Squashed eyeballs was one of the prime concerns for doctors during first U.S orbital spaceflights.
John Glenn became the first American in orbit on Feb. 20, 1962. His flight lasted 4 hours and 55 minutes.
Doctors were worried that weightlessness might cause Glenn's eyes to change shape, preventing him from seeing his instrument panel.
NASA is now dependent on Russia for rides to space until a new generation of commercial spaceships is ready.
When John Glenn blasted off inside his Mercury capsule 50 years ago on Monday to become the first American in orbit, not everyone breathed a sigh of relief that the country had finally matched the Soviet Union's technology.
Doctors were worried that Glenn, a 40-year-old Marine Corps pilot, might not be able to see in space.
"The doctors were literally concerned that your eyes might change shape and your vision might change enough that you couldn't even see the instrument panel," Glenn told reporters at the Kennedy Space Center last week during a series of commemorations to mark the 50th anniversary of his flight.
"They were enough concerned about it we actually put a little miniaturized eye chart on the top of instrument panel," he said.
Three months later when astronaut Scott Carpenter followed Glenn into orbit, NASA scientists wanted to test if he could metabolize food in weightless.
"I was given some radioactive food in a toothpaste tube and I was told to eat that on the first orbit. It was radioactive so they could trace its movement through my body," Carpenter said.
"It was senseless because you can eat food standing on your head and you process it very nicely. Why couldn't we do it the same way in zero gravity? Well, we had to prove it," he added.
Science is not what gave birth to the U.S. human space program -- at the time U.S. leaders were more concerned with the threat of nuclear war with the Soviets -- but Glenn has come to believe that research is the most important reason to pursue space exploration.
"Every bit of progress made by human beings has been made because somebody was curious about the unknown," Glenn said.
"If there's one thing we have learned through the history of our country, it's that money spent on basic research has a way of paying back in the future beyond anything we ever see at the outset," he said.
And while the 50th anniversary of his flight falls at a time when the United States has no vehicles of its own to fly astronauts in space, Glenn and Carpenter, the last two surviving Mercury Seven astronauts, said they are hopeful NASA's efforts to spur commercial space taxis will be successful.
"There are lots of reasons behind our current predicament," Carpenter said. "But what it boils down to is the simple fact that when John and I went to work for this country, the United States was recognized around the world as a can-do nation. We have become viewed around the planet as a can't-do nation and I deplore that."