What happens to the human body after 365 days in zero gravity?

Space · Published 8 October 2026 Swipe more like this →

In short: While six-month stays in orbit are common, NASA's year-long mission pushed the boundaries of human endurance. By studying the physiological shifts in astronauts during extended weightlessness, scientists are gathering the vital data required to protect future crews on the long, arduous journey to Mars.

The mission, featuring NASA astronaut Scott Kelly and Roscosmos cosmonaut Mikhail Kornienko, served as a massive biological laboratory. During their expedition aboard the International Space Station, researchers conducted more than 400 experiments. The primary goal was to determine how extending orbital duration affects various physiological systems, specifically investigating vestibular, sensorimotor, and cardiovascular functions.

One critical area of concern is the impact of fluid shifts on vision. While NASA has observed changes in the shape of astronauts' eyes on shorter flights, the one-year mission allowed scientists to observe whether these ocular shifts intensify after 12 months. Researchers also focused on the preservation of muscle mass and bone density. To combat atrophy, crewmembers typically exercise for approximately two hours every day, but the mission sought to test if specialized workout regimens could sustain health over much longer durations.

Beyond physical mechanics, the mission explored the psychological rigors of long-duration flight and the microscopic changes within the body. A notable component of the research involved the Kelly twins; while the 'twins study' was a NASA initiative rather than a personal request, it allowed scientists to use Mark Kelly as a ground-based control. By using DNA sequencing to compare the gastrointestinal microbiota of both brothers, researchers aimed to understand how a year of zero gravity alters the microbes living in the human gut.

This research is foundational for the 'Journey to Mars.' Because current technology cannot facilitate a trip to the Red Planet in less than a year, understanding the long-term biological adaptations to weightlessness is essential for the survival of deep-space explorers.

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