[Toscoropreark]

They become more economically viable because the price increases and the scale of the operation increases. There have not been too many step change type technological advancements in mining in many years. When we're digging 5km down to get at 0.1% nickel, not much is going to help you make it cheaper.

I agree there haven't been too many step changes, but there has been plenty of scale improvements. These improvements essentially lead to a lowering in operating costs and this is how many operations look to retain their margins. The price factor is largely driven by supply/demand.

Your example isn't a good one, many things could improve the margins seen by deep underground mines.
- improved roof support technologies
- improved drilling technologies
- improved blasting technologies
- changes in labour laws
- etc...

So really the only reason we would go searching off world for minerals is if they were so scarce on Earth that even the extraordinarily high prices they would achieve (due to virtual non supply) couldn't pay for their extraction. Off world mining would be unbelievably difficult, in some instances I would suggest near impossible. This accompanied with the cost of transport is going to blow out the mining of even the purist off world ore body.

It's my bet that if prices got the point we were talking about, then alternatives would be found.

Mining a few km underground is pretty expensive and risky too.

Imagine how risky it would be on an astronomical object with no atmosphere