The black spot is huge, nearly 10,000 kilometers across, making it roughly the size of the Earth. It’s composed of dust, basically vaporized whatever-it-was-that-smacked-into-Jupiter. Probably an asteroid; a comet would’ve been spotted before the impact (because they are brighter), and no one saw it. The impactor itself was probably several hundred meters across, and the explosive energy release (in non-sciencey speak, the gigantic kaboom) would’ve been measured in the tens of thousands of megatons. Given that the largest nuclear weapon ever detonated on Earth was about 50 megatons, you may start to grasp the horrifying power of this event. Had it happened here, well. It wouldn’t be a global extinction event, not quite, but it would’ve sucked mightily.
The Bad Astronomy blog, on the recent impact on Jupiter.
