One of the scariest and most magnificent sights of recent years was the entry and detonation of
the Chelyabinsk superbolide over the Southern Ural Region of Russia on February, 15th, 2013.
The word bolide is not well defined, it can have different contextual meanings,
but in short is basically a large, bright meteor that explodes, which can imply
an object that may drop meteorites to the ground. A superbolide is a bigger version,
typically defined to exceed 100 times the brightness of the full moon. The Chelyabinsk
superbolide in addition to causing at least 1500 injuries, broken glass and thousands of damaged
buildings in the region - a problem for that part of Russia in deep Winter, also dropped meteorites.
The meteorite is classed as a common LL5 type of chondrite. This would be typical
material you might find on the inner solar system’s asteroids, and goes to show that
some of the most common asteroid materials can pose an especially dangerous threat,
even if buildings and people aren’t physically hit by the
falling meteorites. Chelyabinsk began as a roughly 20 meter wide near-earth asteroid,
albeit a small one. It entered the atmosphere at a brisk 42,690 miles per hour, or about
69,000 kilometers per hour. It briefly outshone the sun, and coming in at a relatively shallow
angle it ultimately exploded in a meteor airburst leaving behind a hot cloud of dust and gas.
The earth’s atmosphere absorbed most of the energy, releasing a shock wave on the order
of 500 kilotons yield, or around 30 times the atomic bomb detonation at hiroshima.
This asteroid was not detected before it entered earth’s atmosphere, but not because we couldn’t,
but partly because it was coming roughly from the direction of the sun. Early on there was
a suggestion that the meteor might have been connected to a close approach of another known
asteroid 16 hours after, but this is not the case and the two showed very different trajectories.
But it is known that the impactor is associated with the Apollo group of near-earth asteroids.
The Apollo group are a type of near-earth asteroid that cross Earth’s orbit. They are
the largest class of near-earth asteroids, with about 10,485 currently known. Most of the time
they don’t pose an immediate or even long term threat, they’ve been there a long while, with
only 1,648 identified as potentially hazardous. Chelyabinsk was not previously known, but was
clearly hazardous. This drives home the problem of even small impacts over land on this planet.
As an aside, Chelyabinsk also highlights the impacts that occur to asteroids themselves.
Fragments of the meteorite were collected by people very soon after the impact,
and indeed one chunk made a huge hole in a frozen over lake. Scientists were able
to determine the isotopic changes and shock and determined that since its formation the
asteroid had been impacted by at least 8 major collisions with other asteroids,
some early in its history, but even one as late as 27 million years ago.
But impacts of this class on earth are not rare, they happen relatively frequently.
But most of the time, they occur over the oceans, which form a majority of earth’s
surface area. Russia however forms a very large portion of earth’s land,
which is why it sees impact events more frequently than other parts of the world.
Another such impact occurred in 1947, but it was of a very different nature than Chelyabinsk.
On February 12, 1947, again with February, an enormous meteorite entered earth’s atmosphere
and fell in the Sikhote-Alin mountains in Southeastern Russia. This was not a
stone meteorite, but an iron one. Iron meteorites are rare in the solar system,
only making up around 5 percent of asteroids, but they’re also sturdy and tend to more easily
make it to the ground in falls. Here about 23 tons of material survived to the ground,
and once again eyewitnesses report it to have outshone the sun. It came in at a steeper angle,
about 41 degrees, and began to disintegrate early on, but did
not detonate until it went off like a grenade air burst spewing metal across a mountain side.
This created two different types of meteorite fragments, ones that are
individually fusion crusted and look like individual meteorite falls, but aren’t,
they are fragments where their exterior melted high in the atmosphere. The other
type is shrapnel from the airburst and exhibits torn and twisted forms, sometimes wildly so. The
fall left a smoke trail in the atmosphere that was visible for several hours, and the fragments
buried themselves up to 20 feet deep, while others embedded themselves into trees. Had
this one fallen over a densely inhabited area, it would have been devastating, and probably deadly.
This asteroid probably had its origins in the asteroid belt, and was probably knocked our
way in a collision, and the iron asteroid before entry was something around 100,000
kilograms. Fragments of this meteorite were large enough to create impact craters,
the largest about 26 meters across. As an iron meteorite, it’s of fairly mundane composition,
containing mostly iron and nickel, but with rarer metals such as traces of Germanium and iridium.
Going back further in time however, we come across the largest recorded meteor event in history,
though there are indications of others in unrecorded human history. This one was the
famous Tunguska event of June 30, 1908. Much has been written about this event,
and to this day explanations for it can get pretty far out there, such as a nuclear explosion or an
exploding UFO. There’s no reason to go that far however, it was a natural event, and it was bad.
Tunguska happened once again over a very sparsely populated area in Siberia and may even have caused
at least three deaths. It happened in the morning, and the culprit in a modern context
was very likely a chondritic asteroid, something perhaps like Chelyabinsk. The asteroid was about
50 to 60 meters in diameter, and came in from the south east at an estimated 27 kilometers a second,
or about 60,000 miles per hour. It exploded at altitude, but relatively
close at 5 to 10 kilometers, and laid waste to forestland of over 2000 square kilometers.
This devastating blast, which famously knocked down trees wholesale like a volcanic explosion,
came in at an estimated 12 megatons yield. There have been nuclear tests on earth that yielded
larger, but this would be among the top ten if it were nuclear. But given that it was an airburst,
no crater has ever been discovered. One long standing hypothesis about
the impactor was that it was a comet, and might explain atmospheric disturbances in
the days after the impact. However samples of what seems to be the vaporized meteorite,
and data from the Chelyabinsk fall, point more to a stony or even a high iron meteorite.
No parent body has been identified by the tunguska event, and it was so long ago and
under only visual observation it’s hard to track down. But there is one odd possibility here. There
is an annual meteor shower known as the Beta taurids. This is not typically a meteor shower
that can be seen in the daytime at its peak, as its peak occurs after sunrise. But at peak they
can be observed using radar. They run from June 5 to July 18, Tunguska was June 30, so right in
this time period, and came from the same area of the sky as this meteoroid stream’s radiant.
This is actually the same meteor stream as the more easily viewed Taurid meteor shower
in October and there are several potential asteroids associated with the stream. And
this stream is known to produce periodic meteor storms, though that’s not always guaranteed,
and there have been hypotheses that this stream might be associated with larger materials rather
than the dust that usually comprises most meteor showers. The next chance for a storm,
and perhaps something larger, is slated for June of 2036.
But it’s not likely that there will be another Tunguska level event, at least for a long while,
as it is calculated to be a once in a millennium event at the size it was.
Even still, if Tunguska happened today over a major city, the effects would be similar
to a very large hydrogen bomb detoning over a populated area. The devastation would be
almost total at ground zero under the airburst, worse outside that, and then progressively less
the further out. Loss of life and injuries would be comparable, and in an airburst,
depending on the composition of the impactor it may rain tiny droplets of vaporized iron
over a relatively large area, along with lasting atmospheric effects that might be seen for days
from particulate matter in the atmosphere. But it’s also worth noting that to a cretaceous
period dinosaur, Tunguska was nothing. Little is known about the nature of the dinosaur killer
other than it was either a comet or an asteroid, and conflicting evidence so far has prevented a
full identification. But we do know it left a crater in the Yucatan Peninsula in Mexico,
and the impactor must have been on the order of 100 million megatons,
leaving an iridium rich layer known as the K-T boundary, where the iridium is out of place due
to its rarity in the earth’s crust, it’s a siderophile element that sunk down into the
core with iron when the planet differentiated. What’s interesting here is that this impact and
the subsequent extinction of the dinosaurs was initially met with significant skepticism
within the scientific community, and even with the mountain of evidence for an impact including the
smoking gun and its aftermath, there are still some holdouts against the impact hypothesis.
This goes to show that whenever a suggestion of magnitude is put forth, there is usually a
large pushback against it, until the evidence becomes overwhelming. You can expect this to
be the case for any technosignature or biosignature put forth in astrobiology,
and indeed that’s happened, few things more controversial in science of late has been the
nature of the interstellar asteroid oumuamua. But with the end cretaceous impact event,
there is yet another, newer controversy. Known as the Tanis sight in North Dakota, it’s a part of
the greater Hell Creek Formation that has yielded a huge amount of significant fossil discoveries
from the upper cretaceous period. This particular site is just below the cretaceous-paleogene
boundary and It appears that this site preserves a snapshot of the exact moment,
starting within a few minutes to a few hours after the impact itself in very precise detail.
What seems to have happened here is that seismic waves from the impact arrived in the area about
10 minutes after it happened. This caused a series of waves in standing water, up to several hundred
feet high, that rapidly crashed onto the shores of an interior seaway. The waves hit with great
force in what was a riverway and creek complex and drove all sorts of animal and plant remains miles
inland, along with tektites and debris from the impact itself. The result are some weird fossils,
such as freshwater and salt water animals jumbled together in all orientations along with
microtektites actually in some of the gills of fossilized sturgeon, sucked in during their dying
gasps as it were. There’s even a turtle impaled on a stick. There are also microtektites identical
to those found around the Yucutan preserved in Amber in this formation. There are even
jumbles of feathers, thought to be from dinosaurs, along with severely mangled bones and even eggs.
There have even been reported fossil ant hills clogged with debris, and even the remains of some
early mammals inside burrows, and even mysterious pterosaurs known only from this site, fish in
a position associated with suffocation, with everything appearing to have died from the same
thing at the same time. The site is, of course, controversial, and many scientists are skeptical
and suggest the site needs more interpretation due to it being extraordinarily rare in the fossil
record where you can trace something to a single event like this, which is fair enough, but given
the evidence so far, if it’s not the impact, then some cause other than the asteroid will
have to be found to explain this weird site. But in all of this, it pays to remember that
even the dinosaur killer wasn’t the biggest this planet has ever seen. It’s seen many more
over its billions of years of history, and one very much larger impact with a
Mars sized protoplanet is the currently favored mechanism for the origin of the moon. And, it’s
overwhelmingly likely that this planet has not seen the end of asteroid and cometary impacts.
Thanks for listening! I am futurist and science fiction author John Michael Godier currently
eyeing the mammals suspiciously, including myself. We somehow survived the dinosaur
killer impact and then went on to rule the planet, with some pushback from the birds, after an early
history of being dinosaur food. It all seems too perfect and be sure to check out my books at your
favorite online book retailer and subscribe to my channels for regular, in-depth explorations into
the interesting, weird and unknown aspects of this amazing universe in which we live.