2 NASA astronauts returned to Earth after 9 months. Here are the health impacts

NASA astronauts Barry "Butch" Wilmore and Sunita "Suni" Williams are returning to Earth after spending an unplanned nine months in space.

The pair performed the first astronaut-crewed Boeing's Starliner capsule flight to the International Space Station (ISS) in June 2024.

Wilmore and Williams were only supposed to spend about one week in space. However, issues with Starliner extended their mission by several months and delayed their return until early 2025.

Experts have said that spending a prolonged period in space -- especially many months -- can come with many changes to human physiology and psychology.

Changes due to microgravity

One of the biggest changes comes from spending time in microgravity, which allows astronauts to float inside a spacecraft or outside during spacewalks.

During this period, there is a decrease of muscle mass -- due to decreased use and lack of stimulus through exercise equipment -- and bone loss, according to NASA.

NASA astronauts Suni Williams and Butch Wilmore stand together for a photo enroute to the launch pad at Space Launch Complex 41, June 5, 2024, in Cape Canaveral, Fla.

© Chris OMeara/AP, FILES

Without Earth's gravity, bones that support the body's weight can lose 1% to 1.5% of mineral density on average per month in space, the federal health agency says.

Additionally, without eating a proper diet and getting proper exercise, astronauts lose muscle mass faster in microgravity than they would on Earth.

NASA also says that in microgravity, blood and cerebrospinal fluid often shift upward from the lower extremities to the head and eyes, which is believed to cause eye and brain structural changes.

Crews are at risk of developing kidney stones due to dehydration or excreting calcium from their bones without preventive or countermeasures.

Upon returning to Earth, astronauts are often examined by a medical team as they work to readapt to Earth's gravity and rebalance their equilibriums for everyday tasks such as walking and standing upright.

"A mission of this length definitely poses a much greater risk of long-term muscle atrophy and strength loss, which are often impossible to fully reverse," Dr. Shenhav Shemer, professor of Biology at the Technion - Israel Institute of Technology, told ABC News. "The first priority upon Wilmore and Williams' return will be getting their sense of proprioception - or how the body maintains its sense of position and balance - under control. Usually this takes only a few days, but given the length of their mission it may take longer."

Space radiation

Space radiation is different from radiation experienced on Earth. It's made up of three kinds of radiation: particles trapped in Earth's magnetic field, particles from solar flares and galactic cosmic rays NASA said.

Earth is surrounded by a system of magnetic fields, called the magnetosphere, that protects people from harmful space radiation. However, the higher a person is in altitude, the higher the dose of radiation they are exposed to.

Due to prolonged exposure, astronauts can be at significant risk for radiation sickness and have a higher lifetime risk of cancer, central nervous system effects and degenerative diseases, according to NASA.

Crews aboard the ISS receive an average of 80 mSv to 160 mSv during a six-month stay, according to a 2017 NASA report. Millisieverts (mSv) are units of measurement for how much radiation has been absorbed by the body.

Although the type of radiation is different, 1 mSv of space radiation is roughly the same as receiving three chest X-rays, the federal space agency said.

By comparison, a person on Earth receives an average of 2 mSv every year from just background radiation, NASA said.

"In low earth orbit where the ISS is, astronauts are at least partially protected by the magnetosphere that protects Earth from the radiation exposure of deep space," Dr. Rihana Bokhari, scientific research director at Baylor College of Medicine's Translational Research Institute for Space Health, previously told ABC News.

"However, they do have a greater radiation exposure than those on Earth because the ISS passes through areas of trapped radiation in their orbit," she continued. "Butch and Suni, since they are on the ISS, will not be exposed to enough radiation to seriously cause large impacts on body systems but the long duration exposure to greater radiation than on Earth could lead to an increase in the risk of cancer."

Isolation in space

There are also physical and psychological effects that come from being isolated and in a closed environment for a long duration.

NASA says that crews selected for ISS missions are carefully chosen and trained to make sure they can handle a mission that may last for six months or longer.

NASA's Boeing Crew Flight Test astronauts Butch Wilmore and Suni Williams inside the vestibule between the forward port on the International Space Station's Harmony module and Starliner spacecraft.

© NASA

However, research has shown that this type of environment, regardless of whether someone is in space, can lead to behavioral changes and cause fatigue, stress and sleep loss.

NASA said researchers are investigating ways to help ward off the negative effects of isolation including using virtual reality to "stimulate relaxing environments" or engaging in activities such as learning a language or tending to a space garden.

Dr. Jennifer Fogarty, chief scientific officer at Baylor College of Medicine's Translational Research Institute for Space Health, previously told ABC News that there are also changes to the immune system during this time, often a result of chronic stress, which is why it's important to create as healthy of an environment as possible for astronauts going on missions.