* Gregory Peck as Commander Jim Pratt
* Richard Crenna as Captain "Chip" Conrad
* David Janssen as Major Cooper
* Gene Hackman as "Doc" Kinsella
* James Franciscus as Major "Buzz" Lloyd
* Robert Ginty as Lieutenant "Sparky" Snyder
* Mariette Hartley as Dr. Louisa "Lou" Ross
* H.M. Wynant as Mr. Andrews (NASA Chief)
* Lee Merriweather as "Slim" McMurtry (Astronaut)
* James Whitmore as Narrator
The film also features a number of other supporting actors, including:
* Harry Andrews as Dr. Thompson
* Art Metrano as Technician
* Don Gildner as NASA Official
* Walter Brooke as NASA Official
* Bill Quinn as Newscaster
* Dick Van Patten as Newscaster
I hope this helps!