From: Bruce Johnson

>> PD meson guns.  Pump enough energy into the rock, and it breaks up
>> into gravel.  :)
>
> Near-C gravel, which does pretty much the same amount of damage to your target as the original solid does. This
> is kind of like having a magic anti-bullet that turns an incoming 7.56 round to buckshot :-/

Well, enough energy will turn it into something expanding fast enough that most of it will miss the target, but that usually requires intercepting it quite a ways in advance, it's not 'point defense'.

> A near-c rock is going to have a gargantuan amount of inertial mass that will be well-nigh impossible to deflect. Even turning the damned thing
> into high-speed plasma won’t help you much….you just get a planet-sized plasma cannon hit.

It's not really that hard. If you have a 1,000 ton impactor, and you put a 1 ton object in front of it, the impactor turns into a cloud of material expanding at something like 10,000 km/sec. If you do it right, you may even get a donut-shaped cloud of gas, but even if you don't, blowing something up at the 100d limit will turn it into a cloud three times the size of the planet by the time it hits.