Stranded Starliner: What space does to an astronaut's body

Much has been made of the potential health risks that stranded NASA astronauts Barry Wilmore and Sunita Williams face in space — but how serious are the risks, really?

• Williams and Wilmore have been stuck at the International Space Station (ISS) since June 2024 due to technical issues with Boeing's Starliner spacecraft.They face the effects of radiation and microgravity
• But "they may well be thinking this is pretty grand." (Susan Bailey, radiation biologist)

Humans haven't evolved to live in space — in (near) zero-gravity on the ISS — so those who travel there need highly specialized training and careful health monitoring both before, during and after space travel.

Astronauts selected for human spaceflight are considered capable of not only undertaking their assigned missions but of managing complicated and changing situations.

Wilmore and Williams flew as test pilots for the first crewed flight of Boeing's Starliner spacecraft to the ISS. But propulsion issues with their spacecraft meant their eight-day mission had to be extended and could end up being eight months instead.

While NASA and Boeing will soon decide whether to return the pair to Earth on Starliner or wait until February 2025 for them to hitchhike with a SpaceX crew, the astronauts have been forced to change tack.

"I think it's quite a burden, but they can handle it," said retired German astronaut Thomas Reiter.

Reiter served two missions in space, first on Mir, a Soviet-era space station which deorbited in 2001, and later as a flight engineer on the ISS. Williams replaced him during her first spaceflight.

"Both of them are not inexperienced, they are familiar with the operations on board. However, their last trip to the ISS was many years ago and they [were] focused on a test flight this time, a very specific task, [with] a duration of just a few days," Reiter said.

"Finding out this has been extended, with some uncertainty, to weeks or months and probably now even eight months is something I'm pretty sure they are dealing with," Reiter said.

The health impacts of space travel: background radiation

Wilmore and Williams have completed a number of missions for NASA and accumulated 178 and 322 days in space, respectively. As with all astronauts, this has exposed them to microgravity and space radiation.

Space agencies devote entire departments to study the effects of space on the human body — the German Space Agency (DLR), for instance, runs its lab, :envihab near Cologne.

In June 2024, the journal Nature published more than 40 studies described as the "largest-ever compendium of data for aerospace medicine and space biology."

Among the studies was one known as TWINS. It involved 10 labs that compared astronaut Scott Kelly, who spent a year on the ISS, and their identical twin and astronaut, Mark, who stayed on Earth. And it's that study that points to one of the major risks of a prolonged periods in space — radiation. 

"It's going to be space radiation exposure that's going to be the big limiting factor for how well astronauts do or how long they're going to be able to actually be in space," said Susan Bailey, a radiation biologist from Colorado State University. Bailey led research in the TWINS study into the effect of radiation on telomeres, tiny genetic caps on the end of human chromosomes.

"Radiation exposure really is very damaging to our DNA," said Bailey.

That exposure is what increases cancer risk for astronauts. It also raises oxidative stress within the body.

"That's what all of that is about: Sparing them from those really hazardous late effects and some very acute effects," Bailey said. "We have to come up with countermeasures, some way to protect the astronauts not only during spaceflight, but if they're going to be camped out on the Moon or even Mars."

Space agencies have specific limits on the amount of radiation that astronauts can be exposed to throughout their careers.

The strain of microgravity: From kidney stones to poor sight

Microgravity in space can cause bone demineralization — astronauts lose around 1-1.5% bone density for every month spent in space.

This can also lead to changes in mineral levels in the body and result in health risks. For example, increased levels of calcium in the body's excretory system, which removes waste such as urine, can lead to kidney stones.

"When they come back, they will not be able to go on the dance floor and do heavy dancing, like anyone else who's been for many months in weightlessness," Reiter said.

This environment can also cause changes to vision, with fluids in the body shifting to the head and putting pressure on the eyes. Prolonged pressure can lead to Spaceflight Associated Neuro-ocular Syndrome, which can change the eye's ability to focus, sometimes permanently.

Upon their return to Earth, both Williams and Wilmore will be subject to regular health monitoring.

They won't run out of food and water

Despite the health risks associated with travelling to space, more immediate needs are well met by the ISS. "If there are, all of a sudden, two persons more, they are not running short of water, oxygen or food immediately," Reiter said.

Food, water, oxygen and carbon filtration needs are regularly serviced by resupply missions.

And there are six dorms, two bathrooms, and a gymnasium — so, plenty of room for the crew to spread out.

On top of this, psychological care is aided by integrating the long-stay astronauts into the ongoing aspects at the station. Wilmore and Williams have spent the past two months engaged in scientific and support work with the other seven astronauts at the ISS.

"Health-wise, I anticipate it's going to be very similar to what we've seen with some of our six-month astronauts and the ones that have indeed been up there a year or even a little bit longer," said Bailey, but added: "Astronauts want to be in space, they train their whole life to do that. They may well be thinking this [extension] is pretty grand."