- Pell, S. J., & Mueller, F. (2013). Gravity well: underwater play. In CHI’13 Extended Abstracts on Human Factors in Computing Systems (pp. 3115-3118).