Tom Hanks appears as host of the first 11 episodes, introducing each from in front of a huge relief sculpture of the Greek god Apollo. This format is not used for the final episode, "Le Voyage dans la Lune", in which Hanks appears in character as Jean-Luc Despont, assistant to French filmmaker Georges Méliès.
Lane Smith portrays Emmett Seaborn, a fictional news reporter for a fictitious television network. Seaborn appears in 6 episodes, covering America's space program from the earliest days through the flight of Apollo 13. He also appears in the final episode, reporting on the final Apollo 17 lunar mission, and is himself an interview subject in a mock documentary. The fictional character was added to provide a sense of continuity to the series, often serving as a Greek chorus. Seaborn's personality is similar in some respects to Walter Cronkite, though the real Cronkite is mentioned at times and seen in archive footage. The use of a fictional character also allows for dramatic conflict to be created more easily in episode 8, "We Interrupt This Program," with another fictional reporter Brett Hutchings (Jay Mohr).
Clint Howard as fictional flight controller Paul Lucas in episode 8, "We Interrupt This Program".
Jay Mohr as Brett Hutchings, a fictional young television reporter who competes for Emmett Seaborn's job in episode 8.
Ted Levine as Alan Shepard, one of the original Mercury Seven astronauts. America's first astronaut to fly in episode 1 "Can We Do This?", and commander of the Apollo 14 Moon landing mission in episode 9 "For Miles and Miles".
Mark Rolston as Gus Grissom, Mercury and Gemini veteran who commands the ill-fated Apollo 1 in episode 2 "Apollo One", killed along with Ed White and Roger Chaffee, also seen at CAPCOM station, during White's EVA on Gemini 4 in episode 1.
Mark Harmon as Wally Schirra, Mercury and Gemini veteran who commands Apollo 7 in episode 3, "We Have Cleared the Tower", originally backup commander of Apollo 1.
Robert C. Treveiler as Gordon Cooper, Mercury and Gemini veteran seen at CAPCOM during Gemini 4 in episode 11, "The Original Wives' Club", and in TV interview during episode 9.
David Andrews as Frank Borman, commands his first flight on the Gemini 7 14-day endurance mission, and also commands the first flight to the Moon on Apollo 8 in episode 4, "1968". He also serves on the investigation board of the Apollo 1 fire in episode 2, "Apollo One", and appears in episode 11, "The Original Wives' Club".
Tim Daly as Jim Lovell, who flies with Borman on Gemini 7 and commands Gemini 12 in episode 1, flies with Borman again on Apollo 8 in episode 4, commands Apollo 13 (off-screen) in episode 8, "We Interrupt This Broadcast", and appears in episode 11.
Conor O'Farrell as James McDivitt, commander of Gemini 4 in episode 1, and commander of Apollo 9 in episode 9, "Spider". Later seen as Manager of the Apollo Spacecraft Program in episode 8, and heard briefly in radio broadcast during Gemini 4 in episode 11
Steve Hofvendahl as Thomas P. Stafford, flew on Gemini 6A, and commanded Gemini 9A (both off-screen), seen briefly as commander of Apollo 10 in episode 5. Also backup commander of Apollo 7 in episode 3, and originally slated to fly with Alan Shepard on first manned flight of Project Gemini, before Shepard`s grounding in episode 9. Also seen during astronaut briefing in episode 1.
John Posey as John Young, who flew two Gemini missions and on Apollo 10 (all off-screen), and commands the Apollo 16 lunar landing mission in episode 11. Also backup crew member of Apollo 7 in episode 3. Also seen arrival of astronaut group, and later astronaut briefing, in episode 1.
Daniel Hugh Kelly as Eugene Cernan, Gemini veteran and lunar module pilot of Apollo 10 in episode 5, backup commander of Apollo 14 in episode 9, and commander of Apollo 17 in episode 12, "Le Voyage dans la Lune". Also seen during astronaut briefing in episode 1.
Ben Marley as Roger B. Chaffee, assigned to fly his first mission on Apollo 1 in episode 1, killed in accidental fire along with Grissom and White in episode 2. Seen briefly in episode 7, "That's All There Is".
Jim Leavy as C.C. Williams, originally intended for assignment on Pete Conrad's lunar landing crew (what would become Apollo 12), before being killed in a plane crash and replaced with Alan Bean, in episode 7. Also seen during astronaut briefing in episode 1.
Nick Searcy as Deke Slayton, grounded Project Mercury astronaut who becomes Director of Flight Operations, responsible for supervising the astronauts and determining the flight rotation, appears in all the episodes except "The Original Wives' Club".
^Wendt was not a NASA employee; though he worked at the Florida launch facilities, he was employed by McDonnell Aircraft during the Mercury and Gemini programs, and by North American Rockwell for the Apollo program. Wendt was not Pad Leader at the time of the Apollo 1 fire; Wally Schirra insisted on North American hiring him so he could be the Apollo Pad Leader.
Farmer, Gene; Dora Jane Hamblin (1970). First On the Moon: A Voyage With Neil Armstrong, Michael Collins and Edwin E. Aldrin, Jr. Boston: Little, Brown and Co. pp. 51–54. Library of Congress 76-103950.