On the other hand, it’s exactly what I wouldn’t say if I was building an interplanetary colony ship.

Not if I thought it had any realistic chance of success, anyway.

The Sun shines on Mars too. Solar energy is a practical source of energy as far out as the asteroid belt. Nuclear energy isn't needed to accomplish anything that Musk wants to accomplish in life.

Nuclear propulsion would make colonisation much more practical, much safer, and return trips much cheaper.

Maybe. I understand the physics says there's an ISP benefit. Standard rocket engines are well-understood though, work well, allow for retro-propulsion, and you can synthesize methane easily on Mars.

In the long term when the space transport infrastructure is as well developed as the terrestrial one, I can see the benefit of having different systems for different purposes, so you get the specialized benefits. But I'm not sure we are there yet.

To use an analogy from history, we aren't at the Golden Spike moment yet where California was sufficiently settled to warrant a continental rail road connecting the East and West Coasts. We're still in the exploration and settlement stage, and while trains may be more energy efficient, what we need is a Conestoga Wagon that can be operated and maintained by the settlers themselves while they live off the land and develop the first homesteads.

In the absence of a working fusion reactor, everything is necessarily speculative. That said, going from chemical to nuclear propulsion is estimated to reduce round-trip times from ~600 days (you have to wait at Mars for the orbits to realign), to 30 days.

This is roughly the difference between an 1881 steamliner from London to Syndey, and a 747.

NASA is working on it, but until there's a working engine available for private rockets it seems premature.

https://www.nasa.gov/centers/marshall/news/news/releases/201...

Ordinarily I would agree. However, this is Musk, and both SpaceX and Tesla have made progress beyond what most commentors have thought reasonable when the first press releases went out.

I’d guess a 10%-30% chance SpaceX is already Skunkworks-ing some form of atomic rocket, either fusion or fission.

Edit: and yes, that estimate is despite the public comments.

Well you and I are just speculating, so who knows. But I sort of doubt it. People would have noticed if SpaceX was hiring nuclear engineers. I think they have enough R&D on their plate getting to full reusability.