You have a mass driver which throws a 1 kg slug of iron at your spaceship. The slug reaches your spaceship and at that time it is going 30 km/s relative to the spaceship. You intend for the spaceship to harvest some of the momentum of the slug, allowing the spaceship to accelerate somewhat without spending propellant. The slug will then continue on, somewhat slower than before. What is the best way to harvest momentum from the slug?
The slug must not be melted or otherwise destroyed because it is aimed so that it will eventually orbit back to where it can be caught and re-used.
The catcher can't be too close to the slug because of the danger of a collision. We can suppose that the spaceship tracks the slug and maneuvers very accurately to catch it, but for safety no part of the catcher mechanism should be within 1m of the slug.
A large eddy current brake might harvest some momentum from an iron slug. However, this is somewhat limited for this scenario because it turns the slug's kinetic energy into heat in the slug. Braking an iron slug from 2 km/s to 0 with an eddy current brake would unfortunately melt it.
The slug has to be iron.
A good solution should harvest a lot of momentum from the slug, and also should not weigh too much. We might say the entire spaceship should weigh no more than 1000 tons.