We could have had the ability to fly around the solar system in days or weeks by the 70's if we had embraced nuclear pulsed propulsion ala Project Orion. But the big babies who worried about setting off thousands of nuclear explosions in the Earth's atmosphere nixed it.

We could still build an Orion spacecraft today. Those things would be the size and weight of battleships.

"A nuclear pulse drive starship powered by fusion-antimatter catalyzed nuclear pulse propulsion units would be similarly in the 10% range and pure Matter-antimatter annihilation rockets would be theoretically capable of obtaining a velocity between 50% to 80% of the speed of light. In each case saving fuel for slowing down halves the maximum speed.At 0.1c, Orion thermonuclear starships would require a flight time of at least 44 years to reach Alpha Centauri, not counting time needed to reach that speed (about 36 days at constant acceleration of 1g or 9.8 m/s2). At 0.1c, an Orion starship would require 100 years to travel 10 light years. The astronomer Carl Sagan suggested that this would be an excellent use for current stockpiles of nuclear weapons."