Spaceflight may be changing astronauts’ faces in a subtle but measurable way: their lower eyelids rise in microgravity. A new study examining photographs of 13 NASA astronauts has found that the distance between the center of the cornea and the lower eyelid margin became significantly smaller during spaceflight. The researchers describe the effect as a form of “reverse ptosis,” the opposite of the drooping eyelid position commonly associated with aging or certain neurological and muscular disorders. Although the shift is measured in millimeters, it may have consequences for vision, corneal exposure and the way astronauts adapt to long missions beyond Earth.
The study analyzed 115 photographs, including 52 images taken on Earth and 63 captured during space missions. Researchers used the marginal reflex distance 2, or MRD2, as their primary measurement. MRD2 is the vertical distance from the center of the pupil or cornea to the lower eyelid margin, and it is widely used in ophthalmology to assess lower eyelid position. On Earth, the astronauts had an average MRD2 of 4.9 millimeters, with a standard deviation of 0.7 millimeters. In space, the average fell to 3.9 millimeters, also with a standard deviation of 0.7 millimeters. The mean reduction of 1.0 millimeter was statistically significant, with a probability value below 0.001.
The change was not limited to a small group of unusually responsive astronauts. Every astronaut included in the analysis showed some degree of upward movement in the lower eyelid while in microgravity. In 62 percent of the astronauts, the difference reached at least 1 millimeter, a threshold the researchers regarded as clinically meaningful. The measurements also showed strong agreement between the two eyes. The correlation coefficient was 0.870, with statistical significance below 0.001, suggesting that the phenomenon is generally symmetrical rather than the result of isolated injury or random variation in one eye.
That consistency is important because the lower eyelid is not simply a passive flap of skin. It helps maintain the tear film, protects the ocular surface and contributes to the shape of the space in front of the eye. When its position changes, the distribution of tears across the cornea can also change. The cornea is the transparent outer surface of the eye and must remain smooth and adequately hydrated to focus light accurately. Even a modest alteration in eyelid geometry could influence corneal topography, the pattern of the cornea’s curvature, and potentially affect visual acuity or the peripheral visual field.
The researchers propose that the first major explanation is the fluid shift that occurs when gravity disappears. On Earth, gravity pulls blood and other body fluids toward the lower parts of the body. In microgravity, that familiar gradient is largely lost, and fluid moves toward the chest, neck and head. Approximately two liters of fluid may shift toward the upper body, while an estimated 50 milliliters may redistribute into tissues in the head and neck. This process contributes to the puffy face frequently seen in astronauts during the first days of a mission. Around the eyes, increased tissue fluid can produce periorbital and pretarsal swelling, potentially pushing the lower eyelid upward.
A second mechanism involves the elastic behavior of facial tissues. Skin, connective tissue and muscles are constantly responding to mechanical forces. Under terrestrial gravity, the tissues of the face experience a persistent downward pull. In orbit, that force is greatly reduced, allowing elastic structures to settle into a different configuration. The eyelid may move upward as facial tissues recoil toward a position that reduces mechanical tension. Rather than representing a single anatomical process, the observed change may therefore reflect the combined effects of fluid accumulation, tissue elasticity and altered pressure relationships around the eye.
The results also appear to fit observations from shorter exposures to altered gravity. The magnitude of the eyelid shift was comparable to changes reported during parabolic flights, in which aircraft repeatedly create brief periods of near weightlessness. That comparison suggests the response may begin rapidly rather than emerging only after weeks in orbit. At the same time, the similarity between short-duration and long-duration exposures raises questions about how quickly the eyelid reaches a new equilibrium and whether it returns to its terrestrial position after astronauts come home. The available photographs do not provide enough information to answer those questions.
The findings are relevant to a broader concern in human spaceflight: the visual and ocular changes reported by astronauts. Around 30 percent of astronauts experience dry eye symptoms, although the present study was not designed to determine whether eyelid elevation causes or worsens those complaints. In some cases, altered eyelid position could affect the tear film or increase the area of the cornea exposed between blinks. In other cases, swelling may change the relationship between the eyelid and the ocular surface without producing noticeable symptoms. Establishing a direct connection will require studies that combine eyelid measurements with clinical examinations, tear-film analysis, corneal imaging and detailed reports from crew members.
The researchers emphasize that their conclusions should be interpreted in light of several limitations. The photographs were collected retrospectively from publicly available NASA images rather than through a standardized medical imaging protocol. Many of the subjects were smiling, looking at an angle or photographed under different lighting conditions, all of which can influence the apparent position of the eyelid. The analysis also relied on average corneal diameters for calibration instead of individual anatomical measurements. Because the study did not include systematic symptom data or repeated images from the same astronaut at precisely matched time points, it cannot establish how long the effect lasts or whether it has functional consequences for vision.
Even with those constraints, the study highlights an easily overlooked aspect of living in space. Astronauts do not merely lose bone density, muscle mass and cardiovascular conditioning when gravity is removed; their soft tissues also reorganize in visible and measurable ways. The upward movement of the lower eyelid offers a small but striking example of how the human face responds to a new physical environment. As space agencies prepare for extended missions to the Moon and Mars, researchers say future investigations should use controlled imaging before, during and after flight, record individual eye anatomy and monitor visual symptoms over time. The work, titled “The Effect of Microgravity on Lower Eyelid Position in Astronauts,” was published in Eye & ENT Research on June 17, 2026, and suggests that the face may provide another important window into the biological effects of spaceflight.
Subject of Research: Not applicable
Article Title: The Effect of Microgravity on Lower Eyelid Position in Astronauts
News Publication Date: 17-Jun-2026
Web References: https://doi.org/10.1002/eer3.70043
References: Eye & ENT Research, “The Effect of Microgravity on Lower Eyelid Position in Astronauts,” DOI: 10.1002/eer3.70043
Image Credits: Higher Education Press
Keywords
Microgravity, astronauts, spaceflight, lower eyelid, reverse ptosis, MRD2, ocular health, vision, fluid shift, periorbital edema, NASA, human spaceflight, Eye & ENT Research
