An asteroid (or even just a huge - say 100K tons - space transport barge full of ore) with a huge rocket engine attached - until it is targeted as a weapon, it

An asteroid (or even just a huge - say 100K tons - space transport barge full of ore) with a huge rocket engine attached - until it is targeted as a weapon, it is hardly can be considered a weapon (yet for example 2000 ton coming in at 10km/s would be a Hiroshima TNT equivalent). And for MAD you do need to have credible threat to wipe your opponents Mars/Moon/etc. colonies too.