Some of you may not be old enough to remember this (it's one of my very first memories from childhood), but over a decade ago, starting on July 16th through July 22nd of 1994, fragments from the comet Shoemaker-Levy 9 collided with the most gargantuan monster of our solar system -- the gas-giant, Jupiter. This was truly a remarkable event for astrophysicists and the layperson alike! It marked one of the very first collision between two objects in our solar system to EVER be observed directly!
The comet was named after its founders; Eugene Shoemaker, his wife Carol and David H. Levy, who initially discovered the comet on accident on March 24th of 1993 while they were looking for near-Earth objects. At the time of its discovery, the trio had already discovered eight periodic comets (comets of this designation have an orbital period of 200 years or less) and two non-periodic comets, but curiously, this particular comet was the first to have been discovered in orbit around a planetary body (instead of orbiting the sun directly).
It is believed that the comet was captured by the immense pull of the giant planet's gravity a few decades before its orbit passed within Jupiter's Roche radius (the distance an object can co-exist with another larger object without it being pulled apart due to tidal forces) and was disintegrated into smaller fragments. Some of the portions were quite large, ranging from a few kilometers across to 1.2 miles (2 km) in diameter -- suggesting that the original comet's nucleus may have been up to 3.1 miles (3.1 km) across before breaking apart. If so, the comet might have been slightly larger than Comet Hyakutake, which had its own close encounter with Earth earlier in 1996.
The fragments eventually collided with Jupiter over the span of a week, at speeds that exceeded 37 miles/ps (60 km/s) or around 134,000 mph (216,000 km/h). Astronomers from all over the planet aimed their telescopes to the sky to observe the event along with the Hubble Space Telescope, the Ulysses Spacecraft, and the ROSAT X-ray satellite, but the collision took place on the side of Jupiter that isn't visible on Earth. Thankfully, the famed Galileo spacecraft was in close enough proximity from Jupiter to competently capture the event as it unfolded. It was expected to arrive at the planet in 1995 and was already within 1.6 AU (Earth-Sun distances) of Jupiter at the time. So we received some truly incredible images of the collision as it took place before Jupiter's rapid rotation allowed terrestrial telescopes to get a glance at the collisions soon afterwards.
The first collision took place on July 16th when "fragment A" entered the southern hemisphere of the gas giant at about 60 kilometers per second, shooting a huge fireball 3,000 km high. Said fireball's temperature peaked at 24,000 Kelvin (for comparison, the temperatures in the Jovian atmosphere usually hang around 130 K).
The largest of the collisions came from fragment G, which came crashing into the Jovian atmosphere on July 18th -- subsequently releasing the energy equivalent of 6,000,000 megatons of TNT. The fallout was a HUGE dark spot that was over 12,000 km across (roughly one Earth radius in size). Some of the scars from the various impacts were visible for months and were much more prominent than Jupiter's famed "Great Red Spot," making this a truly once in a lifetime event. It also highlights the importance of the various programs that are in place to search for near-Earth objects that could pose a threat to Earth. Since most of these objects are traveling at very high speeds, even small rock fragments are capable of wreaking havoc on our planet. So.. thanks Jupiter?
Wonder what it would have been like to spend a day on Shoemaker-Levy 9? Check out our article.