"How do they know that?" Modern astronomy has made some astonishing discoveries - how stars burn and how black holes form; galaxies from the edge of the universe and killer rocks right next door; where the elements come from and how the expanding universe is accelerating. But how do we know all that? The truth is that astronomy would be impossible without technology, and every advance in astronomy is really an advance in technology. But the technology by itself is not enough. We have to apply it critically with a knowledge of physics to unlock the secrets of the Universe. Each week we will cover a different aspect of Astronomical technology, matching each piece of technology to a highlight science result. We will explain how the technology works, how it has allowed us to collect astronomical data, and, with some basic physics, how we interpret the data to make scientific discoveries. The class will consist of video lectures, weekly quizzes, and discussion forums. Each week there will be five videos, totalling approximately 40 minutes. They will be in a regular pattern - a short introduction, an example science story, an explanation of the key technology area, a look at how the technology is used in practice, and finally a look at what the future may hold.
W5 Part 4 - Killer Rocks Part II
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AstroTech: The Science and Technology behind Astronomical Discovery
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Computers and Killer Rocks
Why computers are crucial in astronomy, and how they are used to find rare objects.
Conheça os instrutores
Andy Lawrence
Physics and Astronomy
Catherine Heymans
Physics and Astronomy
[BLANK_AUDIO]
So, imagine we've used a giant telescope to survey the whole sky every few nights.
We've used CCD detectors in order to record the light that those telescopes
have collected and we've used computers in order to analyze those
petabytes of data from the CCDs in order to detect objects that are moving.
We've collected all that data.
How are we now going to find out if those
moving objects are going to crash in to planet Earth?
All right, let's imagine that we've tracked an asteroid for seven days.
And in that time, it's moved an angle of half a degree on the sky.
Now let me draw you a quick diagram.
So you get a sense of how far that it is.
I am sure you have all seen the constellation of Orion.
So, in those seven days our asteroid has moved roughly
half way between two of the stars in Orion's belt.
Another the frame of reference is half a degree is about
the size of the full moon on the sky.
Okay, so our asteroid has moved that far in seven days. So, how long
is it going to take it to complete a full orbit around planet Earth.
Well, in physics, we always start off with the simplest model, and the simplest
model is to assume that the asteroid is orbiting planet Earth in a perfect circle.
And I'm also going to assume that it's sufficiently far away that
I can ignore the fact that the earth's going around the sun
and just think that the earth and the sun are in the center of this diagram.
Okay, so in seven days, it's moved half a degree on the sky.
I'm exaggerating that on this diagram
just for you to see it.
How long is it going to take to complete a full orbit all the way around?
Well, a full orbit is going to be 360 degrees.
That's just how many degrees there are in a circle.
So, the time that it's going to take to do a full orbit and I'm calling it capital P
there because it's a time period, to complete the full orbit, that's just going to
be 360 degrees times the time that it's
taken to do that angle on the sky.
Now, if you put in the numbers, that gives
you 5,040 days, which is 14 years.
Okay, so our asteroid that we've tracked is going to take 14 years to
orbit the Earth, in our model that it is orbiting in a perfect circle.
So, how far away is the asteroid from Planet Earth?
Well, to answer that question, we are
going to use some of the formula that we've
used already in this course, our trusty
friend speed is distance over time.
Call the speed v.
The distance is the distance it takes to go one full
circle, so that's the circumference of the circle is 2 pi R.
And the time we calculated here, call that capital P.
All right, we're going to need some other formulae.
Remember back to when we were thinking about the black hole.
You've got that the force of gravity is keeping this asteroid going around our sun.
And the force of gravity is given by GMm/R^2, G is gravitational
constant, M here is the mass at the center and little m is the mass of our asteroid.
And let's not forget the distance, GMm on r squared.
Now, it's gravity that is keeping our asteroid in orbit.
But it's experiencing centripetal force and the
equation for that is mv^2/r.
So, I think we have all of the ingredients that we need now,
in order to be able to calculate the distance that our asteroid is away.
But to do that, I'm going to need another piece of paper.
So, let's equate our force of gravity
with the centripetal force.
And we did this equation before when we were looking at the black holes.
And cancel out the mass of the asteroid. I can cancel out one of these r's.
And so, I've got v squared here, the speed squared,
and I'm going to put in our equation for speed.
2 pi r on P and it's all squared.
Okay, so now I can rearrange this and what I will find is
that I get r cubed (r^3) is
GM P^2/ 4 pi^2
And I'll leave that as an exercise for you at home, to see if
you can rearrange that equation to give you that result.
Now, the mass of our sun is 2x10^30 kg.
We've calculated the time period.
So, you can now calculate the distance to our asteroid
and I'll leave that as an exercise for you to do.
And you can also calculate the speed.
Remember, the speed is just 2pi r on P,
so once you've calculated r you can calculate the speed.
So, just with our simple model of a
asteroid orbiting planet Earth in a perfect circle
and some measurements of how fast it moved, or how far it moved in seven days.
We can calculate how fast it's moving, and how far it is away.
So, we're now inside a very special room in the Royal Observatory of Edinburgh.
Because this room hosts our Crawford collection that contains 15,000 ancient
manuscripts and books that cover maths and physics and astronomy.
And the reason why I've brought you here today is
because I wanted to show a first edition of Newton's Principia.
Now, this book was first published in 1687 and
it is Newton's derivation of the laws of gravity.
Now, the page that I want to show you is going to
help us improve our model of the asteroid going around the earth.
Now, this page shows you a comet going around the sun, and the first
thing that I want you to take away from this is that the orbit is elliptical.
So, we were making an assumption that the orbit was going to be circular,
but actually, most orbits in our solar system are elliptical.
Now, the other thing I want you to take away from this
picture is that the tails of the comet always point towards the sun.
Now, I think a lot of people, when they imagine
a comet, and they see the tail coming out of it.
They think the tail is because of the motion of the comet.
But that's not true.
What happens is the sun evaporates some of the icy comet.
And, it's the radiation pressure of the sun
that pushes out that material to create the tail.
Now, if we want to improve our model of an asteroid as it
comes toward the Earth, we're going to have to add in an elliptical orbit,
as you've seen here in Newton's book.
We're also going to have to add in the motion
of the Earth relative to the Sun, and indeed, all
of the other objects in the solar system that will
effect how the asteroid moves as it travels towards the Earth.
Now, all of these additional complexities are going to be quite hard to put
in just on pen and paper, so you do that with a computational code.
Now, if you do discover a moving object with your
telescope in your garden, then you can upload those positions
and the time that you took those observations. You can
upload that to the minor planet center, it's an online tool,
and they add that data in with all of the other data that's been
taken about moving objects and compute those
orbits for you with that detailed computer code.
And they'll also be able to tell
you whether you've discovered something that's really new.
Now, the final thing that I want to talk to you about today is
how we would model what happens to the Earth if an asteroid did hit.
And to do this, we're going to have to think about energy.
Specifically, kinetic energy, which is the
energy that happens if something is moving.
Now, the asteroids are going to be moving towards
us very fast, and so, the kinetic energy that
they are going to impact into planet earth has
a formula which is E =1/2 mv^2.
Now, m there is the mass of the asteroid and
v is the speed or the velocity that it's moving at.
Now, intuitively, this formula will hopefully make some sense to you.
Imagine that a ten ton truck is coming
toward you, at a really, really high speed.
You're going to be much worse off if that collides with
you, than say, a small toddler, toddling along on a trike.
Now, the detailed modelling of the orbit of
the asteroidal comet will give us quite an accurate
measurement of the speed or velocity with which
that object is going to crash into planet Earth.
But what we don't really have a good
feeling for is the mass of these asteroids.
And so, we're very excited because the NASA Rosetta mission is going to be
landing on a comet in May this year, and we're recording this in 2014.
And that's going to really tell us about the
composition of these objects which will really help
us find out what's going to happen to planet
Earth if something like that crashes into it.
So, we've told you how we can use telescopes, CCD
detectors and computers in order to find potential killer rocks.
I've told you a little bit about the physics of how we
can then use that data to work out really whether these rocks are
going to crash into planet Earth and with future technology, we're going to be
able to find out exactly what's
going to happen when these rocks potentially collide.
Well, let's hope we don't suffer the same fate as the dinosaurs.
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