User blog comment:RandomGuy96/So how do planetary defense cannons work?/@comment-5379655-20130128042915/@comment-4721065-20130129071647

Well, I think the fact that you have to approach planets in ME1 in order to find the mineral deposits (and the industrial-size mineral deposit has to be much larger than PDC) as stated every time you scan planets, while in ME2 this requires special probes being sent in is a pretty unequivocal proof of scanning range being very much limited.

Ships cannot move completely unpredictably through space; nothing ever can. They have to overcome inertia every time they accelerate, decelerate or turn (which also entails decreasing speed to a point) and the faster they move, the greater the inertia. This limits their possible routes a lot, and if you have enough PDCs, you'll eventually close in on the ship, especially given that they have similarly fast rates of fire. For instance, if your ship is stationary at the time of detection and you have 7 PDC's, one will fire directly at it, but another will fire at a point ahead that it can only reach at its max speed about ten volleys away and the rest do the same with other primary directions. The ship will then obviously have to move, but either not at full speed or it cannot make a full turn. The cannons then fire again, this time again at the furthest points it can reach nine volleys away, forcing the ship to slow down further or make another turn. Thus, the net around the ship tightens with every volley, until it is eventually hit. The more PDCs you have, the faster the process becomes, as you can cover more directions at once.

And I finally have the answer regarding re-entry: [this link http://science.howstuffworks.com/question486.htm] says that a meteorite has to be about a size of a marble before a fraction of it (i.e. a grain of sand, most likely) is guaranteed to reach Earth BUT they move at the speeds of 11 to 72 km/s, while mass accelerator rounds move at 1.3% of the speed of light, 4024 km/s. In short, it would be more than realistic for cruiser & smaller rounds (which have to be much lighter than 20 kg dreadnought rounds, as otherwise there is no point to the latter), to get burned up somewhere within the atmosphere.