Not sure if it would be such a economic bomb. Assuming there are many mineable asteroids, others would enter the market. So the long term effect would be that the cost of natural resources approaches the marginal cost of extraction. And markets should have time to adjust as there is increasingly better information on what the initial extractions will look like. It will certianly disrupt the commodities sector, ending many companies and giving rise to incredibly large new ones. And the implications of the new commodity prices would ripple through the rest of the economy. I think it will seem like a bomb to those companies displaced, but for the economy as a whole not as much.