Make Nuclear Pulse Propulsion exempt from the 1963 Partial Nuclear Test Ban Treaty, and resume it's development.

Nuclear Pulse Propulsion involves directing a nuclear blast behind a spacecraft, propelling it forward at speeds far greater and at far less cost than today's chemical combustion engines. This propulsion system was in development and almost completed before it was made illegal in the 1963 Nuclear Test Treaty. This was described by Freeman Dyson as the "first time in modern history that a major expansion of human technology has been suppressed for political reasons".
Using this technology, other worlds would be put within easy reach and human colonisation of the Solar System will finally be easily viable.