You might be thinking of 'The Outer Limits' episode 'The Light Brigade'[1] va juvpu jr yrnea nobhg gur vzcbegnapr bs vapyhqvat orggre snvyfnsrf va bhe cynarg-ohfgref. [rotted for spoilers]

[1] https://en.wikipedia.org/wiki/The_Light_Brigade_%28The_Outer...