But without an atmosphere the shockwave from that 100 kiloton explosion should be pretty minimal (through the ground only), and any debris flung into the atmosp

But without an atmosphere the shockwave from that 100 kiloton explosion should be pretty minimal (through the ground only), and any debris flung into the atmosphere would fall right back down on parabolic trajectories (or go into orbit), not linger in the sky for months/years. The danger posed by such an explosion, off the top of my head, would be getting hit by the object itself, getting hit by falling debris ejected by the explosion, structural damage from the shockwave that traveled through the ground... I'm not sure what else. Of course without an atmosphere everyone would be dead anyway, so I suppose this is kind of a silly thing to think about.