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Lecture 3.15: Asteroid hazards
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From the course by Caltech
The Science of the Solar System
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From the lesson
Unit 3: Big questions from small bodies (week 2)
Meet the Instructors
Mike BrownProfessor
Planetary Astronomy
Today's lecture is a little bit of a digression somewhere between informational
and purely editorial, because we're going to talk about something that is
potentially a concern to to all of the human race, which is that, as we've
talked about meteorites is that they come to the Earth for us to study in detail.
Maybe the less nice thing about meteorites is that they come to the Earth for
us to study in detail.
It's not hard to start to think about the fact that these
things are impacting all the time when you pick them up off the ground or
when you look at the surface of Mars or look at the surface of the moon and
you see them just pocketed by these craters.
They look like the ancient parts on the moon,
there's essentially no part of the moon that hasn't been hit.
You can think of the Earth the same way.
We have an atmosphere that helps us out some, but
the Earth is continuously being bombarded and we are continuously under,
as some people see it, attack by the asteroids.
It's an idea that's compelling enough that we get movies from Hollywood on
asteroids coming to kill us and
people going up to save us from them in thoroughly ridiculous ways.
And for many years we could, we could easily say, look,
you know, no one has been even hurt by one of these things in,
in human memory or maybe there have been occasional things where there have been
reported of a small rock that came from the sky and broke an arm or something.
This was all changed a few years ago in the the, the air burst, the meteor impact
that blew up in the middle of the air in Chelyabinsk over Russia on February 15,
2013.
And as you probably remember it, it exploded in the sky.
Many people saw it.
Many people ran to their windows to, to watch and many people were injured,
because the blast wave from that shock came down from the sky and
shattered windows, cut people up in those ways and it made big news.
And it really drove home the idea that perhaps we really should
be spending some more time thinking about the hazard of asteroid impacts.
These days it's pretty big business.
There's a mandate from Congress to NASA to go find near Earth asteroids
that might hit us.
There are private foundations that are raising money to build telescopes in space
to go track these down.
There are people who lobby very loudly for
countries to develop the capability of deflecting asteroids.
And again, because we see these things happen, we know it must be true.
It's, it makes it a very compelling argument.
The important thing, though is for people when having this argument to have
a realistic assessment of what the potential issues really are.
How do you do that?
Well, it's all in terms of probability.
Of course, we can't tell you that something is not going to hit you or
is going to hit you in a certain amount of time.
If we knew it was coming we could, but we don't know anything so we can't tell you,
but we can tell you probabilities.
We can look back in the past at the Earth and estimate impact rates.
We can look at the number of near Earth asteroids crossing the Earth's orbit and
estimate impact rates.
And when we do that, we can make a plot that looks sort of like this.
I'm sketching it for
you rather than showing you some some fancier version of it.
Because the numbers are sufficiently uncertain that, that I'm, I'm willing to,
you know, let somebody come and erase some of it and move it slightly, but
these are approximately right.
And what do they tell you?
They tell you the time between impacts,
approximate time between impacts in years for things of a certain size.
I'm ignoring the size of the as, actual asteroid let's just instead think of
the explosion when it hits the Earth in terms of megatons of TNT.
One of the other problems with asteroid impact hazards is that people talk about
these numbers like meg, megatons of TNT and, and nuclear bombs and things.
Most of us don't have a very good feel in our heads for
what's a sizable number of megatons.
What's a not sizable mega, number of megatons.
So, I'm going to try to give you some of those feelings to, here too and
let you decide what you think.
So first off, if you wanted to have 10 to the 8th,
that's a 100 hundred million megatons of TNT.
How often does that happen?
Could mean something like 10 to the 8th years, a 100 million years.
That means if the Earth is 4.5 billion years old
not many of those happened in the last bit of the history of the Earth.
I can't really say 4.5 billion divided by 100 million and get the answer,
because as you remember, there were more impacts early on.
But over the last say, billion years, there might have been 10 of these,
10 to the 8th megaton explosions.
Now these are bad news.
Every one of these ten caused serious problems, as we'll talk about in a minute.
The more interesting scale perhaps is to move up here to the once every year scale
or the several times a year or once every ten years.
And when we do that, we see we're somewhere in the megatons.
Smaller than megatons is kilotons.
Let me put some scales on here for context and
we'll start to see what some of these things mean.
Okay.
So, once a year,
there;s something like a 20 kiloton explosion at the Earth coming from space.
Sounds a little bit scary, doesn't it?
Let's see what these sort of things mean.
Chelyabinsk, this is the one that hit Russia in 2013.
It was below a megaton.
So, it was somewhere around in here.
When did it happen?
Well, those sorts of things on this scale happen not every 10 years,
but maybe every 50 years, something like that.
So the fact that we had one last year means that we're particularly lucky but
they don't happen all that frequently.
Other things that you may have heard about, Tunguska was this other big
meteorite impact explosion, air burst that occurred oddly enough over
Russia also in 1908 and it caused a, a moderate area of devastation.
It flattened trees you, you could hear it for,
for many hundreds of kilometers around there.
So, it was quite a big one.
Meteor crater.
Meteor crater, if you've ever been to the Southwestern United States,
meteor crater is in the Northern part of the state of Arizona.
It's just south of the Grand Canyon and it's spectacular, I have to tell you.
It is just a, a tremendous site to see, it's,
this giant crater in the middle of an, otherwise, very
uninteresting desert when that happened it must have been pretty spectacular.
When did that happen?
Well, it was about 50,000 years ago and we put it right around here.
50,000 years ago, this giant iron meteorite slammed into the desert.
Created a pretty big devastation there.
How big do you have to be to make bad things happen?
Bad things you have to be in these sort of 10 to the 5th megatons.
The impact that killed the dinosaur.
Most people have heard the story about the the K-T boundary, 68 million years ago.
A large, in this case, probably comet, hit,
probably off the coast of Mexico and that impact was so
large that it led to wide scale global extinctions of dinosaurs.
That's bad, but that's only once every 100 million years.
So, it's not something that we really need to worry too much about right now.
Let me put a few more scales on there and here you can really see what sort of
destruction we're really talking about in different things.
Chelyabinsk, for all of its webcams that got to see it and for
the reports of injuries, it really is best categorized as essentially no effect.
It's the same as or I should say,
significantly less than a minor tornado hitting a city or
some other thing that happened all the the time across the world.
It is not something that really, we should spend much time thinking about or
trying to prevent unless it is really simple to prevent, which it's not.
Next up on the list would be things that cause severe local destruction.
These are the things that are the size of Tunguska.
Tunguska really did flatten a pretty big area of, of Siberia.
And if Tunguska had then hit over a modern metropolis,
it would flatten that modern metropolis in a pretty severe way.
Now, of course, most of these impacts wouldn't have happened to occur over some
Modern metropolis, they would occur over the ocean,
cause tsunamis, that's bad, too.
Or they would occur over uninhabited land in, like, Siberia, and,
and we would know about it.
But it wouldn't have caused much.
But still, local destruction,
I would call local destruction something that's bad that we would try to prevent.
If I, if I try to think of what other sorts of things cause that level of
local destruction, this is the same sort of local destruction that I would,
I would think of as a truly severe,
major earthquake in an urban area like maybe Los Angeles.
That would be bad.
Next up on the list, regional destruction.
Regional destruction is such a large amount of destruction that we really have
no analog for what that means.
That would mean like continent-scale debris falling out of the sky.
That would mean, fires all over the place.
Might be it, if it hits the water,
it means tsunamis that could wipe out coastal areas, all the way around a basin.
Truly bad.
Truly something we don't want to have happen.
Finally, these that cause global destruction, yeah,
I think we don't want global destruction either.
Okay so, I think its pretty safe to say that we really would like to be able
to absolutely prevent regional destruction, global destruction, and
definitely even local destruction we would like to do something about.
And this level of discretion I'm, I'm not saying we shouldn't worry about
it or that we shouldn't do something about it, but its not at a level that we need
a truly global response to figure out what's going on here.
Okay, so take these, let's say that we do care about these sorts of events here,
and the most common ones are going to be up in here.
Okay, so the smallest local destruction, sort of, the, the, the b,
border between not very destructive and very destructive locally is,
at this point, we have it at something like once every 100 years.
Once every 100 years, there will be an event that could,
if it happened to occur over a populated area, would be as big as, let's say,
th, the biggest swarm of tornadoes that you, you could imagine having hit a city,
or the destruction of, New Orleans, by, by a hurricane.
As an aside, it's probably an obvious statement to say that, you know,
this level of destruction once a century is probably significantly
less than the average that we as humans do to ourselves in bombs and
wars in, in a typical year.
But that's perhaps not relevant to the impact hazard here.
But the important question is, it's somewhere around 100 years,
somewhere around 1,000 years, we would like to prevent these sorts of things.
These are the things we really should be thinking about if we're thinking
about trying to prevent these sorts of events from happening.
Now, these local destruction, even these local destructions, are not likely if they
occurred, on average, not likely to kill very many people.
There have been many different ways of calculating this but I think the, the,
the number that I like is that your probability
of dying from an asteroid impact is something like one in 74 million.
So, one in 74 million because the probability is pretty small, but
when they do happen, a larger number of people are really killed.
So one in 74 million, pretty small probability.
The real question is, what should we do about it?
This is a classic case in, in risk assessment.
You could do it as a, as a cost benefit analysis.
You could decide if the probability of dying in an impact event
is one in 74 million,
how much is it worth to you to retire that probability of dying in an impact event?
You could then compare it to all the other things that you might spend
that money on and decide what is the best use of your money?
And the problem with this is that these sorts of analysis
don't necessarily do very good on these very low probability events.
The probability that you are going to get killed is really small, but
if it happens, it's a potentially major regional event.
It's best to try to prevent those sorts of things.
We here in Los Angeles spend a lot of time and a lot of money thinking about how we
reinforce buildings, how we make roads better, how we keep the probability
of dying in an earthquake or catastrophic destruction as low as possible.
But we do that, I would like to point out, because the probability,
if you live in Los Angeles, the probability of dying in a catastrophic
earthquake in your lifetime, I'm going to make up a number here so
anyone who lives in Los Angeles, don't get freaked out by this number.
I'm going to make up a number and call it one in 100, 1,000 maybe 1,000.
I'll call it 1,000.
There is a one in 1,000 probability that I will die
in a catastrophic earthquake in my lifetime.
When I say that, I then think, you know, maybe I did, I, s, the number's too high,
maybe I really should say one in 100.
There probably will be catastrophic earthquakes in my lifetime
that I live here so I guess the question is will that kill me?
I don't know.
But I'm sitting right here.
Maybe.
But we in Los Angeles spend a lot of money and
time thinking about how to prevent that because, it's kind of likely.
We know that it's bound to happen,
we know that it's bound to happen on human timescales, and we know it's preventable.
One in 74 million, that is a low number.
It's harder to think exactly about what to do.
I have a recommendation.
Here's where we have the opinion part of this lecture.
I have a recommendation on what we should do about the impact hazard.
And the recommendation goes something like this.
First off, it's not going to happen soon.
Yes, there is a probability that maybe this one in a thousand year thing
is just around the corner and we didn't notice it.
But chances are, there's no hurry to figure out if this is a problem soon.
There's no hurry to map out all the asteroids this year or next year.
Astronomers are making pretty good progress mapping out where
the asteroids are.
So far there's nothing that's even vaguely bothersome heading our direction.
And we know the vast majority of the things that would be large enough to
cause these sorts of large scale damages, and there's just nothing coming.
So I would say, let astronomers comfortably find more of these things over
the next decades and we will continue to map them out and
continue to, I suspect, say the universe is safe for now.
Now, there are people who would like to do it yesterday, or
people who would like to do it next year.
I, I just don't see there's any need to have that sort of urgency.
There are also people who would like to
start developing ways of deflecting asteroids.
This, I think, is lunacy.
If there's an asteroid coming, we don't see it coming five minutes from now and
it hitting us.
Well, if we do, then we're dead anyways, so it doesn't matter.
What we see is, decades ahead of time, we see that there's a high probability that
there's going to be a problem with this asteroid.
I have enough confidence that,
if we saw something with a high probability of coming towards us in
decades, we would be able to immobilize to develop something that would deflect it.
I just don't think we need to be prepared for
this once every 1,000 years event right now, just in case.
In fact. I would declare that to be a really
huge waste of money.
That said, my views are not shared by everyone.
There's something about space, and asteroids, and the fact that
finding killer asteroids seems cool, and deflecting them seems really cool that,
that makes people, I think just go t, go a little bit out of what is sensible.
It makes people think, you know, yes, the probabilities are low but
it's a rock from space, we have to be prepared.
And so they spend a lot of time, a lot of effort, worrying about these things.
In fact, there's a recent press release from this, this prominent foundation a,
founded by people like astronauts who are very concerned about this.
Who have this, sort of breathless press release that they have just determined
that we are, in fact, really unsafe.
There have been 26 impacts greater than a kiloton in the past ten years.
They have determined from devices that have been looking for nuclear explosions.
26 impacts greater than one kiloton in ten years,
and the, the, the quote in the press release is that
the fact that a major city hasn't been hit is just blind luck.
Well, okay, let me remind you 26 impacts greater than one kiloton, ten years.
Greater than one kiloton, that's down here.
I didn't, almost didn't even put it on the scale.
And in ten years, well, okay, so we can argue about exactly how this curve looks,
but, you know, I'm already telling you that there are, there,
every year there's a 20 kiloton impact, and I'm not worried about that.
I'm supposed to be even more worried about this because,
you know, they use these big numbers.
But yes, there are 26 impacts greater than one kiloton in ten years.
What effects do those have?
Well, do you remember any of them?
No. No. That's because they really don't have any effect.
This 50 kiloton one, over Chelyabinsk if it had happened over a larger area,
you know, it would have bro, blown out more windows, you know.
It would have been like a really bad storm that hit.
But it's not one that we need to be terrified by, which is, I feel like, part
of the game plan of people who want to really be doing these sorts of projects,
making, flying to asteroids and, and moving them around.
I think that the goal is to terrify people,
to make them think it actually is something to worry about.
And then, finally, there's the wild card version of this, is that we
always talk about asteroid impacts as the thing that we're really worried about.
You rarely hear anybody talking about comet impacts.
Comet impacts are a big problem.
There's a big problem because,
let me just draw you a typical picture like we had before.
Sun is here and let's say the Earth is going around like this.
Near Earth asteroids are in orbits very similar to the Earth.
Maybe they're going like this on an elliptical orbit,
like this on elliptical orbit.
But just like when we talked about planetesimal formation,
near earth asteroids hit the earth with relatively small velocity so
the disruption is not as terrible as it might be.
Comets on the other hand, if you're aware of the orbits of comets,
comets come in screaming in towards the sun like this and come out.
They're going very fast when they come in.
Comets when they hit the Earth, do tremendous destruction.
Think of the poor dinosaurs.
Here's the embarrassing part about worrying about the impact hazard
a little too much.
You go out. You map all the asteroids there are in
the entire inner part of the solar system.
You determine that nothing out there is going to hit us.
Doesn't matter. You can still spend
billions of dollars developing systems to deflect them, just in case.
And then you get warning of about, oh, I don't know a year, two years.
That a comet is coming from the outer solar system and
it's going to scream by you.
It's going to hit.
None of these things that you've been mapping all along,
it's the distant comets that come in fast.
Can you deflect it?
No, they come in too fast and they come in too quickly.
And you don't know they're coming.
Until the last minute, basically.
So it hits, you're all dead, sorry.
Is that going to happen?
No, I don't think so.
Again, the probabilities are low.
Yes, you could do some things to, to make sure that this part of the probability
space that, that all of these regions over here that you might worry about.
We can try to prevent those things from happening and
let's be clear that would cost a lot of money.
But even then you couldn't prevent this part which is as equally bad a thing
happening to you and so you have spent a lot of money to make your probability of
a very, very low risk event, maybe decrease by a factor of two.
But it's still there.
So.
I have advice.
Here's what we do about the impact hazard to the earth.
Nothing.
So, that was supposed to be the end of the lecture.
But when I finished that lecture I was still, you know,
I can still get a little agitated by this whole asteroid hazard thing.
And, I took to my favorite forum when agitated, and that's Twitter,
where I continued my ranting about asteroid impacts a little bit.
And it's, it's quite amazing.
If you, say something that, about asteroid impact hazards being overrated on Twitter,
you get a lot of response from space enthusiasts thinking that you are crazy.
Ad space enthusiasts, we all know who they are.
It's you. It's me.
We're all sort of space enthusiasts and,
and many of us have been sort of programmed to believe that
asteroid impacts are something we should really be thinking about.
Why, why, why does this happen?
How does it happen?
I, I think the answer is because that word, space enthusiast.
We love space.
We love the idea of things that are in space, things that are coming
towards us in space, things that we can do in space to, to solve the problems.
All those things are tremendously exciting.
I think the, the thing that summed it up best was something that came in, also in
Twitter to me, which was, well, I'm not sure if this is an important problem to
worry about or not, but it would be the best thing for the US space program.
And, you know, that statement might be true, but
that's a pretty sad reason to be worrying about the impact hazard.
That is taking something that's not actually a very big problem,
spinning it as a problem and using that as an excuse to go into space.
Have to tell you, I don't want to have any part of that.
Here's what I do want.
I want for us to carefully evaluate the asteroid impact hazard.
I would say I think we have.
I want us to look at that asteroid impact hazard in the context of all the other
hazards that we have on our planet, both man-made and natural things,
some of them we can do things about.
Some of them we can't.
And I want us to give it the appropriate amount of scrutiny and work.
And I want us to not give it extra scrutiny and
extra funding simply because it's space.
And we love space.
You know what if the answer were the best way to mitigate against the asteroid
impact hazard were to install safety glass in houses all around the world.
So that, you know, one of the biggest problems when that, Chelyabinsk
meteorite hit was that windows got blasted and people got cut by the windows.
So, fix that.
That sounds pretty mundane that sounds pretty dull, doesn't it?
And yet it could be significantly cheaper than
coming up with a program to move asteroids in space.
I suspect that the people who are interested in asteroid
impact communication would find that a completely unacceptable solution,
why because its not cool.
Because it doesn't help space.
What would it do?
Might save lives, might save injuries.
Probably not very many.
So, what should we be doing?
Well, okay.
At the first end of the lecture,
I said nothing, and that was a little bit glib, perhaps.
We should be studying the populations out there,
understanding what the impact frequencies are.
We have, we've done.
We should probably check to make sure there are no civilization destroying
asteroids on their way, even though it's a tiny, tiny, tiny problem.
There aren't any.
This is actually one of the other things that you would get from Twitter,
it's like, well asteroid impact hazards are more important than anything else
because civilization ending might happen.
Well, no.
It's not going to happen.
We know there is not an asteroid that is going to destroy civilization in
the next hundreds of years and any solution we can come up with
now we can come up with a much better one hundreds of years from now.
So we've studied the problem.
We'll continue to monitor, to make sure nothing new develops,
which it probably won't, and we should move on.
We should study asteroids, and we should study their impacts because they're
interesting, scientifically they're fascinating.
We should go into space because there are cool things to do there.
But we should not pretend that the asteroid impact
hazard is something that humans really should be worrying about, and
use that as an excuse to go into space.
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