DALLAS—Most of the Apollo astronauts, at one time or another, have voiced grievances at how slowly the American manned spaceflight program has advanced. Why wouldn’t they? Since the end of the focused, breakneck race to the moon during the 1960s and early 70s, NASA lost its ambitious mandate to send humans to explore other worlds. Each new presidential administration had a new plan that required scrapping the old one. And America still hasn’t sent astronauts back to the moon, much less set foot on Mars.