An Orion Future

Backround/History
In OTL, a nuclear propulsion project from the 1950's called Project Orion was in development. It would have used nuclear explosions to power a spacecraft, faster and more efficiently than chemical rockets. Due to the Partial Test Ban Treaty of 1963 (hereafter refered to as PTBT), which banned nuclear explosions in space, in the atmosphere, and underground, the project was canceled. What would have happened had Orion reached its potential?

Point of Divergence
In 1963, as the Soviets and Americans were preparing to sign the PTBT, and the architects of Project Orion faced a seemingly insurmountable obstacle: promoting nuclear powered spacecraft to the increasingly nervous USA and Soviet government officials. There was a desire for a fully nuclear powered vehicle among Orion scientists, however a compromise was reached. Standard engines would bring the spacecraft to space and nuclear power would take over once the ship was far enough from Earth for the fallout to be within an acceptable level. Thus, on October 12th, 1963, the PTBT went into effect, allowing the use of peaceful nuclear powered spacecraft.