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Understanding the effects of microgravity on the human body is essential

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to enabling astronauts to travel through the harsh environment of space for months, or even years.

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Significant changes to the body's skeletal and muscle systems have been studied for decades,

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and strategies to maintain physical fitness are being applied through various countermeasures,

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including vigorous exercise, aboard the International Space Station.

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But scientists and researchers still have a lot to learn - including how time spent in space affects the eyes and brain.

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Vision Changes in Space - A NASA ScienceCast

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Even during a trip as short as two weeks, vision changes occur for about a 1/3rd of American astronauts.

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When the trip is longer - say, four to six months - that figure may double.

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But, before potential solutions can be proposed, scientists first have to understand what's causing these changes.

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On Earth, gravity forces a body's natural blood volume downward, below the waist.

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Our heart forces it back up to the areas above the waist, including our eyes.

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But, what happens to that volume of blood and other fluids when gravity is no longer pulling them down?

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The human body has an amazing ability to adapt.

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Sensors in the upper body note when too much fluid is being received,

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so the body will decrease its overall blood volume in microgravity.

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However, this response doesn't always completely counter these fluid shifts.

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This can sometimes be seen in pictures or videos of astronauts aboard the space station.

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If their faces look puffy, it can indicate there's too much fluid in their head.

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Does this fluid also accumulate in or around the eyes?

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Vision researchers are working to better understand whether the chronic fluids shift

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towards the head during spaceflight are causing the shape of the eye to change,

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or if fluid is accumulating at the back of the eye.

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An imaging technique, called optical coherence tomography,

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uses a special camera to take pictures of the back of the eye

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and helps scientists to better understand the effects of increased fluid accumulation found in the tissue there.

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Dr. Steven Laurie is the lead scientist for Spaceflight Associated Neuro-ocular Syndrome research.

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He says, "We have known since astronauts flew short-duration Space Shuttle missions

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that vision changes during spaceflight occur faster than would be expected during the same time period on Earth.

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However, once we started seeing swelling at the back of the eye surrounding the optic nerve,

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this became more concerning because it has the potential to lead to long-term changes in vision

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that cannot be fixed with new prescription lenses."

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Another challenge for scientists is that astronauts may not conform to a one size fits all treatment approach.

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While all astronauts experience chronic weightlessness,

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about 70% show the earliest signs of fluid accumulating at the back of the eye,

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and only 15% show more concerning signs of this.

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When they return to Earth's gravity, these changes can take up to 1 year to resolve,

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with some changes to the eye never fully returning to how they were before spaceflight.

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Both men and women have been affected, in either or both eyes.

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Dr. Laurie concludes: "Researchers and medical doctors closely monitor astronauts during and after spaceflight

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to determine if any permanent vision changes will emerge,

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while also continuing with research to learn more about the underlying causes of these changes"

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For more information about how astronauts maintain their health on board the Space Station, go to www.nasa.gov/iss-science.

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To discover more about the space on, around, and beyond our planet visit science.nasa.gov