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SPACE (KXAN) — For NASA astronauts Suni Williams and Butch Wilmore, a two-week journey into space could soon become an eight-month stay aboard the International Space Station.

Williams and Wilmore became the first pilots for Boeing’s new Starliner capsule, which launched on June 5, but safety concerns have led NASA to delay a return trip.

The prolonged stay, while not deadly, could have a major impact on their bodies.

Astronauts have spent long stretches in space before. Last March, NASA astronaut Frank Rubio returned from space after 371 days, the single longest spaceflight by an American.

Rubio’s experience could give researchers a better understanding of the impact long-term space travel has on the body.

We do already know a lot about the impacts of space travel on health: NASA’s Human Research Program has studied what happens to the human body for more than 50 years.

So what can Wilmore and Williams expect when it comes to their extended stay?

Radiation in space

According to NASA, the risk of cancer, heart disease, and cataracts increases with prolonged space flight due to radiation in space.

On Earth, our magnetic field protects us from the sun’s radiation. Spaceships, however, are not as protective as the Earth’s atmosphere.

NASA says that these increased health risks come from long-term exposure. This will be especially important once we begin journeys to the moon and Mars within the next decade.

Bones and muscles get weaker

While no gravity in space is, let’s be honest, pretty cool, it comes with some downsides. Without the constant weight of the world on their shoulders, astronauts’ bones and muscles can grow weak.

Bones lose density as a result of zero gravity. Minerals that are typically stored in your bones will also move to other parts of your body, according to Baylor College of Medicine. This increases calcium in an astronaut’s urine and increases the chance of kidney stones.

Muscles, with nothing to pull against, shrink. Astronauts experience decreased muscle mass, strength, and endurance.

While there is exercise equipment aboard the ISS, it’s not enough to perfectly maintain an explorer’s muscles. Astronauts spend at least two hours per day working out to prevent this muscle loss and take supplements to maintain bone health.

The good news is that back pain goes away. Astronauts aboard the ISS reported spines elongating, making people taller and relieving some pressure.

Space eyes

One unusual issue is poorer eyesight after being in space. Without gravity, eyes grow longer and vision changes.

Known as Space-Associate Neuro-ocular Syndrome (SANS), blood collects in an astronaut’s head due to a lack of gravity and rests in the back of the eye near the optic nerve.

This can lead to decreased sharpness and flattening of the eyeball, according to Texas A&M. These changes typically occur after just two weeks in space.

Gravity and our hearts (and stomachs)

Our heart, like other muscles, shrinks while in space. Research has shown that astronauts experience decreased blood volume, lung capacity, and even more arrhythmias (where the heart beats irregularly).

This is mainly an issue once an astronaut returns to Earth. Once gravity is pulling them again, astronauts report a smaller and weaker heart.

Finally, constipation becomes a problem. Without the assistance of gravity, food has a hard time working its way through the body.

According to the Lunar and Planetary Institute, astronauts maintain a special diet, and often lose 5% of their body weight during a four-to-six month stay.

NASA encourages astronauts to eat even when they’re not hungry and to select high-calorie foods. (Food is even taste-tested before the trip). Vitamin D, folate, and calcium supplements must also be taken.

Iron is a whole other issue. It accumulates in the body because blood is just floating around and not pumping as hard. Because of this, astronauts need to be extra careful not to consume too much.

Return to Earth

The astronauts that arrived in space aboard Boeing’s Starliner may have to wait until February to return to Earth.

Space X is launching a mission in September, which was originally scheduled to carry four astronauts on board. NASA may ask two of those astronauts to stay behind so that Wilmore and Williams can take their seats on the return flight.

That flight is scheduled to return to Earth in February 2025.

NASA said Wednesday it is still deciding whether to keep Williams and Wilmore at the International Space Station until then. Officials said they’re analyzing more data before making a decision by the end of next week or the beginning of the next.

The Associated Press contributed to this report.