- This video was sponsored by KiwiCo. More about them at the end of the show. On February 15th, 2013,
over Chelyabinsk Russia, an asteroid heavier than the Eiffel tower slammed into the atmosphere. And then 30 kilometers above
the ground, it exploded. This violent event was
brighter than the sun, but so high up that it was silent, for a full 90 seconds after the blast, which only made the devastation worse. - So you see all these videos, of people, "Look, oh, what was that?" They see the smoke trail in the sky, "Oh, that's amazing!" And then, you know, just when you think
nothing's gonna happen, the shockwave hits and
it blows out the windows, thousand people got glass
in their face and their eyes because they're looking
through the windows. - The shockwave damaged
thousands of buildings and injured 1500 people. What makes the Chelyabinsk
incident kind of embarrassing is that the very same day, scientists had predicted
that an asteroid would make a close fly-by of earth. And they were right. 16 hours after Chelyabinsk,
a similar sized asteroid, known as Duende, came within 27,000 kilometers
of earth's surface. That's closer than satellites
in geosynchronous orbit. But while they correctly
predicted this close approach, they completely missed
the unrelated asteroid that exploded over Russia. And the truth is, this
happens all the time. We're really not that good
at detecting asteroids before they hit us. Since 1988, over 1200
asteroids bigger than a meter have collided with the
earth, and of those, we detected only five before they hit, never with more than a day of warning. With all our technology
and all the telescopes across the earth, not to
mention the ones in space, why do we struggle to
detect dangerous asteroids before they strike? What are the chances that
a big asteroid will hit, wiping out most, if not all life on earth? And if we saw one coming,
what could we do about it? (explosion) Asteroids are the leftover debris from when our solar system formed. Four and a half billion years ago, rocks and dust clumped together
into molten protoplanets. Inside, heavy elements, metals like iron, nickel, and
iridium, sank into the core, leaving lighter silicate
minerals on the surface. Some of these protoplanets grew into the planets we know today. But many more collided with each other, breaking into pieces. These pieces continued orbiting the sun, and smashing into each
other and breaking into even smaller fragments. These became the asteroids, which is why some of them are rocky, loose conglomerates of gravel sized rocks called rubble piles, and others, from the
cores of planetesimals, are mostly metal. - So this is, this is an iron meteorite. And essentially it's the piece of a core of a small planetary body, like basically a small planet, that formed four and a
half billion years ago, differentiated, so the
core material fell out, and then this thing was smashed apart by a collision with another asteroid. That's the oldest thing you'll ever see. - Most of the asteroids have stable orbits between Mars and Jupiter,
in the main asteroid belt. But some have made their
way closer to earth. And these are known as near earth objects. They are of greatest interest to us because of the threat they pose. In his last book, Stephen Hawking considered
an asteroid impact to be the greatest
threat to life on earth. But finding asteroids is
difficult for several reasons. Most are spotted by
ground-based telescopes. - [Prof. Jewitt] So
what you do is you take a sequence of pictures, one, two, three, one, two, three, four, and you look for essentially a moving dot. And it's moving because it's
orbiting around the sun. Whereas the stuff far away, the
stars and galaxies, are not. - But you have to look carefully. Asteroids are not very big. They range from meters
up to kilometers in size. And in the vast expanse of space, rocks like that just don't stand out. And even the small ones can be damaging. The Chelyabinsk meteor was only around 20 meters in diameter, roughly the width of two school buses. Plus, asteroids are rough and dark. They only reflect around 15%
of the light that hits them. So our best chance to see them is when they're fully
illuminated by the sun. And that's why over 85% of the near earth asteroids we've detected were found in the 45 degrees of sky directly opposite the sun. This is called the opposition effect, and it means there are
likely more near earth and potentially hazardous asteroids that haven't been detected yet. Any asteroid approaching
from the direction of the sun just can't be seen. This is exactly what
happened with Chelyabinsk. So far, we have detected and
cataloged a million asteroids, the vast majority of which
are in the main asteroid belt. But 24,000 are near earth objects. Ones that we need to keep a
particularly close eye on. Because even once you've
detected an asteroid, it's hard to tell if
it will hit the earth. - So if you just discover an object, and you only have data from a few days, then you can't really
tell where it's gonna go, because you're tryna take
this little arc of motion and predict it far into the future, so, what you need is observations
over years and years. But even if you have perfect
observations of an asteroid, there's kind of a fundamental limit to how far in the future you can predict. And that's because of a couple of effects, but one is that, you know, they're not just orbiting the
sun with no other influence. All of the planets have gravity, and all of the planets are pulling on near earth asteroids and can
change the orbit significantly. So there is something
called dynamical chaos, which basically means, after
a certain amount of time, you don't know where the
asteroid is gonna be. And in practice, what that means is, we can't do any work more
than 100 years in the future. So the maximum time you can
predict with any accuracy at all where a body will be is about 100 years. - And this is pretty important, because we know with certainty, if one does hit, the
results will be dramatic. (gentle music) This is Barringer crater in Arizona. It's named after a mining
engineer, Daniel Barringer, who was the first to suggest it was formed by a meteorite impact. The prevailing view,
even up until the 1950s, was that it was created
by volcanic activity. But Barringer was
convinced it was the site of an iron meteorite impact. So in 1903, he staked a mining claim, and began drilling for
the metallic meteorite, which he believed to be worth more than a billion 1903 dollars. - Yeah. So people are motivated by money, right? So they thought, "Hey, we can get some
iron for free!" basically. So they started to drill
in the bottom of the crater and found nothing. And then they started to do
other exploratory drills. And this went on for years and decades. They started to drill sideways. Somebody said, you know, maybe it came in from
an angle, which it did. And maybe the iron is not under the middle but maybe it's over there under the wall. So he was doing drilling, if you go there, you
can see the drills now. He was drilling around the
wall, he found nothing. So what they didn't realize is, when you have an impact at high speed, it's not like you're throwing
a stone into a brick wall, you know, and it makes a
hole and sticks in there. Or just bounces off. It's explosive. It's like totally explosive. So the kinetic energy of the projectile comes in maybe 30 kilometers per second. The kinetic energy of the
projectile is big enough to completely vaporize the projectile. It turns it into a gas. And that gas is super hot
and super high pressure, and it explodes and it
blows out the crater. So the projectile doesn't
really exist after the impact. I mean, little pieces can survive. But this 50 meter body
was basically obliterated. So he was looking for
something that did not exist. - He spent 27 years mining the crater, drilling down to a depth
of over 400 meters. But what he was searching
for had vaporized on impact 50,000 years earlier. The 50 meter asteroid, not that much bigger than Chelyabinsk, released the energy equivalent
of 10 megatons of TNT. That's over 600 times the
energy of the Hiroshima bomb. So, the thing that most closely resembles a meteorite impact is a very
large nuclear explosion. (explosion) - This is the actual
size of the T-Rex skull. And I thought, this is such a
cool thing, I gotta have it. So, I bought the T-Rex. The dinosaurs were wiped out by a 10 kilometer size asteroid, that hit about 65 million years ago. So, above a critical size, which is probably a couple of kilometers, an impacter delivers so much energy that it has a global effect. (explosion) So essentially, it launches
a whole bunch of debris into sub orbital trajectory. So the ejector goes around the earth, falls back into the earth, all over, even on the other side of the planet from where the impact occurred. And what that means is
the whole sky lights up with wall-to-wall meteors. So you can imagine the sky
turning from, you know, a nice blue day like today, into essentially a red, hot glow, like being inside a toaster oven. So the first effect of this impact apart from the initial blast near where the actual impact occurred, the first effect is the sky turns into a great source of heat, and it cooks everything on the ground. So these guys were basically cooked. - Cooked alive. - Cooked alive, as they
were walking around. The only animals that had a chance were the ones living in
tunnels under the ground or maybe in the water. And they were able to
come back and take over without having to deal with the dinosaurs as a major obstacle. - What are our chances that earth gets hit by another 10 kilometer
or bigger asteroid? - In your lifetime, assuming
you live to be 100 years old, you have a 10 kilometer impacter like the KT extinction event every hundred million years
or something like that. So the probability of
getting it in one year is one in a hundred million. So you have one in a million chance of dying from a 10 kilometer impact. But, because we know that there are no 10 kilometer impacters with a path that intersects the earth for the next hundred years, your chance of dying from
that is actually zero. So work done already has
reduced that down, you know, from one in a million to nothing. - So the good news is, there won't be another
dinosaur style extinction event in our lifetimes. But, there are
exponentially more asteroids of smaller sizes. For every 10 kilometer asteroid, there are roughly a thousand
one kilometer asteroids, and they're still capable
of doing a lot of damage. - One or two kilometers is capable of causing
local, but massive damage. So that means, you know, instead of wiping out the entire world, you would wipe out the equivalent of some European country,
like France or Germany, to mention two of my favorites. So you would obliterate those countries with the impact of a one
or two kilometer size body. - Do we know about all the one to two kilometer
bodies that could hit us? - We think that we know
90-something percent. Maybe 98% of those bodies
have been identified, and we have their orbits, and we can make reasonable predictions for the next 10 years of
something about where there'll be. And we seem to be okay at
the moment, but you know, what about the ones that are just a little bit less than a kilometer? What about the ones that are 800 meters? That's still pretty,
pretty savage if it hits. - And this is possibly where the greatest threat of asteroids remains. A few hundred meters is large enough to obliterate a large city. But small enough that we
haven't detected them all yet. - We're missing a lot of
hundred meter size projectiles. And those guys are big enough
to cause substantial damage on the earth, depending on where they hit. - So it could destroy a city. - Yeah. It would knock down
the buildings in the city. It would cause city-wide fire. And if it hit the ground, it would throw up ejecta
that would come back down, rain on the ground, it
would be high-speed ejecta, that would obliterate a hundred
kilometer zone around it. - And this could happen tomorrow? - Well it could, yeah. (chuckling nervously) - If we saw a big one coming, what's our best bet for, I mean, could we do anything about it? What would we do about it? - [Prof. Jewitt] No. - Is there anything we
can do to actively ... - No. There's nothing we can do. I was on a committee that
looked at that, okay, like 10 years ago, like, what could we do? One option would be to try to bomb it. It's a standard thing. We don't know how that would work out. Even when you got it there, and even if you could explode it, on the surface or in the surface, it's not clear what you would do, because typically what happens is you blow up a body, and
the fragments move out. They expand out, but not very quickly. And then gravity pulls
them back together again. So it would reform as a rubble pile. If it was not already a
rubble pile to begin with, which it probably would be
because of past impacts. So blowing up a rubble pile is something that we
don't really know about. Another idea is you could attach, you could be all gentle, and attach a rocket to the asteroid, and just try to push it aside. Just nudge it aside, instead
of trying to blow it up, let's just push it gently aside, so that it deflects it and
it doesn't hit the earth. The trouble is, when you
work out the numbers, none of the rockets that we
have can push it around enough. You would have to keep the
rockets attached to the surface, which we don't know how to do. Remember, it's a rotating body, for centuries, to have
a significant effect on the motion of the asteroids. So forget bombs, forget attaching rockets. Ablating the surface, basically you boil the
surface with a laser. We don't have any lasers powerful enough and probably can't make
lasers powerful enough to do that from the earth. We would have to take
the lasers to the object, which is even more difficult. The idea that you could wrap an asteroid in cooking foil, aluminum
cooking foils, another nice one. It may be a good one, the best one. But it still doesn't really work because we don't know how to do that. We don't have a way to launch enough cooking foil to wrap up an asteroid and change its radiative properties which would itself move
the asteroid around. So the truth is, to be honest, we do not have a way now to deflect a kilometer size asteroid, at all. - That could destroy a country. - [Prof. Jewitt] Yeah,
we just don't have a way. - And 10 kilometers? - 10 kilometers is absolutely a thousand times more hopeless. (laughing) So, when we discuss this, you know, we had all these grand ideas. "Oh, we could do this and
this," and none of them worked. We came down to the most basic idea, well, maybe if we could figure out where the asteroid is gonna hit, like which city is it gonna explode over, we can evacuate that city. And then we looked at the
history of city evacuations, and we looked at cases, you know where, for example, you have
like a week's warning, where some hurricane
system is gonna come in and flood a city. And evacuation just doesn't work either. And the reason is very, very simple. Going into a city, there
are not that many freeways. If you have millions of people
trying to get on a freeway, the first time a car breaks
down, you block that freeway. So instantly, you have millions of people trying to get out of the target zone, and they won't be able to
because all of the roads will be instantly blocked. So again, even that, even
evacuation of a city, is probably the most hopeful
thing that we could try to do. Even that's really, really difficult, because of the large
numbers of people involved. What I think all reasonable
people would conclude is, let's do the thing that we can do first. So let's look for them. Let's do the surveys. Let's build the telescopes. Let's put this telescope in space. That will be a major contribution to understanding the
threat from the asteroids. And then when we find a particular object that looks especially dangerous,
then we can focus on it. We can focus everything we have on it, and we can begin to think
seriously and with real motivation about ways to deflect it. - Now, if you're concerned
about the world ending in an asteroid impact, let
me set your mind at ease. There are many other
potential global catastrophes summarized in this map of
doom made by my friend Dom, over at Domain of Science. So if you wanna see which
of these horrible scenarios is likeliest to be our downfall, well go check out the
video on his channel. (sci-fi sound effects) My oldest now knows how
to do the sponsor message. Do you wanna say it? This episode ... - [Kid] Is sponsored by KiwiCo. (chuckling) - Very good. KiwiCo creates awesome projects
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