On this date 21 years ago, telescopes all over the world were trained on the planet Jupiter, which was about to experience the first in a week-long series of impacts caused by tidally torn Comet Shoemaker-Levy 9. Going into the event, about the only things that were known was that this would be the first cosmic impact event ever to be witnessed by scientists and that no one really knew what would happen when the comet 'impacted' the gassy planet.
What did happen was spectacular.
On July 16, 1994, the first in a series of more than 20 fragments hit the planetary king, creating a large fireball and leaving a dark black area of gasses at the impact site. In time, Jupiter would come to have a virtual necklace of impact sites in its Southern Hemisphere, which also went on to have serious impact here on Earth in that, for the first time, the threat of planetary collisions from large objects shooting through the solar system was taken seriously.