Thirty years ago a comet exploded on the surface of Jupiter , some 40,000 kilometers above its highest cloud cover. However, we could not observe this directly, since the comet would not be discovered until eight months later, when the marriage of American astronomers Shoemaker and his partner Levy observed a peculiar object in the vicinity of Jupiter.
This happened on March 24, 1993. They observed a comet that seemed to have a very elongated shape and was very close to Jupiter. As the days passed, it was discovered that the comet orbited the gas giant , becoming the first comet discovered that did not directly orbit the Sun. It was also observed that its elongated shape was due to the fact that it actually consisted of a series of fragments that followed exactly the same orbit . Studying this orbit in detail revealed several interesting facts.
The comet would have been captured by Jupiter in the late 1960s or early 1970s, when it was at the aphelion of its orbit around the Sun (moment of maximum distance) and having a very low speed with respect to the giant planet. This led the comet, called Shoemaker-Levy 9 , as it was the ninth discovered by this team, to describe a very elliptical orbit around Jupiter , which took it from a perihelion of barely a hundred thousand kilometers on Jupiter to an aphelion of almost fifty million kilometers. Given this huge orbit, it took the comet about two years to complete one revolution around the planet.
This highly elliptical orbit brought Shoemaker-Levy 9 to closest approach to Jupiter in July 1992 , when it came just 40,000 kilometers above its atmosphere. This distance, less than the radius of the planet and its Roche limit , fragmented the comet into at least 21 pieces . The Roche limit is the distance to a planet, star or astronomical body in general, at which it is able to overcome the forces that hold together a smaller object that orbits it. That is, at this distance, Jupiter’s gravity was strong enough to overcome the comet’s own internal gravity .
The 21 pieces followed their orbit without excessive disturbance and were detected in March 1993 . But in studying the comet’s orbit we were not only able to learn about its past, realizing that it would collide with Jupiter just 16 months after being discovered, in July 1994. The prediction specifically said that Shoemaker-Levy 9 would pass about 45,000 kilometers from the center of Jupiter on its approach in July 1994, Jupiter’s radius being almost 70,000 kilometers. The day did indeed come and we were able to observe how these 21 fragments, which had sizes between 300 meters and just over a kilometer , fell into the Jovian atmosphere one by one, over five days.
Given the unusual nature of the occasion, since it was the first time that we would see two bodies in the solar system collide, a multitude of observatories and telescopes were directed towards Jupiter . The ROSAT space telescope, the Keck observatory in Hawaii, the Hubble telescope, or the Galileo probe, which was on its way to Jupiter, formed the main body of observation. Also the Ulysses probe, which was designed to observe the Sun, or the Voyager 2 probe, which was beyond the orbit of Neptune, turned their instruments to observe the impact.
As night fell over the Iberian Peninsula on July 16, 1994 , the first of 21 fragments reached the upper layers of Jupiter, burning up in a brilliant fireball and leaving behind scars in the Jovian atmosphere . These fragments were named with letters, from A to W, with fragments G, K and L creating the greatest disturbance. After each impact, dark spots formed on the clouds , the three largest reaching sizes of about 12,000 kilometers in diameter , comparable to the size of our own planet. The energy released in the largest of these impacts, that of fragment G, is estimated to be several million megatons of TNT, hundreds of times more powerful than the entire existing nuclear arsenal today.
These collisions disturbed Jupiter’s atmosphere , bringing large amounts of sulfur compounds to the surface, creating pressure waves that propagated at about 450 meters per second and creating fireballs that reached 24,000 ºC in temperature. The marks left by the successive impacts lasted for months and some of them were more easily distinguishable than even the Great Red Spot, a giant storm that was observed four centuries ago by Galileo Galilei.
This event put the focus on the importance of Jupiter as a “space vacuum cleaner” , deflecting and attracting multitudes of asteroids and comets on their way to the inner solar system and, specifically, to Earth. Jupiter, due to its great mass (greater than the sum of the rest of the planets) receives at least 2,000 times more impacts per year than the Earth . In addition, it is capable of fragmenting objects that pass excessively close to its surface, as it did with Shoemaker-Levy 9. Evidence has been found of past events on the surface of Callisto and Ganymede , which show chains of craters probably formed after the collision of fragmented comets.
References:
Comet Shoemaker–Levy 9 Collision with Jupiter, 2005, National Space Science Data Center, NASA
JC Solem, 1994, Density and size of Comet Shoemaker–Levy 9 deduced from a tidal breakup model, Nature. 370, doi:10.1038/370349a0
