by Patricia Jeffery © 2022, Calgary Centre of the Royal Astronomical Society of Canada
On March 24, 1993, American astronomers Eugene and Carolyn Shoemaker and Canadian David Levy were at the Palomar Observatory in California studying a telescope generated photograph in search of NEOs (Near Earth Objects). What they discovered was not an asteroid but a very strange comet. They named it Shoemaker-Levy 9 as it was the ninth comet that they had discovered with an orbit of 200 years or less.
SL9 wasn’t orbiting the Sun, it was orbiting Jupiter and instead of one nucleus, it seemed to have several of them. Only after reviewing older photos did the astronomers learn that their new comet had been kidnapped by Jupiter 20 to 30 years earlier when it had ventured too close to the massive planet while on its way toward the Sun. SL9’s orbit was egg-shaped, bringing it very near to Jupiter at times.
On July 7, 1992, as it passed just 40,000 kilometres above its captor’s cloud tops, Jupiter’s powerful gravitational forces tore the comet apart. In a dangerously unstable orbit now, the string of 23 fragments, ranging in diameter from a few 100 metres to 2 kilometres, eventually collided one after the other with Jupiter’s southern hemisphere between July 16 and 22, 1994, at a speed of approximately 216,000 km/hr.
SL9’s demise highlighted Jupiter’s role in protecting the inner planets from space debris by acting as a ‘cosmic vacuum cleaner.’ Earth’s collision with a city-sized asteroid 65 million years ago resulted in the extinction of the dinosaurs plus 75% of mammals larger than a rat and half of all plant species. Astronomers speculate that without Jupiter’s help, extinction events might’ve been more frequent on Earth and complex life might never have been able to develop at all.
On July 19, 2009, exactly 15 years after the SL9 impacts, a new spot the size of the Pacific Ocean appeared in Jupiter’s southern hemisphere. Scientists believe yet another comet or asteroid had fallen victim to Jupiter’s powerful suction.
