By Stephanie Dwilson | 2026-01-01

Living in space for a year or longer can be extremely challenging for astronauts. From dealing with isolation and disrupted sleep, to combating bone and muscle loss, the demands on the human body are rigorous and unexpected.

As humanity continues to push the boundaries of space exploration by planning longer crewed missions, understanding and addressing these issues becomes increasingly important.

The longest single spaceflight to date lasted 438 days.

On March 2, 2016, US astronaut Scott Kelly and Russian cosmonaut Mikhail Kornienko exited their space capsule in Kazakhstan after 340 days in space, completing a NASA mission to study long-duration space flight effects.

Research teams studied changes to Kelly’s body after his return, and compared the data to his identical twin brother, Mark Kelly, who stayed on Earth.

While astronauts traditionally remain on the International Space Station (ISS) for about six months, extended assignments are becoming more commonplace to help prepare for longer-duration crewed missions.

NASA astronauts Butch Wilmore and Sunita Williams gained international attention in 2025 when their eight-day mission unexpectedly turned into a more than nine month stay aboard the ISS.

On November 27, NASA astronaut Chris Williams and Russian cosmonauts Sergey Kud-Sverchkov and Sergei Mikaev blasted off for the ISS for an eight-month stay as part of Expedition 73.

Aboard the ISS, Williams is studying technology that can help make human space exploration safer, including a new workout system designed for longer missions.

Even longer space missions are planned for the near future, with NASA’s Artemis program planning a Gateway space station that orbits the moon, supporting lunar surface missions and serving as a staging point for Mars missions.

This renewed focus on traveling to Mars and beyond makes it more important than ever to understand just how space impacts the human body and mind.

Impact of extended missions

Spending many months in space has a significant impact on the human body.

Stressors astronauts might encounter include readjusting to Earth’s gravity, which can take a few weeks. Muscle mass loss can take a few months to recover. Bone mass loss can take a couple of years to recover.

Structural changes to the brain may be permanent after fluids move differently in the body while in space. For example, the brain may sit higher in the skull and the brain’s ventricles may expand.

Blurred vision is potentially irreversible. Poor cardiovascular function can occur, along with blood clots. Poor digestion and lower nutrient absorption may occur.

Space radiation exposure is likely.

And being isolated and confined aboard a spacecraft can cause stress, fatigue and sleep loss.

Studies to protect astronauts

To better understand the toll that an extended stay in space takes on the human body, NASA is conducting a series of 14 studies on the ISS.

These are designed to protect astronauts on future, long duration Artemis missions, including to the moon and Mars.

The Complement of Integrated Protocols for Human Exploration Research (CIPHER) program seeks to understand how the human body adapts to long-duration spaceflight, by examining risks like microgravity and isolation.

The studies cover bone, brain, vision, cardiovascular and sensorimotor health.

Participating astronauts on missions lasting up to a year will provide blood and urine samples that scientists will analyze for calcium levels and biomarkers of joint health, according to NASA.

They also will have high-resolution 3D images taken of their leg and arm bones before and after their mission.

They will undergo advanced imaging tests, including MRI, CT, and ultrasound scans ahead of their spaceflight to help researchers track how the heart and arteries respond to missions lasting up to a year, among other tests.

Vision tests will hep researchers evaluate how long-term spaceflight alters the structure and function of the eyes, with scientists also assessing venous blood flow and brain structural changes which may impact vision.

Additionally, NASA is collaborating with scientists at the University of Florida and elsewhere to learn more about Spaceflight-Associated Neuro-Ocular Syndrome (SANS).

SANS impacts about 70% of astronauts, leading to potentially permanent optic nerve and eye damage.

The European Space Agency (ESA) also continues to study the risks that long space flights pose to humans.

The agency is working to support astronauts through its Space Medicine Team, which aims to improve their health and better prepare them for longer missions.

The interdisciplinary team includes biomedical engineers, exercise physiologists and psychologists.