Once you're in orbit, why waste valuable delta-V on gravity wells? Carbonaceous chondrites, according to the paper I did years back, are up to 12 percent water, so volatiles can be had without going to Mars. This may not be true for Apollo asteroids, which could have a lot of volatiles "cooked out" during close approaches to the sun, but it needs to be researched.

I would really like to see a nuclear-powered "Prospector" probe whose sole job was to spend a few years hopping from asteroid to asteroid and dropping a little analysis probe on each one as it goes by. Fit it with the mother of all radars and telescopic sights, to find asteroids on its own and track them.

In fact, if it was up to me, I'd build several such probes. If one were clever, you could have them return to Earth orbit every so often for refueling, and use them again.

Asteroids that are easily accessible from Earth for mining purposes also tend to be asteroids that swing near Earth, and pose potential threats. It won't suck to have transponders on a few hundred of these things, either.