[Axenolith]

1 gram of matter converted to energy yields about 21+- Kt.

Even if the efficiency is crap, say travelling at 20 or 30% C, a couple 10's of pounds is going to yield a shitaceous bang.

Something travelling toward the planet at an appreciable fraction of C can also not be "broken up" without the aggregate mass still carrying pretty much the same yield but over a wider area. Particles the size of sand would transit the atmosphere as if it weren't there, like a cosmic ray and impact solid matter like artillery rounds.

You'd better watch out
You'd better beware
'Cause Albert says that E
= MC²

[Thud]

"Quantity has a quality all its own." - some Soviet admiral.

A small acceleration rate over a long enough distance can produce a very high velocity. Throw in the Earth's relative motion and you can get an impressive energy release. Plus the spacecraft could easily be a hundred tons plus in mass.