I would add one more thing which could spark a rebirth in America's space program: our current or near-future generation of probes discovering any hints which may indicate the chance of life elsewhere in our solar system. Perhaps some kind of fossil evidence or existing organic materials, either on or underground at Mars, or out farther at the moons of Saturn or Jupiter.
Presumably we won't always have such an anti-science bent administration, and such a resumption of discovery and exploration could well spark new interest in space programs (and funding of them).
Given how risky and expensive manned spaceflights are, I think it would take an "wow" moment to get people looking up to the skies with wonder and curiosity... not to mention purpose... again.
MISSION:TREK's in-depth review of STAR TREK
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