> You know at a deep level that a cat is sentient and a rock isn’t.

An axiom is not a proof. I BELIEVE cats are sentient and rocks aren’t, but without a test, I can’t really prove it. Even if we could understand completely the sentience of a cat, to the point we knew for sure what if feels to be a cat from the inside, we can’t rule out other forms of sentience based on principles completely different from an organic brain and even embodied experience.

I believe the biggest issue creating a testable definition for conscientiousness. Unless we can prove we are sentient (and we really can't - I could just be faking it), this is not a discussion we can have in scientific terms.

> Interestingly where carbon fiber's failure mode is instant, failing catastrophically (like say chalk), basalt will be more gradual (like say wood), in some use cases that's an advantage.

So, should we use it to make a submarine to visit the Titanic?

If the nuke is already in orbit, harvesting it for fissile fuel seems like a sensible way of decommissioning it. They you can power your NTR (or RTG if you must) from its fuel. It'll require some in-orbit metallurgy work, to get it in the proper shape and composition.