Apophis is a stony asteroid about 1,100 feet across, weighing around 60 million tons. If it struck the Earth it would release the equivalent of 1,200 megatons of TNT -- that's 75,000 Hiroshima bombs, or 24 of the biggest nuclear weapon ever exploded, Tsar Bomba. It would, I'm guessing, make for a really bad year, and a pretty bad next five years or so, but wouldn't be a civilization-killer. By contrast, the Chicxulub impact, which wiped out the dinosaurs and 75% of everybody else, is rated at one hundred million megatons.
It won't miss by much, though -- about 20,000 miles. Which, in astronomical terms, is about one-fifth of a cat's whisker. In fact, we have hundreds of active satellites (those blue dots in the video above) in geosynchronous orbit, which is 22,500 miles out. I want to know what the chances are that Apophis will perturb some of their orbits and make them useless, or even hit one. That would be fun to watch.
