I'm working on a story involving life on Europa being upturned dramatically by contact from a probe sent from Earth. I know the chances of life, let alone intelligent life, are pretty slim on Europa, but I'm ignoring that for the sake of the story. The driving force of the story is that a probe that's come from Earth crashes/malfunctions after melting through the ice.The probe sinks to the bottom of a trench and sets off some sort of ecological disaster/collapse, that then has cascading effects on the civilizations living there. In retaliation, a fairly advanced society that has adapted to living in the ice (most of society is less advanced and lives on the sea floor), makes plans to retaliate against the Earth by breaking off a chunk of the crust and hurling it at Earth.
How big of a piece of ice would be needed to cause considerable damage to Earth? I'm thinking even if it burns up entirely on impact, if it adds enough water to our atmosphere it could really do some damage, but I'm not really sure what sort of size would make sense? I'm thinking it would need to be as little as possible while still big enough to ensure it does real harm to Earth. It would probably not be going very fast since they don't have engines attached or anything like that, and are purely working with controlling water pressure to push it the way they want (my idea is that they are basically creating their own plumes as initial propulsion out of orbit, and then just riding that momentum or possibly sling-shotting around other celestial bodies to compensate). I'm also just going to assume they have the capacity to calculate the trajectory correctly. So they are reasonably sure it will hit Earth.
I don't know much about physics, so I'm not sure how much Jupiter's gravity would have an impact, or whether they would be able to use that somehow to slingshot it around at their target and maybe increase it's speed that way, or if it would most likely just never be able to break free of Jupiter's orbit at all without a more powerful propulsion.
Thank you for your help!