Alan Shepard should have been the first man in space
Two weeks before Yuri Gagarin became the first human in space, NASA flew a successful unmanned Mercury flight. But it could have been a manned flight; NASA was overly cautious, and Shepard eventually flew into space two weeks after Gagarin.
Gus Grissom almost drowned after re-entry
When Gus Grissom splashed down in the Atlantic Ocean on 21 July 1961 on the second Project Mercury flight, the explosive bolts on his hatch accidentally fired. Grissom scrambled out into the water before the spacecraft sank (it was recovered 38 years later), barely staying afloat until he was rescued.
20 Mercury spacecraft were built, but only six were used for manned flights
NASA tasked the McDonnell Aircraft company to build 20 separate Mercury spacecraft. Of these, six were used for manned flights (a seventh flight was cancelled), nine were used for unmanned flights (two of which were destroyed) and five never flew.
The first Mercury “astronaut” was a monkey
On 4 December 1959, a rhesus monkey called Sam became the first “astronaut” in the Mercury programme. He flew to a height of 85 kilometres (53 miles), 15 kilometres (9 miles) short of space, on a prototype spacecraft called Little Joe 2.
The astronauts insisted on having a window
Early prototype Mercury spacecraft had just two portholes, but the Mercury astronauts demanded that a larger window be installed, not only for the view but also for orientation. The latter proved invaluable on the last mission, when instrument failure required visual orientation.
Image courtesy of NASA