Amateur astronomers from Australia and Japan may have detected the results of an asteroid or comet impact on Jupiter. This would be only the 2nd recored impact on Jupiter and the first since Comet Shoemaker-Levy 9 just over 15 years ago.
On the night of July 19, Anthony Wesley of Murrumbateman, Australia noticed an unusual dark spot in Jupiter’s South Polar Region. An independent discovery was also made by Toshirou Mishina of Yokohama, Japan. Though dark spots are common on Jupiter, they are rarely this dark and almost never appear in the polar regions.
Professional astronomers from JPL/Caltech used NASA’s Infrared Telescope Facility (IRTF) on Mauna Kea in Hawaii to study the spot. According to a Twitter post made by one of the astronomers, the spot appears to be a legitimate impact feature. “Live” comments on the observations can be read on Leigh Fletcher’s Twitter page.
Wesley has produced a webpage with pictures (which are excellent) and updates. His page can be found here.
Additional pictures and news can be found at Spaceweather.com.
The Transient Sky - Comets, Asteroids, Meteors 
It is not too promising that such an impact was not known in advance. It makes one skeptical about our actual knowledge of the position and trajectory of asteroids and comets, and that earth may not be as safe from impact as we like to think we are.