Early in the morning on Monday September 10th, Oregon-based amateur astronomer Dan Peterson had his telescope trained on Jupiter — the largest planet in our solar system — and spotted a bright flash in the planet's upper atmosphere. He reported the sighting to the Association of Lunar and Planetary Observers (ALPO), and after the news quickly made its way around the Internet, the sighting was confirmed by another amateur astronomer in Dallas, Texas. George Hall had also been observing Jupiter that morning, and found that his webcam had recorded images of the impact.
After the news and images spread, astronomers eagerly waited for nightfall so that they could aim their telescopes at the giant planet, in hopes of spotting any scar left by the impact. The images of Comet Shoemaker-Levy 9 as it broke up and impacted with Jupiter back in July 1994 show huge dark scars in the planet's upper atmosphere as dramatic evidence of the power of the impact.
Images of the planet poured in from around the world, and comparing them with pictures taken before the impact, astronomers were able to find what appears to be a small impact scar, but the results were nowhere near as dramatic as with Shoemaker-Levy 9.
The reason for such a big difference? Relative size.
When Shoemaker-Levy 9 hit Jupiter, the giant planet's gravity first ripped the comet apart into a total of 21 fragments. The largest fragment was still over 2 kilometres in diameter though, and its impact left an enormous swirling scar in Jupiter's atmosphere twice the size of Earth. However, finding impact scars from the smallest fragments, in the constantly swirling, churning clouds of Jupiter's upper atmosphere would not be easy.
Even with the less-than-dramatic effect on Jupiter's atmosphere, we have to think about how often Jupiter saves us from these objects, and what such an impact would have done to the Earth.
"It's kind of a scary proposition to see how often Jupiter gets hit," Hall said in an interview with NBC.
Due to its powerful gravitational pull, so close to the inner solar system, Jupiter has been called a "cosmic vacuum cleaner", as it is probably the planet that receives the most comet and meteor impacts in the solar system. It's possible that the astronomer Cassini recorded an impact in 1690. Voyager 1 recorded a hit as it passed by in March of 1979. Fragments of Shoemaker-Levy 9 impacted over a 6-day period in July of 1994. The latest hits were once in 2009, twice in 2010, and the latest one on Monday, and those are only the known hits, as we aren't watching Jupiter all the time.
Earth gets its fair-share of meteorite impacts, as Jupiter can't catch everything that comes out way, and there are still plenty of 'near-Earth objects' floating around out in space. Most of these meteorites are quite small and burn up on entering our atmosphere. However, there is evidence of some powerful strikes in our past.
Barringer Crater, in Arizona, is an excellent example. Officially named 'Meteor Crater' by the U.S. Government — based on the nearest US post office, 'Meteor' — it is 1,200 meters wide and 170 meters deep. It was formed around 50,000 years ago when a 50 meter wide iron asteroid slammed into ground at over 38,000 kilometers per hour, causing an explosion estimated at being the equivalent of a 10 megaton bomb.
With speculation that this impact on Jupiter was a small undetected comet, using the Imperial College of London's Earth Impact Effects Program and entering in a diameter of 50 metres — the estimated size of the smallest fragments of Shoemaker-Levy 9 — if this object hit the Earth, it would have been roughly the equivalent of a setting off a 20 megaton bomb, which could level a small city.