Just think about what would happen if one of these collided with Earth, not being a solid object it would spill in from the polar regions forming ice sheets up to a mile thick, then the cooling of the atmosphere would cause rains and flooding for about a month after. you would then have a rapid rise in the level of the oceans as the ice receded from the more temperate zones. this would only have to be a small one about one quarter the size of our moon. this would not even take in to consideration the massive tides up to two miles high that would sweep from one side of the globe to the other and accompanying earth quakes. you may wish you had an ark to float around in while this was happening.