Towing Asteroids by Gravity

I'm intrigued as to why there are no references to the above in any of the
'space' newsgroups. Do you guys know something that the public doesn't? It
seemed such an interesting idea to me.
At risk of embarrassing myself before some clever people, I played with some
figures in a spreadsheet and was startled as to how little gravitational
attraction the '20 ton space craft' would have. But what if it had hydraulic
grabs (very long arms, to fragile to be deployed in Earth gravity) and
picked a rock from the asteroid mass of a few hundred tons?
Thoughts please.

Dave W.