re: Use MADMEN- not a joke.
I would think one of the real engineering challenges to this novel approach is the microgravity on the asteroids/comets these are to land on. Trying to drill into something with any torque and pressure behind the drill bit is going to be tough unless the robots can securely anchor themselves to the surface. Let's say they use a ballistic anchor device. Wouldn't the impact for the anchors exert exactly opposite the force to the comet as desired?