 Greetings and welcome to the Introduction to Astronomy. In this video we are going to discuss comets. Now these are a solar system object that has been known since ancient times, but not really been able to be studied close until recently with the dawn of the space age that we could actually go and study what a comet is like. So we'll take a look a little bit about those over the course of this lecture. So let's look at some of the early observations first. What do we mean by some of these comets? Well, comets have been seen for thousands of years. They can be very easily visible in the sky, and here is one shown in a little tapestry here, where we can see it. And one of the problems was that it would seen as a sign of disaster. And why was that? Because it was a change in the heavens. Nowadays, we know that the heavens change all the time. But long ago, even just hundreds, five hundred to a thousand years ago, any sign of change in the heavens was a sign of disaster coming. So this is one example seen here. But what does a comet actually look like? And we can see that in this image that it has parts that are visible in the sky, which we call the head and the tails. We can see the head of the comet right at the center here. And we have one tail going back this way, kind of curving off to one side, and one going straight back here a little bit harder to see. So actually two tails to a comet, and most comets do have two tails. Now a comet does not flash across the sky. So it does not behave like a meteor or a shooting star. They will slowly change position just like a planet. They are orbiting through the solar system so that over the course of a night, it might follow a specific path through the sky and pass through it. But that will take time. It is not flashing through the sky like a shooting star. So if you see a comet one night, you can go back out the next night and it will be in roughly the same position. It won't change drastically from just one day to the next, just like any other solar system object. Jupiter or Venus will remain in essentially the same spot over the course of a couple of days. So what do we look at? When we look at cometary orbits, how do they orbit? Well, the difference with these in planets is the shape. The cometary orbits are very elliptical, and that means that they spend most of their time invisible in the outer solar system. So as a comet comes in close to here, it moves very quickly. Kepler's second law tells us that an object close to the sun moves faster, and an object far away from the sun out here moves very slow. So the vast majority of time a comet is invisible. It's only seen when it passes close to the sun, which is when it's moving its fastest so it can rush through here in just a couple of years and it can spend many decades out here in the depths of the solar system. We also know that comets come back over and over again, and Sir Edmund Halley discussed this, and in fact what he looked at was the orbits of comets that occurred in 1531, 1607, and 1682, and he noticed how similar their orbits were. So he made a prediction that said that maybe this comet would return in 1758. Unfortunately he died before that, and was never able to see that yes his prediction came true, but the comet was named in his honor as Halley's comet that comes back about once every 76 years and has been visible for over a thousand years now. It just constantly comes back, comes in close to the sun, and we see it for a couple of years, and then it heads back out and spends most of its time out here in the depths of the solar system. So comets have very elliptical orbits, and they do orbit in a way like a planet in that they come back to in a continuous orbit over and over again. So let's look at the structures of a comet. What is a comet made up of? Well, the pieces of the comet include the nucleus. That's the actual piece of the comet. That's this little tiny dot down here. So that is the cometary nucleus, which is where the actual material is. That is the solid piece of the comet, and that is sometimes what is called a dirty snowball because it is primarily made of ices, which would be the snowy part, and dirty carbon compounds that make up the remainder of it, so giving it the dirty snowball area name. And we have the coma of the comet, which is the larger area around this. Sometimes we see that as the head of the comet. That is where the material that has been vaporized from the nucleus resides. So some material is vaporized off this icy surface and then continues around the comet. It remains in a coma around the comet. That can include things like hydrogen, the hydroxyl molecule, carbon monoxide molecules, and various other objects that would be left there. And finally, we see the tails of a comet. We have two tails. Comets will have two. They will have a plasma or gas tail and a dust tail. So they are a little bit different here. We can see the plasma tail here going straight back and the dust tail kind of curving off to the side. Let's look at those in a little bit more detail here. So when we look at these tails, the comet tails will always point away from the sun. So you can always tell where the sun is in the sky relative to a comet by the direction that the tails are pointing. So here they're pointing away from the sun. As it's coming into the solar system, they lag behind the comet. Here they're pointing away from the sun. Here they're pointing away from the sun. So as the comet is leaving back into the outer solar system, the tail goes first. So the tail leads the comet. Now there are two types of tails, and we just mentioned those, but let's look at them in a little bit more detail. There is the plasma or gas tail, which consists of atoms or ions, atoms with the electrons removed. Those are very light particles and are pushed straight back from the sun. So we see this one going straight back. That is what we call the plasma or gas tail. It is very light particles, so they're very easily pushed away from the sun. There is also the dust tail, which is larger particles. These lag behind the comet in its orbit. So they're heavier particles, so you'll see them kind of curving away from this and kind of showing the direction with which the comet is moving. They're going to lag behind. I'm not sure when you see that, it tells you something about the direction the comet is moving. So just by looking at a comet's tails, we can tell the direction of the sun and the direction that the comet is moving at that time. So how have we explored comets and we have begun to explore them now? We have seen the problem is that the nucleus cannot be studied from the Earth. It's too small to be able to see any detail. The first explorations were of Halley's Comet in 1985, the last time it came close to the inner solar system, and gave us the first images of a comet nucleus. There was also the Stardust Mission in 1999, which went out to actually gather samples of cometary material and bring them back to Earth for study. However, the best study of a comet to date has been the Rosetta Mission. And we'll see that here. Rosetta was a mission to orbit and land on a comet. So in 2014, the Rosetta spacecraft went into orbit around the comet and we see the comet here, and you can start to see some of the structures. This is the nucleus that we never really had a chance to see before the mid-1980s. So we can actually now see what this cometary surface looks like, and we expect it's very icy. So most of this is made up of icy material with some dust and rock bits kind of thrown in. So what it was able to do was give us, first of all, detailed images of the surface. So not just images like this, but we're actually able to see far more detail of the surface, some very close-up images. And we got to see it as it approached the sun so we could watch for changes as material was being vaporized off the surface. So Rosetta was able to study that as this comet made its approach to the sun. It did have a lander, the filet lander, which was designed to land on the surface and kind of harpoon itself into the surface to hold itself in place. Unfortunately, some of the mechanisms failed, and when it landed on the surface, it bounced and went into, not this specific part, but into a very shadowed area, which meant that unfortunately it was unable to recharge its batteries using solar cells, so it was only able to work until those batteries died. So it was able to give us a little bit of information from the surface of this comet, but not as much as we would have hoped. But that was really our very good up-close study of a comet and has given us a lot of the knowledge that we have of comets overall today. So where did the comets come from? Well, we note that there are two types of comets, and we call these short period and long period comets. We typically divide these by their orbital periods. So short period comets are orbits of less than 200 years. Long period comets are those with more than 200 years. But we also see other differences in their structures. In their orbits. And in fact, the short period comets orbit in the same plane and direction as the planets. So they act just like planets and asteroids and other objects. They're all orbiting around the sun in the same direction. So they all orbit around counterclockwise. The planets go counterclockwise. And even those comets that we see that come in with more elliptical orbits will also orbit counterclockwise around the sun. So they orbit in the same plane roughly, meaning that they're not tilted up or down out of the image here and essentially in the same direction. So they're kind of a part of the solar system. The others are what we call the long period comets. They have random directions, meaning that they can go in any direction that you could have them coming around in a clockwise direction. And they could also come very much in a three-dimensional sense that they could come from way up above the image here and go down below it. So they can have a random direction and a random orientation. They are spread out all over. So where do these come from? Well, typically we find two locations for them. They can come from the Kuiper Belt and the Oort Cloud. The short period comets come from the Kuiper Belt out beyond Neptune. This is that distribution of icy bodies. Pluto exists out there as well as some of the other dwarf planets. And Neptune is very close to them, meaning that its gravity can cause deviation to the orbits and bring them into the inner solar system. So it can actually bring comets that were orbiting way out beyond it down into the inner part of the solar system. So it can bring them in for us to see. The others are long period comets. Now those exist out in what we call the Oort Cloud. That's a very large spherical distribution of cometary nuclei. They may have been pushed out there and probably were when the solar system was forming. As icy objects came close to Jupiter or Saturn or one of the other giant planets, they could get kicked out of the solar system but would end up in this great Oort Cloud. Meaning that they were still orbiting around the Sun but on very large orbits. And what you could have is that a passing star could come close to the Oort Cloud. It goes out to 100,000 astronomical units and we could have stars that actually pass within that distance from time to time. So we could have a star that passes close and that could deviate their orbits, bringing them into the inner part of the solar system. So passing star here could bring that comet and send it into the inner solar system. Neptune in the Kuiper Belt could do the same thing. So what happens to comets? Well, many of them will remain in orbit but eventually just fade out. They'll keep coming back but they do not have enough material to create a coma and a tail. So there could be essentially dead comets still orbiting around. They're not really visible because they do not come close to the Sun but there's not enough material left there to be vaporized. So we're not going to see a coma. We're not going to see the tails and the comet will be essentially invisible. I see rocky ball that orbits around the Sun. Some can actually be broken apart if they come too close to the Sun or one of the planets. So some can be broken apart, torn apart, and we see here one, as it passed close to Jupiter, that was actually ripped into a number of pieces by the tidal forces of Jupiter. So some of them can be destroyed by the Sun. Some of them can be destroyed by a planet or can actually impact into a planet. As we see here, comet Shoemaker-Levy in 1994 actually crashed into Jupiter and we saw all the pieces of it there and here we can see the scarred remnants of where it struck into Jupiter, of where it crashed into Jupiter. So cometary impacts can occur on other objects as well. This is just one relatively recent one that has been seen. So let's finish up with our summary of what we've gone over here and we've talked about comets. They have very elliptical orbits so they pass close to the Sun, but most of their time is spent in the depths of the solar system. The nuclei have been studied up close by spacecraft and even samples of cometary material being returned to the Earth. And finally, comet orbits can be deviated by more massive objects such as the planets, sometimes causing collisions so causing them to collide into planets. So that concludes our lecture on comets. We'll be back again next time for another topic in astronomy. So until then, have a great day, everyone and I will see you in class.