Nuclear thermal rocket engines by NERVA (Nuclear Engine for Rocket Vehicle Application) were considered to be exceptionally reliable and safe for space exploration back to 60s and 70s.
However, although the latest NERVA engine NRX/XE was tested to be appropriate for a Mars mission in 1972 and was deemed ready for integration into a spacecraft, the mission was cancelled.
The program met or exceeded the expectations for a Mars mission and it is unknown why it was cancelled by the Nixon Administration.
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