I don't know what we could really do against something that's 20km across though. Tow it out of the way. If you see its approach early enough, which should be

I don't know what we could really do against something that's 20km across though. Tow it out of the way. If you see its approach early enough, which should be appreciably easier with a rock that size, it doesn't take much of a nudge for it to miss us — on the current pass, at least — and you don't even need to make contact with it. https://en.wikipedia.org/wiki/Gravity_tractor https://en.wikipedia.org/wiki/Gravity_tractor If, OTOH, we don't have sufficient time to tractor it out of the way, let's hope we've established an offworld colony, 'cos we're probably gonna need it with a rock that size...