Astronauts who have spent more than a month in space have shown evidence of damage to their eyeballs and brain tissue.
MRI scans on 27 Nasa astronauts revealed a pattern of deformities in their eyeballs, optic nerves and pituitary glands, it was revealed in the journal Radiology.
Seven of the astronauts had a flattening of one or both of the eyeballs, causing them to become long-sighted. Four had swelling around the optic nerve and three had deformed pituitary glands.
The study was led by Larry Kramer at the University of Texas Health Science Centre in Houston, who says the findings could be explained by a build-up of cerebrospinal fluid in the brains of the astronauts, caused by exposure to the micro-gravity of space.
He added: “Microgravity-induced intracranial hypertension represents a hypothetical risk factor and a potential limitation to long-duration space travel.
"Consider the possible impact on proposed manned missions to Mars or even the concept of space tourism. Can risks be eventually mitigated? Can abnormalities detected be completely reversed?
"The next step is confirming the findings, defining causation and working towards a solution based on solid evidence."
The findings have not rendered any astronauts ineligible for future space travel.
Shuttle missions typically last a couple of weeks, AFP reports, while International Space Station journeys can last more than six months.
A Mars mission could potentially last a year-and-a-half.
Last month Nasa published a feature about vision changes experienced by astronauts on board the ISS. It referenced research from the October 2011 issue of Ophthalmology and referred to tests on seven astronaut test subjects who all reported blurred vision.
Astronaut Bob Thirsk, who spent six months as a member of the Expedition 20 and 21 crews in 2007, told a post-flight survey: “After a few weeks aboard I noticed that my visual acuity had changed.
"My distant vision was not too bad, but I found that it was more difficult to read procedures. I also had trouble manually focusing cameras, so I would ask a crewmate to verify my focus setting on critical experiments."
Nasa provides space anticipation glasses (spectacles with a stronger prescription) for crew members over the age of 40
Crews also have access to SuperFocus glasses - adjustable focus glasses eliminating the need for bi- and tri-focal lens associated with multiple vision adjustments. These specialised glasses are in addition to an astronaut's regular prescription glasses.