BigNick
Thinks s/he gets paid by the post
I presume you're talking about getting minerals, etc from other planets.We are not limited to the Earths resources. There are unlimited resources available. All we have to do is go get them.
For that to be possible, you have to be able to demonstrate that we have, or will have at a given specific point, the resources available to embark on a project which is capable of putting enough machinery and/or people into space and bringing those resources back to earth. Given our inability to provide clean drinking water for a billion people, I would suggest that that day is some way off, and when it approaches, I'm not sure if anyone's going to be prepared to pay the taxes which will be required to get the project off the ground, even if the requisite quantities of energy and knowhow were actually available.
We've all been watching scifi movies for too long. There's a reason why we haven't been back to the moon for nearly 40 years, and it's not just because the TV viewing public got bored.