Small space tug

Recently, NASA recommended using a specialized spacecraft to adjust a killer-asteroid's orbit. The idea was to position a massive spacecraft some 50 meters from a small asteroid (200 m daimeter) and use gravity to, over several months, attract the asteroid in the direction of the spacecraft.

Of course, being NASA, they figured, might as well make the craft weigh 20 tonnes, so as to better attract the asteroid. It seems to me, this is a little overkill. Lets say that NASA intends to 'station keep' the spacecraft with one of their new ion engines (firing a beam of Xenon atoms continuously for months). It seems to me, that all they need to do is find a suitably large bolder on (or near) the asteroid and lift that to the 50 meter point.

Perhaps its best to imagine a diminutive spacecraft wedging itself between asteroid and boulder, and then, much like superman, pushing the boulder off the surface. Given the miniscule gravity of the asteroid, such a feat probably doesn't require a chemical rocket (if it does, maybe just use a small thruster).

P.S. The idea above probably doesn't make much sense until you look at NASAs original concept in the links below.