No grappling hooks and just sit and 'lurk'?
Being 'teh doctor', I sat down and did a few simple calculations:
Whereas: The average asteroid has a density of 2.0g/cm^3 = 2 x 10^6 g/m^3, or 2 metric tons (2000kg).
Assuming a spherical body (more likely it will be a rough cylinder) of 200 meters in diameter:
Volume of a sphere = 4/3 x Pi x radius^3, r = 100m
= 4.2 x 10^6 meters cubed, or ~ 8.4 x 10^6 metric tons.
A 20 ton 'gravitational tractor' probe will be outmassed by 420,000 times. The probe should not go into orbit as both objects would revolve around a common balance point called the barycenter - sort of like an elephant and a small mouse on a teeter-totter. *g* Or, in other words - this deflection would be negligible.
Nope - it would have to stay close to the target and slowly change its orbit by very small changes in the thrust direction. It would need to be applied continuously - which would require an ion propulsion system. This would be quite tricky to pull off to say the least.
But being a man of action - I'd recommend a modified solution. Being this damned close - slowly touch down a small device on the surface and NUKE the bastard. (or liberal use of velcro and duct tape..)
Certainly more bang for the buck.