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.