[underground]

It will happen when venture capital realizes the net worth of the resources available in the asteroid belt alone. Trick is to build orbital colonies so you don't constant consume planetary fuel to escape gravity.

Anyway, its all covered in this book Mining the Sky

The resources described show how even such a high cost, high risk venture could return nearly unlimited profit. Earth's entire "domestic" GDP would be insignificant.

[Prospero]

The high Delta V for mining the Asteroids, as inviting as that may be, can be more easily accomplished by using God's natural pier, the moon, and a few rail guns...

And don't give up on the Moon. The Japanese Kayuga has discovered Uranium on the surface... turns out lunar morphology ain't what was suspected.

There's a solid metal asteroid out there, by the way... no one is quite sure what kind of metal, but it's a half-mile of something heavy, and it ain't carbon.