ARM was going to bring back a moderate sized asteroid the size of a small car or so. The reason this works is that there are no deep gravity wells involved. The delta-v needed to move things over long periods of time is feasible if difficult.

To be explicit, the ARM mission propulsion bus, canceled in 2017, is now to be the main component of the lunar gateway program. But it's like putting the cart before the horse with no intention of getting a horse ever. All this talk about lunar gravity well exploration is just sour grapes.

https://en.wikipedia.org/wiki/Power_and_Propulsion_Element#A...