Impact modelers were likewise ecstatic with their data. With Hubble, we observed plume development and dissipation for four impacts, ranging in size from small through medium to large. Surprisingly (and contrary to predictions), all plumes rose to the same height above the cloud tops. Subsequent sleuthing suggested that plume height was linked to a level in the atmosphere below which the explosion couldn’t escape. This information helped refine models of extremely large atmospheric explosions. Why would we care about that? The answer lies in the third lesson.

We’ve known for hundreds of years that impacts have happened in the past in our Solar System. One glance at the pock-scarred face of our Moon reveals a panoply of craters large and small. It was only in recent decades, however, that we started to accept that truly cataclysmic impacts had occurred on Earth. More important and more recent is our understanding that those impacts can fundamentally change our environment.

The third sobering lesson from Shoemaker-Levy 9 was that giant impacts are not a thing of the past; that they are a real and present threat. In the wake of Shoemaker-Levy 9, NASA recommitted to discovering unseen objects that are potentially hazardous to us as a species. We are developing tools to find Earth-crossing asteroids and discussing the means to deflect one should it be discovered to be on a collision course.

Nature could not have fired a bigger shot across the bow than the full week of catastrophic collisions on Jupiter that took place in July 1994. In planetary defense circles, we joke about the dinosaurs getting done in because they did not have a space program to protect themselves. Fortunately, we had the mighty Hubble ready to watch the Shoemaker-Levy 9 impacts on Jupiter. People ask if I worry about asteroids. I do not. I have personal car insurance and I have personal home owners insurance, but we all have comet/asteroid impact insurance. It is our world’s space programs, and groups like The Planetary Society. We watch the sky, and we are preparing, just in case…