Chapter 1: Unforeseen Health Risks in Space

Astronauts serve not only as explorers of the cosmos but also as subjects for vital research aimed at understanding the effects of space travel on the human body. While on Earth, scientists monitor various health metrics, collect blood samples, and perform cheek swabs. In contrast, astronauts are tasked with conducting these assessments themselves while in orbit.

During one such examination, an astronaut discovered a blood clot while performing an ultrasound on their own body, with guidance from a ground-based specialist. Prior tests before the astronaut's mission had shown normal results, so the presence of a blood clot was unexpected. “This is something we hadn’t anticipated,” remarks Karina Marshall-Goebel, a senior scientist at NASA and author of the study published recently. “Such findings have never been documented before.”

Upon discovering the clot, NASA medical personnel intervened. Although the astronaut exhibited no symptoms related to the clot, they were removed from the study and placed on blood-thinning medication for the remainder of their mission. This incident unveiled a new health risk associated with human spaceflight.

The research aimed to explore various known effects of space travel. Ten years ago, scientists noted that astronauts returning from extended stays at the International Space Station (ISS) displayed symptoms such as swollen optic nerves, flattened eyeballs, and altered vision. Consequently, NASA began providing glasses for those experiencing vision deterioration. Researchers suspect that fluid accumulation in the body—blood and water—may be responsible for these changes. In a microgravity environment, fluids tend to shift toward the head, raising intracranial pressure.

To test this hypothesis, Marshall-Goebel's team focused on the jugular vein, responsible for transporting blood from the head back to the heart. The study involved nine male and two female astronauts (their identities were not disclosed). Prior to launching, researchers measured the astronauts’ jugular blood flow in various positions—seated, lying down, and tilted. All measurements appeared normal. The astronauts subsequently repeated ultrasound exams during their ISS missions.

The results revealed that blood flow in the jugular vein had stagnated in five out of eleven astronauts. “There was some back-and-forth movement, but no consistent forward flow,” explains Marshall-Goebel. Stagnant blood flow in this vein is uncommon; it typically occurs in the legs when individuals remain seated for extended periods, such as during long flights. This finding was troubling, as stagnant blood can clot. Although clots may dissolve naturally or with anticoagulant treatment, they can also lead to severe complications, including lung damage.

In two astronauts, blood flow actually reversed direction, moving from the heart toward the head—an “extremely abnormal” occurrence for this vein, according to Marshall-Goebel. Researchers suspect that a blockage somewhere downstream caused this unusual reversal. Similar phenomena have been observed in non-astronauts with tumors or masses that redirect blood flow.

“It’s akin to taking a detour in your car when the road is blocked,” Marshall-Goebel explains. The astronauts’ bodily fluids and organs may have shifted upward in the absence of gravity, obstructing certain veins. Marshall-Goebel expressed admiration for the body’s adaptability under these unprecedented conditions.

The research team had astronauts wear specialized suits, similar to those used by Russian cosmonauts to prepare for re-entry to Earth. These suits utilize suction to mimic gravity and help draw fluids back toward the lower body. While some astronauts showed improved blood flow, others did not.

The researchers link the observed effects to the unique environment of space. All astronauts had been in good health prior to launching, and most experienced a return to normal conditions upon their return. Analysis of the data indicated that a second astronaut may have developed a clot that went unnoticed during their mission. Fortunately, none of the crew members faced adverse clinical outcomes, as noted by Marshall-Goebel.

Experts studying astronaut health who were not involved in this research find the results intriguing and warranting further exploration. “There is enough evidence to recognize this as a significant health risk in space travel that demands additional investigation,” states Virginia Wotring, an associate professor of space medicine at the International Space University in France. “We need to understand these risks before embarking on long-duration missions where medical assistance would be far away.”

Wotring also advocates for a study examining potential differences in health risks between male and female astronauts. The researchers caution that their findings could have implications for women using hormonal birth control, which can increase the risk of blood clots—especially if they choose to skip their periods during missions. “There are many effects of space on the human body that we have yet to uncover,” Wotring adds.

The researchers emphasize that altered blood flow in astronauts at 260 miles above Earth is not as alarming as it might seem. It’s possible that many other space travelers have experienced similar conditions without detection. “Clots can develop quickly when blood flow stagnates, and it doesn’t take months for this to happen,” explains Michael Stenger, head of the Cardiovascular and Vision Laboratory at NASA’s Johnson Space Center and a co-author of the study. “This can occur within days or even hours.” Current missions typically involve astronauts spending six months on the ISS, with some extending their stay to nearly a year.

Nonetheless, the discovery of the clot was an unsettling surprise. “It likely caused concern for everyone involved,” says Marshall-Goebel. “However, identifying this risk factor is a positive development because it allows for monitoring and preventive measures.” Should complications arise now, astronauts can return to Earth within hours; on missions to the moon, they would be days away, and months away on Mars.

A NASA representative indicates that the agency is now developing a formal ultrasound program to monitor all astronauts while in orbit. They are also ensuring that the ISS is adequately stocked with medical supplies for crew members.

After nearly six decades of human space exploration, scientists have gained significant insights into how the body functions in space. Concerns once existed regarding the impact of weightlessness on basic bodily functions, but research indicates that while the human body does not thrive in prolonged weightlessness, it can adapt relatively well.

However, uncertainties linger, particularly as commercial entities like SpaceX prepare to send non-professional astronauts into space. Christopher Mason, a geneticist at Weill Cornell Medicine who has studied changes in astronauts Scott and Mark Kelly, raises questions about how the general population might respond. “Our super-fit astronauts can adapt well, but what will happen when we send two everyday individuals?” he wonders. “We can only hope they will fare well, but without data, it remains uncertain.”