The proposed Asteroid Redirect Mission would capture a small astroid, then tow it back to the moon and place it in orbit - in a bag - if I understand correctly.
I'm all for creative thinking applied to finding a justification for a (manned?) NASA mission to deep(-ish) space that's easier than anything involving humans and the surface of Mars, because more and more people are realizing how difficult (and expensive) that would really be in the short term. But this one worries me for the following reason.
If the asteroid somehow fragmented, through an unanticipated impact or explosion (I said "if") or even through mechanical instability within the asteroid itself, it could possibly put many small particles, pebbles etc. into orbit around the moon.
There are meteorites from far, and suborbital debris released from impacts on the surface, but this scenario could put many small objects in a relatively stable orbits around the moon, since the event is presumably starting from a stable orbit. By relatively stable, I mean 10 or 100 years at least.
Without a substantial atmosphere like the Earth, objects in lunar orbit wouldn't safely "burn up" like the space needles are said to have mostly done. (see also Orbital Debris Quarterly News). Forces in space like photon pressure and perturbations from Earth will cause these orbits to evolve over time, but without an atmosphere, I'm guessing that either transport into space via open-ended orbits induced by perturbations, or collision with the lunar surface will have to be the only removal mechanisms.
If you happen to be standing on the moon, an orbiting mote at it's apoapsis could be cruising across the lunar surface and hit you in the head at 6000 kph!
It's unlikely of course! And standing on the moon implies many risks already, but has this been worked out somewhere?
I'm looking either for a review or calculation that addresses this technically, or if that's not forthcoming, some quantitative argument why it is more, or less dangerous than I'm suggesting here.