To investigate this theory, Marshall-Goebel and her team targeted the jugular vein, which delivers blood from the head to the heart. The study’s astronaut subjects included nine men and two women. (The study did not disclose their identities.) Before the astronauts launched, researchers measured blood flow in their jugular vein in seated, supine, and tilted positions. The readings looked normal. The researchers had the astronauts repeat the ultrasounds during their missions on the ISS.

Scans showed that blood flow in the vein stalled in five of the 11 astronauts. “Sometimes it was sloshing back and forth a bit, but there was no net-forward movement,” Marshall-Goebel says. Seeing stagnant blood flow in this kind of vein is rare, she says; the condition usually occurs in the legs, such as when people sit still for hours on a plane. The finding was concerning. Stagnant blood, whether it’s in the neck or in the legs, can clot. Blood clots can dissolve on their own or with the help of anticoagulants, but the blockages can also cause serious problems, such as lung damage.

In two astronauts, blood in the vessel actually started moving in the opposite direction, from the heart toward the head, which is “extremely abnormal” for this vein, according to Marshall-Goebel. The researchers think the blood switched directions because of a blockage somewhere downstream. The phenomenon has been reported in non-astronauts with tumors or masses that forced blood to find a different path to the heart.