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....