Doctors have documented the third known case of a blood clot in an astronaut’s jugular vein in space. The finding was published in Nature Scientific Reports.

The jugular vein carries blood from the brain back to the heart. In weightlessness, blood can pool in the upper body and flow backwards.

A study of 11 astronauts found that 55 per cent had stagnant or reversed jugular blood flow around day 50 of their missions.

The clot was found during routine ultrasound scans. The astronaut was treated and the clot resolved without lasting harm.

NASA now screens astronauts’ neck veins during long missions. Blood thinners and special collars are among the countermeasures being tested.

The finding matters for planned Mars missions, where crews will spend months far from hospitals.

Blood normally flows down from the brain through the jugular veins. Without gravity, fluid shifts toward the head and the flow can stall or even reverse. Understanding this process is key to keeping crews healthy on long missions.