> This probably can happen as soon as there’s some manufacturing and mining capacity beyond LEA, so that it’s safe to use nuclear technology without any environmental impact
The idea pursued nowadays is you launch using chemical propulsion and then only turn on the nuclear propulsion once you reach a safe distance from Earth. This is different from the original 1950s Project Orion which proposed to use nuclear pulse propulsion (i.e. repurposing nuclear weapons for propulsion) from the surface to orbit, which would have produced enough fallout to likely kill a handful of people per launch (in the long-run through higher cancer rates). The question then is - is it safe to launch nuclear material to orbit using chemical propulsion? Yes, we can secure it in containers designed to survive catastrophic loss of the launch vehicle. But, will the general public believe it is safe, even if it actually is? Possibly not-which is a political obstacle rather than a technical one.
The other issue is that nuclear propulsion systems can be too large/heavy to launch on a single chemical rocket, but you could launch them as multiple modules assembled together in orbit.
I don’t think this need or should depend on off-Earth manufacturing or mining capacity. I think it is going to be a long time before the highly complex manufacturing supply chains needed to turn raw materials into cutting edge technology like nuclear space propulsion systems exists off Earth. But we should be able to manufacture them modularly on Earth, such that in space we’d be doing module assembly rather than manufacturing.