Spaceflight can halt and even reverse blood flow in astronauts' upper bodies, a NASA report said Wednesday, a startling discovery that has important implications for future trips to Mars and other long-duration missions.

NASA has known for decades that spaceflight is rough on the body, and that spending lengthy periods of time in orbit, unburdened by gravity, can cause astronauts’ muscles to lose mass and their bones to become more brittle.

Now, in the unexpected discovery, researchers have found that spending time in space can affect how blood flows through a major blood vessel in the upper body, causing it to halt or even flow backwards — a health risk that was unknown until now.

In the study, published Wednesday in JAMA Network Open, a medical journal published by the American Medical Association, researchers examined 11 healthy astronauts who had stayed aboard the International Space Station for an average of six months. During routine ultrasound assessments, by the 50th day of their missions, seven crew members were found to have stagnant or reverse blood flow in their left internal jugular vein, a major blood vessel that runs down the side of the neck and is responsible for draining blood from the brain, face and neck.

One astronaut was also found to have developed a clot in the internal jugular vein during spaceflight, and a partial clot was discovered in another crew member after returning to Earth, according to the study.

But the discovery is not necessarily a death knell for long-duration space travel, according to some experts, who said the findings would eventually lead to the development of treatments and interventions to treat the health risks.

“This was an unexpected finding,” said Michael Stenger, manager of the Cardiovascular and Vision Laboratory at NASA’s Johnson Space Center in Houston, and the study’s senior author. “We did not expect to see stasis and reverse flow. That is very abnormal. On Earth, you would immediately suspect a massive blockage or a tumor or something like that.”

The findings have major implications for long-duration space missions, including flights to Mars, which would require an interplanetary journey that lasts up to eight months.

“It’s potentially a serious problem,” said Dr. Andrew Feinberg, a professor of medicine at the Johns Hopkins University School of Medicine, who was not involved with the new study but who previously collaborated on research on the health effects of NASA astronaut Scott Kelly’s one-year mission at the space station. “If you get a clot in the internal jugular vein, the clot could travel to the lungs and cause a pulmonary embolism — that’s very dangerous. If that happens on a long-term mission, it could be calamitous.”

A medical mystery

The research began several years ago, as NASA attempted to investigate why nearly two-thirds of astronauts reported blurry vision and impaired eyesight after spending months at the space station. In some cases, those vision problems persisted even after the astronauts had returned to Earth.

Stenger and his colleagues were tasked with understanding how the weightless environment affects the circulation of fluids in the upper body. They found that without the ever-present tug of gravity on Earth, some astronauts’ bodies struggled to drain fluids normally.

“This is why some astronauts get puffy faces, because there’s no gravity to pull down those fluids circulating in the upper body,” Stenger said. “You’ll sometimes also see veins popping out in the neck, or in the head — which you can see with bald astronauts, in particular.”

In the study, the astronauts underwent ultrasound assessments to measure their left internal jugular vein before launch, at about 50 days into their spaceflight, 150 days into spaceflight and again approximately 40 days after returning to Earth.

During spaceflight, the internal jugular vein becomes engorged, but Stenger said he’s most concerned about crew members who experienced stagnant blood flow in this blood vessel.

“If those blood cells aren’t moving, they start sticking to each other, and that’s what we call a blood clot,” he said. “It’s the same risk factor when you sit on an airplane for too long and could get clots in your legs.”

Potential consequences

While the health risks of blood clots are serious, the discovery does not necessarily doom long-duration space travel.

Dr. Ben Levine, a cardiologist and professor of internal medicine at the University of Texas Southwestern Medical Center in Dallas, who was a consultant on the study, said that the engorgement seen in the blood vessels did not necessarily mean that the pressure inside them was high.

“Astronauts use their arms a lot to move around, so I think there’s plenty of blood flowing through the big veins,” he said. “I’m intrigued by the study’s findings but not overly alarmed.”

Levine did acknowledge, however, that blood clots in the upper body could be potentially devastating, and added that more research is needed, particularly with regard to possible interventions.

In the study, the researchers saw improved blood flow when the astronauts wore a lower-body vacuum suit that essentially pulls blood down from the head into the lower extremities — akin to what the human body experiences on Earth thanks to gravity.

“Medicine in space is a journey into extreme physiology — discovering what happens when we leave Earth and how our bodies adapt,” said Dr. Jan Stepanek, a physician at the Mayo Clinic in Phoenix who specializes in aerospace medicine. “If we send people on missions to Mars, we need to find appropriate countermeasures, so they maintain muscle mass and bone density and cardiovascular fitness.”

Dr. Scott Parazynski, a physician and retired NASA astronaut who flew on five space shuttle missions but was not involved with the research, said that although he’s concerned about the potential consequences of the study’s findings, it would not deter him from flying in space.

“I don’t know that it would change my desire to go to Mars, but it does add to the list of risks,” he said. “I’m encouraged, though, that we may be getting to the root cause and that it’ll be something that we can eventually treat.”

Stepanek added that the research demonstrates how much more scientists have to learn about the health impacts of journeying into space.

“It’s humbling,” he said. “After 50 years of human spaceflight, we may think we know everything, but it turns out that nature has a way of surprising us.”