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Astronauts’ blood flows backwards in space

Sean Keach, November 27, 2019 6:45PM The Sun

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The observation that astronauts’ blood sometimes clots and flows in reverse will need to be better understood before we head off on long space trips in the future. media_cameraThe observation that astronauts’ blood sometimes clots and flows in reverse will need to be better understood before we head off on long space trips in the future.


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NASA has observed a bizarre effect of human space flight: blood going backwards in astronauts’ veins and clotting*.

The observations could have serious implications* for plans to send astronauts to Mars and to allow tourism in space.

An unnamed astronaut on the International Space Station was carrying out an ultrasound on their body – guided by experts on the ground. An ultrasound is a scan to study inside the body.

Similar tests before the astronaut went to space came back normal.

But the scan taken in space revealed a clot* of blood, shocking NASA doctors.

“We were not expecting this. This has never been reported before,” said NASA scientist Karina Marshall-Goebel, speaking to The Atlantic.

The astronaut didn’t have any clot-related symptoms, but was given blood-thinning medication for the rest of their time in orbit to prevent more clots forming.

media_cameraMembers of the International Space Station (ISS) expedition 59/60, NASA astronauts Christina HKoch and Nick Hague and Russian cosmonaut Alexey Ovchinin on their way to launch on March 14, 2019. Picture: AFP

This clotting and reverse flow was declared a new risk for humans in spaceflight, which will have to be better understood and planned for before astronauts can spend months or years in space, or before regular people can go to space for holidays or to live and spend extended periods in reduced gravity.

NASA researchers observed the jugular veins of 11 astronauts on the ISS using ultrasounds.

And in five of those 11 astronauts, blood flow in the jugular vein had stalled*.

“Sometimes it was sloshing back and forth a bit, but there was no net-forward movement,” Ms Marshall-Goebel explained.

The jugular vein is one of the most important parts of the body.

It runs between the heart and the head, draining deoxygenated* blood from the brain.

This “draining” process is essential to reduce pressure in the brain.

medical illustration media_cameraThe jugular vein runs down the neck to carry deoxygenated blood from the brain back to the heart. Picture: iStock/Getty Images

Stalled blood flow in veins is really rare, and is typically only seen in legs – often after long plane flights.

Blood that doesn’t move normally through the body is a major concern because it can lead to clotting, which can cause more serious problems like damage to the lungs.

The blood in the jugular vein also began moving in the opposite direction (from the heart towards the head) for two astronauts.

NASA scientists described this as “extremely abnormal”, and said that the blood may have switched directions due to a blockage.

A similar phenomenon* has been observed on Earth for patients with tumours* that force blood to find a different route to the heart.

“It’s almost like a detour,” explained Ms Marshall-Goebel.

This strange condition vanished when the astronauts returned to Earth.

“I think it was probably scary for everybody.

“But I think the fact that we found this now is really, really good news, because if you know this is a risk factor of spaceflight, it’s something that you can monitor* and prevent,” said Ms Marshall-Goebel.

media_cameraAstronauts Kjell Lindgren and Eric Boe about to train underwater on July 3, 2019 in preparation for their work on the ISS without gravity. Picture: Michael Wyke

As part of the same research, astronauts on the space station placed their legs in a special chamber with lower air pressure to suck more blood into their lower legs as would happen in Earth’s gravity. The chamber improved blood flow to the legs in 10 of 17 tests but worsened it in two.

This article was first published in The Sun and is republished here with permission.

Record-breaking Russian cosmonaut Valery Polyakov (52), is greeted at Star City, the cosmonaut training center near Moscow after spending 439 cosecutive days in space. General media_cameraRecord-breaking Russian cosmonaut Valery Polyakov (then 52), is greeted at Star City, the cosmonaut training centre near Moscow, Russia, after spending 439 consecutive days in space in 1994-1995.

The finding that space travel can do weird things to humans’ blood and the way it flows around the body will have to be planned for as we look to space tourism and long-distance space travel in the future.

US President Donald Trump has ordered NASA to get humans to Mars by 2033.

The closest that Earth and Mars would ever be to each other as they separately orbit the Sun is a distance of 54.5 million kilometres but that’s rare. The average distance is 225 million kilometres.

Spacecraft that have already been to Mars without astronauts have taken 128-333 days, which would be a very long time for a human to be in a cramped spacecraft.

SpaceX boss Elon Musk believes his Interplanetary Transport System (ITS) could manage the journey in just 80 days, eventually reducing travel time to just 30 days.

Russian cosmonaut Valery Polyakov holds the record for the longest continuous trip into space, with nearly 438 consecutive* days in orbit aboard the Mir space station, from January 1994 to March 1995. He trained very hard before the trip to be as fit and strong as possible and wore a pressure suit while in space to force blood into his legs.

Curiosity - Robot Geologist and Chemist in One! This artist's concept features NASA's Mars Science Laboratory Curiosity rover, a mobile robot for investigating Mars' past or present ability to sustain microbial life. 

Curiosity will land near the Martian equator about 10:31 p.m., Aug. 5 PDT (1:31 a.m. Aug. 6 EDT). 

In this picture, the rover examines a rock on Mars with a set of tools at the end of the rover's arm, which extends about 7 feet (2 meters). Two instruments on the arm can study rocks up close. A drill can collect sample material from inside of rocks and a scoop can pick up samples of soil. The arm can sieve the samples and deliver fine powder to instruments inside the rover for thorough analysis. 

The mast, or rover's "head," rises to about 6.9 feet (2.1 meters) above ground level, about as tall as a basketball player. This mast supports two remote-sensing science instruments: the Mast Camera, or "eyes," for stereo color viewing of surrounding terrain and material collected by the arm; and, the Chemistry and Camera instrument, which uses a laser to vaporize a speck of material on rocks up to about 23 feet (7 meters) away and determines what elements the rocks are made of. 

NASA's Jet Propulsion Laboratory, a division of the California Institute of Technology, Pasadena, manages the Mars Science Laboratory Project for the NASA Science Mission Directorate, Washington. 

For more information about Curiosity is at and . 

Image credit: NASA/JPL-Caltech media_cameraCuriosity, the NASA rover exploring Mars. Curiosity left Earth on November 26, 2011 and arrived on Mars on August 6, 2012. Picture: NASA/JPL-Caltech


  • clotting: the action of thickening to form a lump
  • implications: the conclusions or answers you can arrive at without actually being told
  • clot: liquid thickened to form a lump
  • stalled: slowed or stopped completely
  • deoxygenated: without oxygen in it
  • phenomenon: thing we notice
  • tumours: a type of growth or lump
  • monitor: watch
  • consecutive: one after another


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  1. What did they give the astronauts to prevent another clot forming?
  2. In what situation would blood go in reverse on Earth?
  3. What were the results of the trial of the pressure chamber for astronauts’ legs?
  4. How far is it from Earth to Mars?
  5. How long did Valery Polyakov stay in space for on his long trip?


1. Create a space first aid kit
If you were an astronaut on the ISS, what would you pack in a first aid kit? Think about the health issues in today’s story, then think about other effects that space travel and living on the space station could have on your health. Also think about what types of accidents you could have in zero gravity. What would you pack to help you when things go wrong? List all of the items you would pack and, for each item, write a sentence explaining why you would pack this and what might happen to make you need it.

Time: allow 25 minutes to complete this activity
Curriculum Links: English, Science, Health and Physical Education

2. Extension
After reading the story, do you think that it is safe to start space tourism? Write arguments for and against people going on space holidays.

Time: allow at least 20 minutes to complete this activity.
Curriculum Links: English, Science, Critical and Creative Thinking

Wondrous Wow Words
After reading the article, with a partner, highlight as many wow words or ambitious pieces of vocabulary that you can find in yellow. Discuss the meanings of these words and see if you can use them orally in another sentence.

HAVE YOUR SAY: Does this make you more or less keen to go to space? Would you go if you could?
No one-word answers. Use full sentences to explain your thinking. No comments will be published until approved by editors.

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