Research on astronauts has found that blood flow in the jugular vein can slow down or even reverse during spaceflight.

In microgravity, blood shifts towards the head. The jugular veins, which carry blood from the brain back to the heart, can become distended and sluggish.

Doctors worry this raises the risk of blood clots. A clot in the jugular vein can be dangerous, and treating it in space is far harder than on Earth.

NASA has studied the problem since clots were spotted in astronauts on the International Space Station. Ultrasound scans showed stagnant flow in some crew members.

The findings matter for long missions. Space agencies are testing countermeasures, from exercise routines to compression devices, to keep blood moving normally.

Longer missions to the Moon and Mars make this line of research urgent. Ultrasound scans taken aboard the station helped doctors spot the sluggish flow in crew members. The research will continue on upcoming long-duration missions.