In July of 1994, scientists from around the globe observed in awe as the comet Shoemaker-Levy 9 collided with Jupiter. The force of the impact’s explosion was equivalent to three hundred million atomic bombs. Jupiter’s six days were consumed by 21 separate impacts from the comet’s fragments, which produced giant plumes of debris that rose 3,000 kilometers above the cloud tops—an impressive feat considering Jupiter’s immense gravity—and heated Jupiter’s atmosphere to 30,000 degrees Celsius. At the time of impact, the comet was moving at 216,000 kilometers per hour, and its largest fragment measured 2 kilometers in diameter. The impact generated enormous clouds of debris that were visible for months and left a scar in Jupiter’s atmosphere that was more prominent than the Great Red Spot. Now, collisions of this magnitude are not completely unprecedented. Our Solar System is littered with the remnants of comet and asteroid impacts. Scientists believe a massive asteroid struck Earth at the end of the Cretaceous Period, causing the extinction of nonavian dinosaurs.
However, these occurrences are so rare that the opportunity to witness one in action is once in a lifetime. So, what did it look like? And does the event shed light on the likelihood of a similar occurrence occurring on Earth? I am Alex McColgan, and you are currently viewing Astrum. Join me as we examine what we learned about Jupiter and planetary collisions from the largest planetary explosion ever observed from space. Carolyn and Eugene Shoemaker and David Levy were conducting research at the Palomar Observatory in California in 1993 when they discovered a periodic comet captured by Jupiter’s gravitational pull. (“periodic” indicates that the comet’s orbital period is less than 200 years.) The majority of comets in our solar system orbit the Sun, making this a rare occurrence. Jupiter is by far the largest of the eight planets, so it is not surprising that it has the ability to capture objects that approach its orbit. Many of Jupiter’s irregular moons are likely captured asteroids and comets whose surfaces have since lost their volatile material. However, this comet possessed additional peculiar characteristics. For one, it was enormous. It was so massive that scientists estimate that similar impacts occur once every 6,000 years. However, the comet was also fragmented, most likely due to an earlier encounter with Jupiter’s tidal forces. However, its highly eccentric orbit was the most notable feature. Eccentricity measures the deviation of an orbit from a circle, with zero representing a perfect circle and one representing the upper limit at which an elliptical orbit transforms into a hyperbolic orbit. In other words, the orbit of Shoemaker-Levy 9 was extremely eccentric, with an eccentricity of over 0.98.
Almost immediately, astronomers realized there was a chance the comet would collide with Jupiter, but their suspicion turned to certainty as they gathered more precise data. As soon as astronomers determined that the impact would occur in July 1994, the entire world began to eagerly anticipate the event. Astronomers monitored SL9’s movements with the Keck Observatory, Germany’s ROSAT X-ray telescope, and NASA’s Hubble Space Telescope, among other instruments, in anticipation of its impact. But when the first fragment of the comet struck on July 16, 1994, the worst-case scenario occurred, and it appeared we would miss the spectacle! The trajectory of SL9 indicated that the impact would occur on the Jupiter side facing away from us. This meant that none of Earth’s powerful telescopes were able to observe the initial impact. This would have been a devastating disappointment for scientists. Fortunately, however, not all of our cameras are located on Earth. The 1989-launched Galileo spacecraft was only one year away from Jupiter at the time of SL9’s final approach, purely by chance. It happened to be in the ideal position to capture the impact as it occurred. However, Galileo was not our only distant aid. The 1990-launched Ulysses spacecraft, which had been designed to monitor the Sun, was also aimed at Jupiter. And even NASA’s Voyager 2, located 44 AU (or 6.6 billion kilometers) away, was programmed to monitor radio emissions from the crash site using its ultraviolet spectrometer.
Each probe halted its own mission in order to assist us in witnessing an extraordinary event. Galileo witnessed a massive fireball erupt shortly after fragment A collided with Jupiter, with temperatures reaching 24,000 degrees Celsius. Its plume quickly rose 3,000 kilometers, covering the entire length of Australia from north to south! This was unexpected, as scientists had not anticipated seeing fireballs following the collision. A few minutes later, massive amounts of ejected debris returned to Jupiter’s surface and burned up, once again transforming Jupiter’s atmosphere into a raging inferno. Jupiter’s rotation quickly brought the impact site into view from Earth, allowing Hubble and other powerful telescopes to observe a massive dark spot on Jupiter. ( As it happens, Jupiter’s days are only 10 hours long, and its rotation is rapid. Contrary to popular belief, larger planets have shorter days than smaller planets. The impact of the comet generated shockwaves that rippled across Jupiter’s dense atmosphere at 450 meters per second. And this was only the initial impact. The comet’s fragments bombarded Jupiter for six days, from the 16th to the 22nd of July, with the largest impact occurring on the 18th, when Fragment G struck. Its impact alone generated a blast 600 times more powerful than the world’s entire nuclear arsenal, leaving a massive, one-diameter-wide dark spot. Nevertheless, as impressive as the initial impact was, the comet’s aftermath proved to be just as beneficial.
By studying Jupiter’s debris clouds, scientists have gained unprecedented insight into Jupiter’s atmosphere and its motions. In addition, spectroscopic readings provided a never-before-seen glimpse of Jupiter’s composition beneath its dense cloud tops, identifying material that had been splashed upward by the comet’s impact. They detected heavy elements such as silicon, iron, and magnesium in addition to diatomic sulfur and carbon disulfide. Unexpectedly, they also detected significant quantities of water, which was an intriguing discovery. In fact, one of the primary objectives of the Juno probe is to discover where this water is hidden in Jupiter’s atmosphere. However, one of the most unsettling consequences of the collision was the realization that large celestial bodies could still strike planets. According to one school of thought, comet and asteroid collisions were much more frequent in the early days of the Solar System. However, Shoemaker-Levy 9 demonstrated that extremely destructive collisions were still a possibility. Had it occurred by chance, and had we witnessed a remarkably rare occurrence? Or does it occur more frequently than expected? Remember that we have only had the technology to observe this type of event for the past 80 years or so. If a comet as large as SL9 collided with Earth, the vast majority of life on the planet would perish. Anyone who lived through the 1990s can attest to the profound impact this had on our collective psyche! It also served as a wake-up call for NASA and other defense agencies. Before SL9, there was no concept of “planetary defense.” Following that, NASA established the mission of monitoring Near Earth Objects, or NEOs, with the goal of identifying up to 90% of asteroids 1 km in diameter or larger in our neighborhood.
NASA is now well on its way to identifying asteroids greater than 140 meters in diameter, having achieved this objective. But before you spend the entire night fretting, understand that these occurrences are unquestionably uncommon. And perhaps there is one more positive aspect to SL9’s impact… Jupiter is a massive planet with a strong gravitational pull, and because it is also one of the outer planets, some scientists believe it may act as a sort of cosmic “vacuum cleaner.” We know that Jupiter experiences between 2,000 and 8,000 times more cometary impacts than Earth. Jupiter has acted as a magnet for these types of comets and asteroids, which may be one of the reasons why extinction-level impacts are so uncommon on Earth. This argument has even been incorporated into the Rare Earth Hypothesis, which suggests that complex life could not have evolved on Earth absent a unique set of conditions.
However, not everyone agrees with this hypothesis, and we still have a long way to go in proving it. So, while we may not know the exact probability of a massive comet or asteroid striking Earth, the collision of SL9 with Jupiter has undoubtedly improved our understanding of these occurrences. In addition, it was without a doubt one of the most spectacular action scenes ever witnessed by human eyes. Perhaps one day we will have the opportunity to observe something larger, but hopefully not up close! While there have been other explosive events, such as the 2022 Tonga volcanic eruption, Shoemaker-Levy 9 is the largest explosion ever observed on a planet. In contrast, the competition appears to be a drop in the bucket.
