Trivia

According to NASA, the mysterious "fireflies" observed by John Glenn on his first orbital flight were actually condensed ice crystals from the small hydrogen peroxide rockets used for altitude control illuminated by sunlight. Upon uses many of them formed a particulate cloud around the spacecraft and many attached themselves to the skin of the vehicle as well.. A This was confirmed by astronaut Scott Carpenter on the next Mercury flight when he banged on the craft's side, causing more of the flakes to break free and become visible.

It is generally believed that Gus Grissom was not at fault in the real-life hatch-blowing incident on the Liberty Bell 7 capsule. Kickback from the manual activation switch caused a tell-tale bruise to form on the hand activating it, and Grissom never developed the bruise. Wally Schirra, at the end of his Mercury 8 space flight, deliberately activated his own hatch to demonstrate how the bruise formed and exonerate his comrade. The most likely explanation for Grissom's hatch blowing is that the external release lanyard came loose as it was only held in place with a single screw - a design that was changed to be more secure for subsequent flights. N.A.S.A. apparently believed in Grissom's innocence as well, as he remained in a prime rotation spot for subsequent Gemini and Apollo flights. There is also significant belief among astronauts of the time that, had he not been killed in the Apollo 1 fire, Grissom would have been the first man to walk on the moon.

While several of the lead actors chose to meet their real-life counterparts, Scott Glenn elected not to meet with Alan Shepard. Scott said he wanted to get down Shepard's character and nuances by observation and by hearing others' points of view. After filming, the real Alan Shepard wrote writer and director Philip Kaufman and commented on Scott Glenn's "spot-on" performance - except for "not being nearly as good-looking as he was."

In the film, Alan Shepard says "Louise, I'm going to the moon, I swear to God. I'm on my way." Of the Mercury Seven, Shepard was the only one that did go there, on Apollo 14, becoming the fifth person to walk on the moon (and the only person to ever play golf on the moon) on February 5-6, 1971.

Upon his death on December 7 2020 (the 79th anniversary of the Japanese attack on Pearl Harbor), Chuck Yeager was the last surviving real character.

Chuck Yeager: The legendary pilot appears as Fred, the elderly bartender with a fedora in Pancho's bar (at around 55 minutes). Yeager said this was appropriate because "if all the hours were ever totalled, I reckon I spent more time at her place than in a cockpit over those years."