Comet Shoemaker-Levy 9: The backstory and its impact

Over six dramatic days in July 1994, a shimmering practice of ice-encrusted particles slammed into Jupiter, the solar system’s largest and most large planet. Like a celestial string of opalescent pearls, they impacted its swirling environment with the explosive equivalence of 10 million megatons of TNT, hundreds of occasions extra energetically potent than Earth’s total nuclear arsenal.

The direct hit on Jupiter by Comet Shoemaker-Levy 9 that summer time sat on a harmful par with the Cretaceous-Paleogene extinction occasion that decimated three-quarters of earthly life, together with all nonavian dinosaurs, 66 million years in the past. And humanity was afforded a ringside seat as telescopes, spacecraft, astronomers, and a 1.6-million-strong on-line viewers watched awestruck as nature did its ferociously brutal work.

A 12 months earlier, the comet (and its intriguing backstory) had been completely unknown.

Recognizing Comet Shoemaker-Levy 9

On March 24, 1993, husband-and-wife skywatchers Eugene and Carolyn Shoemaker and fellow astronomer David Levy had been at California’s Palomar Observatory, in search of deep-space objects that may pose collisional dangers to Earth. As the nice and cozy spring day surrendered to a light dusk in San Diego County, the observatory’s growing older Schmidt telescope digicam whirred and clanked obligingly into motion.

Throughout six many years of service, this 18-inch (46 centimeters) instrument helped uncover 100-plus supernova remnants, increasing astronomers’ information of the stellar evolutionary lifecycle from cradle to grave. It additionally facilitated the excellent survey, images, and mapping of a whole bunch of asteroids and dozens of comets. But when the Shoemakers and Levy sought potential Earth impactors that evening, a shock lay in retailer.

The 13.8-magnitude comet they discovered was uncommon within the excessive. A squashed blob, it exhibited “a dense, linear bar … oriented roughly east-west; no central condensation was observable,” they wrote, “however a fainter, wispy ‘tail’ prolonged north of the bar and to the west.” Because the ninth periodic comet discovered by the trio, it earned the moniker Shoemaker-Levy 9 and entered the Worldwide Astronomical Union (IAU) Round 5725 on March 26.

A necklace of objects

By then, it had prompted fairly a stir. Observations by Jim Scotti at Kitt Peak, close to Tucson, Arizona, revealed an extended, slim practice of objects, shattered into fragments with not less than 5 discernible groupings that brightened close to their southwestern finish. Forty-seven arcseconds lengthy — about 710,000 miles (1.1 million kilometers) or thrice the Earth-Moon distance — it was a thousand occasions too faint to be seen with the bare eye.

And when quite a lot of precovery observations from the mid-March timeframe additionally got here to gentle, IAU Central Bureau for Astronomical Telegrams Director Brian Marsden mapped Shoemaker-Levy 9’s orbital parameters, revealing its tantalisingly shut proximity to Jupiter. “The comet is positioned some 4 levels from Jupiter,” he wrote, “and the movement suggests it could be close to Jupiter’s distance.”

As these parameters firmed-up with larger confidence, it turned obvious that Shoemaker-Levy 9 occupied a unfastened, extremely eccentric orbit, circling Jupiter each two years. It doubtless originated as a short-period comet orbiting from barely inside of the asteroid belt at perihelion to only inside Jupiter’s orbit at aphelion.

An oddball orbit

Nudged out of solar orbit and into the jovian sphere of affect between the mid-Sixties and early Seventies — astronomers Ed Bowell and Lawrence Wasserman calculated a date round 1966 — this Jupiter-grazing trajectory set it on an irreversible path towards its doom.

Over the succeeding years, Shoemaker-Levy 9 made a number of approaches to its enormous host, the closest and most catastrophic in July 1992, when it swept 25,000 miles (40,000 km) above Jupiter’s cloud-tops.

Passing inside of the orbit of the innermost jovian moon, Metis, that shut shave sounded the comet’s death-knell. For it additionally carved a fateful path deep inside Jupiter’s Roche Restrict — the world inside which the planet’s tidal forces are sufficiently sturdy to disrupt and break aside smaller celestial interlopers held collectively by gravity alone.

Between 1.5 and a couple of.2 hours after its shut graze, the comet fractured into not less than 21 items, which after its March 1993 discovery had been named A by means of W, excluding I and O, with Q —close to the middle — recognized because the brightest and certain the largest. However actual numbers proved unknowable, for imaging limitations and the fragments’ obscurement by dust made it tough to resolve tinier chunks or confirm if bigger ones had been really multiples.

The approaching collision of Comet Shoemaker-Levy 9

In Might 1993, Marsden predicted the comet’s future trajectory would carry it inside 28,000 miles (45,000 km) of Jupiter’s middle, a distance smaller than the planet’s radius. And from that prediction sprang a single, indeniable conclusion: Shoemaker-Levy 9 would hit Jupiter in July 1994 — the primary time humanity had predicted a collision between two solar system objects various minutes upfront.

The affect would shed new gentle on each planet and comet. It will inject matter from the deep jovian inside into the higher environment, furnishing spectroscopic insights of Jupiter’s unexplored depths and yielding clues about cometary dynamics and evolution. Some gaseous species may condense into clouds, their spreading traits churning up volatiles like water, ammonia, and hydrogen sulphide.

Understanding the longevity of Jupiter’s centuries-old storms, spots, and eddies drew acute curiosity. Some argued the affect may besmirch its environment for months, maybe creating a brand new storm as much as 1,500 miles (2,400 km) broad. Others countered that the comet’s diminutive measurement and density would impart few long-term results.

Figuring out that measurement and density would allow astronomers to pin down how a lot punch Shoemaker-Levy 9 may pack. Computational dynamics, reflectivity traits and the comet’s noticed habits implied a density of 0.018 kilos per cubic inch (0.5 grams per cubic cm) — much less dense than water. However hypothesis that the fragments had been smaller than 1,600 toes (500 meters) was quickly known as into query.

Information from the Hubble House Telescope in July 1993 prompt fragment sizes of 1.8 miles to 2.5 miles (2.9 to 4 km) that permitted an extrapolation of the comet’s pre-breakup dimensions of as much as 7 miles (10 km). Later Hubble pictures refined these fragment-size limits downward to the order of 0.6 miles (1 km) or much less.

With Jupiter some 534 million miles (860 million km) from Earth on the level of affect, the consequences of the collision would show arduous to see. However one factor was sure: large Jupiter was in for a beating.

Capturing the crash

Approaching from the south at 130,000 mph (210,000 km/h) and colliding south of the planet’s South South Temperate Belt at 45 levels South latitude, all Shoemaker-Levy 9’s fragments had been predicted (with 99.9-percent certainty) to satisfy their finish on the jovian farside, agonisingly out of view of earthly telescopes.

However fast-spinning Jupiter revolves on its axis each 9.5 hours, which means the carnage would rotate shortly into view minutes after every collision. As ground-based skywatchers hoped to see affect flashes mirrored off the planet’s innermost moons, Hubble intently watched the comet and Jupiter’s environment earlier than and after the occasion.

Elsewhere, the Galileo spacecraft — en path to a December 1995 arrival at Jupiter — had direct visibility of these affect flashes from a distance of 150 million miles (240 million km). The Solar-circling Ulysses probe, 240 million miles (390 million km) south of Jupiter, additionally had line-of-sight visibility for radio observations. And Voyager 2, some 4.1 billion miles (6.6 billion km) from the motion, would nonetheless have the ability to watch from afar.

The large crash

Late on July 16, 1994, Fragment A penetrated the Jovian clouds at 35 miles (60 km) per second. Moments later, Galileo detected a fireball that peaked at 42,700 levels Fahrenheit (23,700 levels Celsius) and Hubble witnessed a mushroomlike plume rising 1,900 miles (3,000 km).

On Earth, keen-eyed observers witnessed the fireball at Jupiter’s limb simply after the collision, nixing predictions that astronomers could be unable to see them. A darkish, spotlike characteristic shortly shaped on the affect website.

By far the largest-yield collision was Fragment G, whose July 18 affect carried a possible of 6 megatons of TNT, 600 occasions stronger than all humanity’s nuclear munitions. It left a darkish spot 7,500 miles (12,000 km) throughout, virtually as broad as Earth. Two impacts a half-day aside on the nineteenth additionally left scars and the final — Fragment W — ended the onslaught on July 22.

Planet-wide results had been profound, from intense auroral exercise within the northern hemisphere to colossal waves that swept Jupiter at 1,000 mph (1,600 km/h) for 2 hours after the larger impacts. Elevated temperatures oddly dropped to regular ranges extra shortly at bigger affect websites than smaller ones. And international stratospheric temperatures returned to regular inside two to 3 weeks.

Shoemaker-Levy 9 revealed clues to Jupiter’s inside. Ammonia and hydrogen sulphide had been present in larger portions than could be anticipated from a comet. Vital quantities of water had been detected spectroscopically and ammonia and carbon disulphide remained within the jovian stratosphere — increased than their typical location within the troposphere — over 14 months after the affect.

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Influence dynamics confirmed Shoemaker-Levy 9’s structural power was fairly low, implying the unique comet was just one.1 miles (1.8 km) throughout. Hubble revealed its fragments had been strong, large objects. Astronomers at Mauna Kea, Hawaii, recognized carbon monoxide, sodium, magnesium, calcium, and manganese — all affording proof concerning the chemical make-up of this historical physique.

Absorbing asteroids

In the meantime, mighty Jupiter weathered the storm. World maps from Hubble in August 1994 confirmed easterly and westerly atmospheric jets steadily softening the darkish spots’ edges. Influence websites remained however scarring slowly receded. But for months, they remained extra simply seen even than the well-known Nice Pink Spot. At this time, Shoemaker-Levy 9 stays essentially the most outstanding transient characteristic ever seen on the planet.   

It was neither the primary nor final time Jupiter was thus brutalised. In 2009, a Pacific-sized darkish spot in its southern hemisphere appeared, doubtless as a consequence of an asteroid twice as large and hundreds of occasions extra highly effective than the Tunguska impactor which hit Siberia in 1908. Extra just lately, the Jupiter-orbiting Juno probe witnessed an affect fireball in 2020 and newbie skywatchers captured one other on video in 2023.

Just like the biblical David, Shoemaker-Levy 9 struck Jupiter a glancing, punishing blow over these six dramatic days of July 1994. However Jupiter’s Goliath has lengthy been a benevolent large: For eons, its measurement, mass, and immense gravity have drawn and consumed comets and asteroids. In so doing, it has eliminated hazardous impactors that may have savagely pummelled the inside solar system and eradicated the slimmest probability of advanced life ever arising on Earth.