Unless you are familiar with meteors, and I am not. It is difficult field to delve into.
Here is something that I would think to be reliable enough for the process. Based off what I am seeing, it will be next to impossible to determine original mass of a bolide (meteor that causes damage). So I think that it will be best to use nuclear bomb energy to compare destructiveness, but there is likely a lot of stuff I am 'glossing over' with this decision.
I will use the hiroshima bomb's yield. Which is 15 Kilotons of energy. Ugh, just realized I have that in english units...And the conversion is easy to do, thankfully (133,446,648 Newtons).
So we can assume, safely that the 500,000 ton force was terrifying. The question is how terrifying would a 400 ton (363 kg) force be? Kinetic Energy is 1/2 of the mass time velocity squared.
So 181kg * v2 is equal to 133,447 kN. Divide by 181 and we get 737 take that by the sqrt and we get... 27 m/s...
So that harmless 400 tons is going to be WAY worse than hiroshima, assuming we can get the mass to survive the heat. Oh terminal velocity for a person is found here. 50 m/s to 90 m/s for a sky diver.
Without surface area, I can't speculate further. I need to go 27 m/s in order to equal a crude nuke with a 400 ton mass.
Now what most people don't understand about atmospheric entry is that it entails the object burning up, because it is slowing down. Usually we ant the object to slow down, but because I am making a weapon, I don't want it to slow down.
The ISS is travelling at 7,670 m/s. if we can preserve that speed long enough to reach the surface, we will have successfully created a 'significant' impact. supersonic speed is 1,440 m/s divide that by our target of 27 m/s and we get = 53. Please note that this value isn't squared, but 50 times the Hiroshima bomb is very viable.
However, I wasn't intending to use all the mass of the station, but rather a tiny portion, just really efficiently. Say I was in a really bad mood.
1 kg travelling at 7,670 m/s is my ideal target. It is theoretically possible, and will be way more exciting on the ground. I would get 29,414,450,000 Newtons of energy. That is 2952 kilotons. Hiroshima is 15.
Oh, I found this. which has a nice chart dictating impact events energy discharge.
3.8 Mt impact energy - 1.2 km (0.75 mi) crater size - 5,200 mean year occurance
Keep in mind a 1 kg object is 2 lbs.