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In July of 1994, scientists around the world watched in amazement as the comet Shoemaker-Levy
9 smashed into Jupiter. The impact’s blast was so powerful that it unleashed a force equivalent to
300 million atom bombs. For six days, Jupiter was throttled by 21 separate impacts from the
comet’s fragments, which produced giant plumes of debris that rose 3,000 kilometres above the
cloud tops – an impressive feat considering Jupiter’s immense gravity - and heated Jupiter’s
atmosphere to temperatures reaching 30,000 degrees Celsius. At the moment of impact,
the comet was travelling at a blistering speed of 216,000 kilometres per hour, with its largest
fragment spanning 2 kilometres in diameter. The impact raised huge clouds of debris that were
visible for months and left a scar in Jupiter’s atmosphere more prominent than its Great Red Spot.
Now, collisions of this magnitude aren’t entirely unheard of. Our Solar System is
littered with evidence of major impacts from comets and asteroids. Scientists
believe Earth was hit by a massive asteroid at the end of the Cretaceous Period, which
likely led to the extinction of the non-avian dinosaurs. But these events are extremely rare,
meaning the chance to see one in action is a once-in-a-lifetime opportunity. So, what did it
look like? And does the incident shed light on the odds of a similar event happening here on Earth?
I’m Alex McColgan, and you are watching Astrum. Join me today as we relive the biggest planetary
explosion ever witnessed from space – and unpack what it taught us about Jupiter and planetary
collisions. In 1993, astronomers Carolyn and Eugene Shoemaker and David Levy were conducting
research at California’s Palomar Observatory when they discovered a periodic comet that had
been captured by Jupiter’s gravitational pull. (“Periodic” means the comet has an orbital period
of fewer than 200 years.) This was unusual, as most comets in our Solar System orbit the Sun.
However, Jupiter is so massive, being the largest of the eight planets by far, that
its ability to capture other objects approaching its orbit isn’t surprising. A lot of Jupiter’s
irregular moons are likely captured asteroids and comets that have since burned off their volatile
material on their surfaces. But this comet also had other unusual characteristics. For one, it was
big. So big that scientists think the frequency of similar impacts is a one-in-6,000-year occurrence.
But the comet was also fragmented, most likely torn apart by Jupiter’s tidal
forces on a previous approach. Most striking of all, however, was its highly eccentric orbit.
Eccentricity measures the deviation of an orbit from a circle, with zero being the value of a
perfect circle and one being the upper limit when an elliptical orbit becomes hyperbolic.
Shoemaker-Levy 9’s orbit had an eccentricity of over .998: in other words, extremely eccentric.
Almost immediately, astronomers realized there was a possibility the comet could collide
with Jupiter, but their suspicion turned into certainty once they collected more precise data.
Before long, astronomers knew the impact would occur sometime in July 1994, and pretty soon the
whole world was waiting for the event with bated breath. Anticipating SL9’s impact, astronomers
monitored its movements from the Keck Observatory, Germany’s ROSAT X-ray telescope and NASA’s Hubble
Space Telescope among other instruments. But when the first of the comet’s fragments hit on July 16,
1994, the worst case scenario occurred, it looked like we would miss the spectacle!
You see, SL9’s trajectory meant the impact would occur on the side of Jupiter facing away from us.
That meant none of Earth’s high-powered telescopes were in position to view the initial impact.
For scientists, this would have been a crushing disappointment. But as luck would have it,
not all our cameras were located here on Earth. By sheer chance, NASA’s Galileo spacecraft,
launched in 1989, was only one year out from Jupiter at the time of SL9’s final approach.
It just so happened to be in perfect position to record the impact as it happened.
But Galileo wasn’t our only helper from afar. The Ulysses spacecraft,
which had been launched in 1990 to monitor the Sun, was also pointed at Jupiter.
And even NASA’s Voyager 2, located 44 Astronomical Units away (or 6.6 billion kilometers), was
programmed to monitor radio emissions from the crash site with its ultraviolet spectrometer. Each
probe paused its own missions to work together on this to help us witness an extraordinary event.
Shortly after Fragment A impacted Jupiter, Galileo saw a massive fireball erupt,
reaching temperatures as high as 24,000 degrees Celsius. Its plume quickly rose 3,000 kilometres,
which would make it as big as Australia from north to south! This was surprising, as scientists
hadn’t expected to see fireballs in the aftermath of the collision. A few minutes later, masses of
ejected debris plummeted back toward Jupiter’s surface and burned up, again turning Jupiter’s
atmosphere into a raging furnace. Before long, Jupiter’s rotation brought the impact site into
view of Earth, allowing high-powered telescopes like Hubble to view a huge dark spot on Jupiter.
(As it happens, Jupiter’s rotation is fast, with days that last only 10 hours. Contrary
to what you might think, larger planets tend to have shorter days than smaller ones!)
The comet’s impact set off shockwaves, which rippled across Jupiter’s dense atmosphere at
a speed of 450 meters per second. And all this was just from the first impact. For six days,
between July 16th and July 22nd, the comet’s fragments bombarded Jupiter, the largest coming
on July 18th, when Fragment G hit. Its impact alone produced a blast 600 times more powerful
than the world’s entire nuclear arsenal, leaving a huge dark spot one Earth diameter across.
However, as spectacular as the initial impact was, the comet’s aftermath proved just as valuable. By
studying the clouds of debris, scientists gained an unprecedented window into Jupiter’s atmosphere
and its movements. In addition, they caught a never-before-seen glimpse of Jupiter’s composition
beneath its dense cloud tops, as spectroscopic readings were able to identify material that had
been splashed upward by the comet’s impact. They detected diatomic sulfur and carbon disulfide, and
heavy elements like silicon, iron and magnesium. Interestingly, they also detected substantial
amounts of water, something they weren’t necessarily expecting. In fact, one of NASA’s Juno
probe’s primary objectives is to locate where this water is hiding in Jupiter’s atmosphere. However,
one of the more disturbing implications of the impact was the realization that large celestial
bodies could still hit planets. One school of thought theorized that major comet and asteroid
collisions had been a lot more frequent earlier in the Solar System’s existence. But Shoemaker-Levy
9 made it clear that very destructive collisions were still possible. Had it happened by chance,
and we witnessed an extremely rare event? Or does it happen more than we thought? Remember,
we’ve only had the technology to see this kind of event within the last 80 or so years.
If a comet as large as SL9 were to crash here on Earth, it would lead to the extinction of
most life on the planet. This had a dramatic effect on our collective psyche, as anyone who
lived through the 1990s can attest! It was also a wakeup call for NASA and for various defence
agencies. Before SL9, the term “planetary defence” didn’t exist. But in its wake, NASA took up the
mission of monitoring Near Earth Objects, or NEOs, with the goal of identifying upwards of 90% of
asteroids in our celestial neighbourhood greater than 1 kilometre in diameter. Having achieved
this goal, NASA is now well on its way toward identifying asteroids greater than 140 meters.
But before you stay up all night worrying, be aware that these events are undoubtedly rare.
And there is, perhaps, one other silver lining to SL9’s impact... You see, Jupiter is a massive
planet with a powerful gravitational influence, and since it is also one of the outer planets,
some scientists now think it might act as a cosmic “vacuum cleaner” of sorts.
We know that Jupiter gets approximately 2,000 to 8,000 times as many cometary impacts as Earth.
So, perhaps one of the reasons extinction-level impacts are so uncommon here on Earth is that
Jupiter has been a magnet for these kinds of comets and asteroids. This argument has even
become part of the Rare Earth Hypothesis, which suggests that Earth is host to a unique set
of conditions, without which the evolution of complex life would be impossible. Not everyone
agrees with this hypothesis though, and in any event, we’re still a long way from proving it.
So, while we may not know the exact likelihood of a massive comet or asteroid hitting the Earth, the
impact of SL9 with Jupiter has certainly advanced our understanding of these events. Moreover, it
was, without question, a spectacular moment that treated watchers to one of the most impressive
action scenes ever witnessed by human eyes. Maybe one day we’ll have the chance to see something
bigger, but hopefully not from too close! While there have been other explosive events, like the
2022 Tonga volcanic eruption, for now, the winner is clear: the biggest explosion ever seen on a
planet is Shoemaker-Levy 9. And by comparison, the competition looks like a drop in the bucket.
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Thanks for watching! If you enjoyed this video, check out my other comet
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