Throughout our history, humanity’s threats have primarily come from terrestrial entities.
But in 1994, we witnessed something that shook us to our core. Comet Shoemaker-Levy 9
slammed into Jupiter’s surface, causing massive damage that still affects the planet to this day.
What if Jupiter’s gravity hadn’t caught the comet? What if it sailed past the gas
giant and collided with us instead? This is Unveiled, and today we’re
answering the extraordinary question: What if Comet Shoemaker-Levy 9 hit Earth instead?
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Spotted in March of 1993 by astronomers Eugene and Carolyn Shoemaker, and David Levy,
Shoemaker-Levy 9 was the first comet found orbiting Jupiter. By that point, it had already
been torn into 21 pieces and was near the end of a potentially decades-long orbit. It’s estimated
that the original intact comet had a nucleus of over one mile across. For perspective,
it’s thought that the asteroid that wiped out the dinosaurs was roughly 6.2 miles in diameter.
But the size of SL9 was more than large enough to cause massive damage. Indeed, we had front
row seats to watch it rain destruction down on Jupiter. Over the course of six days in July of
1994, astronomers watched in awe as pieces of SL9 slammed into the planet. This event shook up both
the scientific community, but also governments and the public, hammering home the possibility of
a world-ending event coming from the sky. It’s no coincidence that only a few years later, Hollywood
produced not one but two movies that dealt with that scenario—Armageddon and Deep Impact.
It’s no surprise that SL9 captured the imagination when you look at the effects it had,
and continues to have, on Jupiter. In total, there were 21 impacts on the planet’s surface,
with a combined force of six million megatons of TNT. To put that in perspective,
the entire world’s nuclear arsenal at its peak in 1974 had a potential yield of 25,000 megatons.
These impacts left huge bruises on the planet, including one the size of Earth. While these
scars gradually faded over time, their influence still affects Jupiter to this day.
During SL9’s bombardment of Jupiter, the Galileo probe was also on its way to the
planet and captured some truly stunning images of the destruction. However, once Galileo got closer
to Jupiter and began studying its atmosphere, we started to truly understand the extent of SL9’s
carnage. First in 1996, and then again in 2000, Galileo took images of a pair of superimposed
ripple patterns in Jupiter’s faint rings. Upon studying the pictures in 2011, researchers from
SETI and Cornell University determined that the massive impacts from SL9 had caused the
entire ring to tilt by over one mile. The comet itself never crossed through the ring. Instead,
the collisions alone shot enough cometary dust into the atmosphere to alter the ring permanently.
What’s more, in 2013, US and European scientists discovered that a whopping 95% of the water
found in Jupiter’s upper atmosphere came from SL9. So clearly, the comet has had
lasting effects on the planet. But what if Jupiter’s gravity hadn’t captured the comet,
and it instead headed our way? In short: it wouldn’t be good.
Astrophysicist and radio astronomer Gerrit L. Verschuur predicted that any impact from a comet
two thirds of a mile across would likely spell the end of civilization. And anything three miles
or larger would probably mean the end of our species. Given that an intact SL9 was about 1.1
miles across, its collision with Earth would be disastrous. The comet would be blown into pieces
on its initial strike, creating a gigantic fireball engulfing the surrounding area.
Mechanical engineer Michael Paine published a paper in 2004 exploring the environmental damage
from asteroid and comet impacts. According to him, a comet the size of SL9 would create
a massive fireball with a 150 mile radius, effectively incinerating anything in its path.
In addition, 300 mph winds would rip apart anything within a 110 mile distance. Trees
would be torn from their roots, and windows would shatter up to 250 miles away from the impact site.
Flaming debris would be spewed into the atmosphere, raining down countless small
comets that would burn up in the air. The immense heat would ignite forests and buildings up to 370
miles away. Earthquakes, tsunamis and hurricanes would follow, hitting the area within hours and
lasting months. That’s some truly apocalyptic stuff. But that wouldn’t be the end of it.
The dust, debris and smoke would plunge us into what’s known as an impact winter. The sun would
disappear for at least a month, with regional freezing lasting weeks. This could plunge
worldwide temperatures by more than 46 degrees Fahrenheit, disrupting things
on a global scale—you can say goodbye to summer that year. You can also say goodbye
to the environment. The explosion of the comet would send billions of tons of sulphur-laden
dust into our skies, while the heat of the impact would create nitrogen oxides;
together, they would generate a toxic acid rain that would destroy plant life.
Moreover, that acid rain would find its way into our oceans and lakes, decimating marine life.
This deadly downpour would last for months in the regions surrounding the comet impact.
But that wouldn’t be our only atmospheric concern.
Given a sizable enough comet, our ozone layer would be destroyed within days,
which would result in a greenhouse effect perhaps lasting for centuries.
Needless to say, this would put enormous pressure on our food production. We would likely see a
global crop failure and worldwide food supplies being disrupted for years to come. Food shortages,
coupled with staggering environmental and infrastructure damage, would likely mean the
end of civilization as we know it. Humanity would probably survive, but we would need to rebuild.
However, these doomsday scenarios are all based on a land impact. Water makes up 70% of the Earth’s
surface. What if the comet struck the ocean instead of land? Well, it wouldn’t be as bad.
In 2010, the late Elisabetta Pierazzo, a senior scientist at the Planetary Science Institute,
created a model studying an oceanic impact from an asteroid.
While it’s still not pretty, it certainly paints a better picture than a land collision.
If an asteroid about two thirds of a mile across smashed into the ocean, it would spew tons of
saltwater into our atmosphere, seriously damaging the ozone layer. This would send our surface
UV index skyrocketing to an unheard of 56. To put that in perspective, a UV index rating of 11 is
considered extremely dangerous. This incredibly high level of radiation would force people to
remain indoors during the day. While that would still have a negative effect on our world,
the kind of decimation that we would see from a land impact would largely be avoided.
But wait, what about those massive tsunamis from comet collisions that we see in movies?
Wouldn’t coasts be battered with those? Well, actually, no. Like tossing a rock into a lake,
an impact from a meteor or asteroid would only affect the immediate area surrounding it.
It would certainly send out large waves, but they would quickly lose energy. Tsunamis are only
created from giant shifts in the seabed, like from landslides or underwater earthquakes. So, we would
definitely be dodging a bit of a bullet if a comet struck the ocean, but it still is far from ideal.
That’s why, in the aftermath of SL9, governments and scientists began a concerted effort to
monitor our skies for any impending dangers. After witnessing the damage done to Jupiter,
the US Congress in 1998 mandated that NASA find at least 90% of near-Earth objects (or
NEOs) 0.62 miles or more in diameter. And, in 2011, NASA announced that it had done
just that. However, NEOs smaller than 0.62 miles still pose a danger. In 2005,
that search was refined to include NEOs larger than 459 feet and to find 90% of them by 2020.
NASA, unfortunately, wasn’t able to meet that goal. The search for objects that small requires
even more resources and coordination that NASA alone couldn’t handle. In 2016, the agency sought
to remedy that by establishing the Planetary Defense Coordination Office. Its goal is to
help centralize and coordinate the search for NEOs across different agencies and governments.
NASA currently estimates there are more than 878 NEOs 0.62 miles or more in size.
While they believe they’ve found approximately 90% of them, there are still a great deal of unknown,
smaller NEOs that could cause havoc on our planet if they collided with it. This is why scientists
still remain vigilant, watching the skies for any potential apocalyptic object that may strike.
And that’s what would happen if Comet Shoemaker-Levy 9 hit Earth instead.
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