Two NASA astronauts will endure months of intense radiation exposure after Boeing's Starliner spacecraft stranded them on the International Space Station.

Astronauts Sunita Williams and Barry Wilmore have been stuck on the ISS for three months. This has already put them at risk of receiving about 40 mSv to 80 mSv of radiation, which is roughly equivalent to 120 to 240 chest x-rays. 

They won't return to Earth until February 2025 at the earliest, and will have spent at least eight months in space and risked radiation exposure roughly equivalent to 310 to 630 chest x-rays.

Studies have shown that doses of radiation at that level increase long-term risk of cancer, cardiovascular disease and central nervous system damage, according to NASA.

'Space is a really inhospitable place to live – there’s no breathable air, microgravity wastes away your bones and muscles and you’re subjected to increased doses of radiation in the form of high-energy charged particles,' space plasma physicist Martin Archer wrote in an article for The Conversation. 

'These can cause damage to the cells in your body by breaking up the atoms and molecules that they’re made of,' he continued. 

NASA reported that astronauts spending six months on the ISS experience between 80 millisieverts to 160 millisieverts of radiation. 

Millisieverts (mSv) are units of measurement for the amount of radiation absorbed by the human body, and one mSv of space radiation is roughly equivalent to receiving three chest X-rays.

Williams and Wilmore have already endured a cumulative average risk of receiving roughly 40 mSv to 80 mSv of radiation, according to NASA.

'Having multiple kinds of radiation bombard their bodies puts them at risk for cancer, central nervous system damage, bone loss, and some cardiovascular diseases,' NASA has stated. 

Space radiation can increase risk of cancer by damaging cellular DNA. 

Cells will attempt to repair these damages, and sometimes they succeed. But when the DNA is repaired incorrectly it can result in genetic mutations that can lead to cancer over time.

Radiation can also alter the cardiovascular system by damaging the heart, hardening and narrowing the arteries, and eliminating cells in the linings of blood vessels. These damages can ultimately lead to cardiovascular disease. 

It impacts the brain too. Radiation exposure can hinder neurogenesis, which is the process of generating new brain cells. This could lead to cognitive impairment and memory deficits.

The ISS has shielding to reduce the amount of space radiation that astronauts are exposed to, but crews still endure about 365 times more radiation than we do here on Earth, according to Archer's calculations. 

People on Earth who receive an average of 2 mSv per year from background radiation.

Most of the health risks associated with space radiation exposure manifest over the long-term. 

But astronauts are also at risk of experiencing acute radiation syndrome (ARS), which occurs when a person receives a high dose of radiation over a short period of time. 

In severe cases this can be fatal, but no astronaut has ever died from ARS before.

Williams and Wilmore won't be returning to Earth until February 2025 at the earliest, when SpaceX's Crew Dragon mission is scheduled to bring them home from.

'Butch and Suni, since they are on the ISS, will not be exposed to enough radiation to seriously cause large impacts on body systems,' Bokhari said. 

'But the long duration exposure to greater radiation than on Earth could lead to an increase in the risk of cancer,' she added. 

Scientists have also warned that there's an increased episodic risk from solar flares - intense bursts of high energy radiation from the sun's surface. 

When flares occur, 'there's spike risks because there's episodic waves of solar radiation and deep space ionic radiation that come through the magnetic field, and luckily Earth has a strong magnetic field that blocks a lot of that,' oncologist Stanton Gerson told ABC News. 

But, 'If you're on the other side of the moon, you don't have that.' 

The sun is currently approaching solar maximum, which is the peak of it's 11 year cycle when the sun is most active and solar flares happen more frequently. 

It's difficult to estimate how this will impact the astronauts' risk of radiation exposure, Gerson said, but it is an additional risk factor to consider.

Before Starliner even launched toward the ISS, the spacecraft was already experiencing technical issues that delayed the mission several times. 

Even on the day of liftoff, Starliner experienced minor helium leaks that engineers determined were not severe enough to delay the launch again. 

It was all downhill from there. By the time Starliner reached the ISS, it had sprung more helium leaks and five of its 18 thrusters failed. 

Wilmore and Williams were able to safely board the ISS, but in a press conference on August 24, NASA officials announced that allowing the astronauts to return home on Starliner would be too risky. 

Instead, they will await the arrival of SpaceX's Dragon Capsule, which is scheduled to launch toward the ISS no earlier than September 24. 

Astronauts aboard the ISS are subjected to more radiation than we are on Earth because our planet is surrounded by a system of magnetic fields called the magnetosphere, which protects us from space radiation. 

Space radiation is made up of three different types: particles trapped in the Earth's magnetic field, particles shot into space during solar flares, and galactic cosmic rays, which are high-energy protons and heavy ions from outside our solar system, according to NASA. 

The ISS is located in low-Earth orbit, which means it receives some protection from space radiation thanks to the planet's magnetosphere, space physiologist Rihana Bokhari 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 said. 

And a spacecraft or space station's mechanical shielding can only block out so much of it.