Quote Originally Posted by TeChameleon View Post
... wait, in-atmosphere?

From what little I understand of physics, wouldn't that be less a case of 'rocks fall, everyone dies', and more a case of 'atmosphere is stripped away in a hellish firestorm and entire hemisphere is sterilized with hard radiation, everything dies'?
A baseball at 0.9 C takes out a baseball stadium, maybe a small town. Given that energy increases exponentially as you approach C, something at 0.5 C would definitely do a lot of damage, but it's not causing in atmosphere fusion with its mere passage. Something at 0.6 C is "just" a strategic nuke, not a country wiper.