ANALYSIS: Impact Observed in Jupiter's Atmosphere
Although seeing a bright flash across millions of miles of interplanetary space may give the impression that Jupiter was hit by something pretty big, as Plait mentions in his blog, the impactor wasn't likely more than a few tens of meters wide. As Jupiter has a more powerful gravitational field than Earth, objects will hit the Jovian atmosphere around five-times faster than they hit Earth's atmosphere. Greater velocity means more energy, so (from the kinetic energy equation E=1/2mv2) we'd expect an object hitting Jupiter to be carrying 25 times more energy than a comparable object hitting Earth's atmosphere. This means 25 times more energy will be released on impact, producing a way bigger flash.
If you're experiencing a little deja vu right now, you're right, this certainly isn't the first time amateur astronomers have witnessed Jupiter flashing.
In 2009, a significant impact was witnessed by amateur astronomer Anthony Wesley in Australia that, after some detective work, was found to be an asteroid impact. Then in 2010, Wesley was again looking in the right place at the right time to spot another large impact and confirmed by Philippines-based amateur astronomer Christopher Go.