If astronauts make it to Mars, they may not be able to see it very well, a NASA study has shown.
Space flights lasting six months or more appear to cause a number of problems with astronauts' eyesight, some of which, including blurry vision, persist long after their return to Earth.
The researchers studied seven astronauts, all aged around 50, who had spent at least six continuous months in space. All reported that their vision became blurry, to varying degrees, while on the space station.
Vision changes usually began around six weeks into the mission and persisted in some astronauts for months after their return to Earth. The eye abnormalities appear to be unrelated to launch or re-entry, since they were found only in astronauts who spent extended time in microgravity.
Five of the astronauts showed a flattening of the back of the eyeball, and five showed folds in the choroid - the light-sensitive vascular tissue behind the retina. Five also showed excess fluid around and presumed swelling of the optic nerve.
These abnormalities could potentially be caused by increased intracranial pressure. However, none of the affected astronauts experienced the usual symptoms, such as chronic headache, double vision, or ringing in the ears.
The researchers' best guess is that the problem may be the fluid shifts toward the head that occur when astronauts spend extended time in microgravity. This could be leading to abnormal flow of spinal fluid around the optic nerve, changes in blood flow in the choroid, or changes related to chronic low pressure within the eye.
Another recent NASA survey of 300 astronauts found that correctible problems with both near and distance vision were reported by about a quarter of astronauts on brief missions and half those on extended missions with, again, some problems persisting for years.
"In astronauts over age 40, like non-astronauts of the same age, the eye's lens may have lost some of its ability to change focus," says ophthalmologist Dr Thomas H Mader of Alaska Native Medical Center.
"In the space program's early days most astronauts were younger, military test-pilots who had excellent vision. Today's astronauts tend to be in their 40s or older. This may be one reason we've seen an uptick in vision problems. Also, we suspect many of the younger astronauts were more likely to 'tough out' any problems they experienced, rather than reporting them."