Re: impact fallout
Justin -
While it is not widely known, the Rio Cuarto impacts of ca. 2100 BCE did exactly that. Their total explosive force was ca. 350 megatons, and it wasn't that they were that big, it was just that they came in low and the heat they threw off as they flew through the atmosphere managed to set most of South America on fire.
I can't get the cut and past feature to work, but there's one article on them over at MSNBC in the space section and some Argentinians have a new internet site online which you can find through Yahoo search.
EP