How Asteroid Mining exactly works can not be explained in one image, but Alberto Cuadra was able to capture the first plans their Asteroid Redirect Mission (ARM) into a single Infographic back in 2013.
The ARM Missions was first mentioned in 2012 and at that time had two options for asteroid retrieval. Option A is depicted here using a large (15m) inflatable bag to capture the asteroid of up to 500 tons. In this scenario the asteroid would have had to have been a single boulder up to 8m (26ft) in diameter.
Infographic explaining NASA’s proposed mission to retrieve an asteroid and bring it to a lunar orbit.
The second option, option B, was selected as for the mission in 2015. In this scenario a vehicle will land on the asteroid and deploy robotic arms to extract a 20 ton boulder (4m wide). This option was chosen because it was more relevant for other NASA objectives like gaining more experience with sampling, landers, autonomous docking, planetary defense, mining, and spacecraft servicing technology.
The bag approach (Option A) was thus not discarded due to it being infeasible, instead it may have been to simple for NASA’s other scientific objectives. The bagging technology is still in the books and it can possibly capture a much larger boulder. Furthermore, extraction of volatiles would be possible during transport by solar heating the bag and it’s asteroid contents.