Never is a really long time. Very small and light craft could take chucks out of rocks and direct them elsewhere. Allowing the process to take a long time per

Never is a really long time. Very small and light craft could take chucks out of rocks and direct them elsewhere. Allowing the process to take a long time period substantially changes the physics. The source of this idea is not science fiction, but industry. Valuable materials get sought after and rational bounds do not always apply, at least not at first. Remember that many said man would never fly, and then it was commonly said we would never walk on the moon. Never really gets around. Remember not needing more than 640k memory?