Note that in real life, the colony and moon version of this is very, very difficult. It's nearly as hard to take something out of orbit as it is to put it in in the first place, as all the momentum must be shed. The energy necessary to do this is actually more than will be released in the collision. Note that most of the force would come from Aerobraking in the atmosphere, so its not all that hard for something in low earth orbit where "stationkeeping" burns are required to to keep the object in orbit.
One case where this would work would be if the object being dropped is in an extremely eccentric orbit, as the object's orbital speed at or near apoapsis would be extremely low, such that even a small amount of delta-V would produce a large change in trajectory, especially around the opposite end of the orbit (you know, the end that, with a little tweaking, would intersect the planet).