 Greetings and welcome to the Introduction to Astronomy. In this lecture we are going to talk about comets, some of the icy debris in the solar system as compared to asteroids which are more rocky material. So comets have been known since ancient times, and what they were considered at the time was that there was a change in the heavens. Remember that Aristotle had told us that the heavens were perfect and unchanging, so something new appearing in the sky, such as a comet, would be a sign of disaster coming, and here we see an image of that from a medieval tapestry. And that again is something that one of the objects that we have known since ancient times that did exist. Now comets do, we do see comets, and when we look at a comet in the sky we will see the head, here we see the head down towards the bottom, and we see the tail stretching back up. Now a comet does not flash across the sky, it is very different than a meteor. So meteors we will talk about in a future lecture, meteors or shooting stars are just flash across the sky, a comet would just sit there and would rise and set with the other objects. So it would slowly change its position much like a planet. So here we see the comet with its head pointing toward the sun down below the horizon and the tail stretching back away. Now when we look at the orbits of comets we find that they are different than the orbits of the planets. Not completely, they are elliptical, but that because of their elliptical they follow Kepler's second law, which means that they come in close to the sun and they spend most of their time far away from the sun. So a comet like Halley's Comet mentioned here would have an orbit of about 76 years and it would spend the vast majority of its time out in the depths of the solar system and would only spend a very short time in close to the sun where it would be visible. So most of the time a comet is invisible, only visible when it comes close to the sun. As I mentioned Halley's Comets, or Edmund Halley, was researching comets and noted that there were some similarities in orbits of comets that appeared in 1531, 1607, and 1682. And he made the prediction that this comet would return in 1758. And while he never lived to see this prediction come true, the comet did appear toward the end of that year and was then named in his honor. Its next arrival is scheduled for the summer of 2061, so now it is out at the very outer depths of the solar system, getting ready to head back into the inner portions. But again it will be decades before it comes close enough to be able to be visible again. Now when we look at a comet, what is the structure of a comet? Well a comet has a nucleus, which is the central portion, that is a ball of dirty ice, sometimes called a dirty snowball, and that is the material of the comet, everything comes from there. The coma around it is material that has been vaporized, removed from the nucleus by the heat of the sun. So this only occurs when it is close to the sun. Same thing with the tails. So it is material that has been vaporized, it is mostly hydrogen, hydroxyl molecules, carbon monoxide molecules, and similar very simple compounds. The nucleus itself is invisible. We cannot see the nucleus from Earth. We can see the coma when it gets close enough to the sun, that becomes much larger. The nucleus is only a few kilometers in size and would be invisible to us. The tails stretching back and the comets have two tails. They have a plasma or gas tail, which goes straight back, also called the ion tail here, and it has a dust tail which kind of curves. Now the tails tell us a little bit about the motion of the comet, and let's take a look at that here. The tails first of all always point away from the sun. So when the comet is coming in on its orbit, they lag behind. When the comet is heading back out, they lead. There is the plasma or gas tail, which is individual atoms or ions, and that is pushed straight back from the sun. So the gas tail goes straight back. The dust tail is slightly larger particles that lag behind in their orbit. So we can see those two, the one is curved backwards, those are the heavier particles that will always fall behind in the orbit as the comet moves. Now we can see this in a comet here, and there we can see very distinctly the bluish color of the ion tail heading back this way, and then the sweeping curve of the dust tail. Now that does tell us what direction the sun is and what direction the comet is moving. So the sun is in this direction heading down, and the comet must then be moving this way because the material in the tail, the heavier particles, are lagging behind it. Now how can we explore comets? We've seen them forever, but how can we actually go and explore comets? Well, the nucleus cannot be studied from Earth, it is far too small to see anything. The very first attempts were the flyby of things like Halley's Comet. Then that occurred in 1985, and those gave us the first images of a comet nucleus. The stardust mission allowed us to collect material, and as it flew through to catch those little particles in a special material designed to collect these, and the individual particles would be retained here and then brought back to Earth and could then be studied. So we could have individual particles here that were actually some of the icy material from comets, the debris left behind by comets, and we can actually study some little bits of that now, so we have samples of cometary material to study as well. We had the deep impact mission as an impact to look at the structure of a comet to study how sturdy they were, and that was the deep impact mission in 2005, which crashed into Comet Temple I, so making a new impact there, and again, a chance to study how that held up to this large impact, looking at what happened. The biggest mission was the Rosetta mission to study a comet, and this was orbiting the comet in 2014, giving us detailed high-resolution images of its surface, and it studied the comet as it approached the Sun to see how the material was vaporized off the surface, how that affected things, so it was actually in there in orbit right around the comet. And here we can see kind of the double-lobed structure of this comet where there's two pieces, one off to the left, one off to the right, and again, to try to look at those and try to understand it as we watched it come in close to the Sun as that material began to be vaporized. It also brought a lander to land on the surface, which did work. It landed on the surface, unfortunately, it bounced, and it bounced into a shady area and was not able to recharge its solar cells, so it only was able to get a limited amount of information. Very low gravity of the comet made it very difficult to just land easily, and the harpoon type system that was intended to anchor it did not quite work right, so because of that location it was unable to recharge its batteries and give us all of the information that we might have hoped to have obtained. Where do comets come from? Well, we're going to look at two types of comets. There are the short period, which is defined to be less than 200 years, and the long period comets, which are more than 200 years, so that's just kind of a cutoff, a rough cutoff as to where they are. We note that the short period comets orbit in the same plane in direction as the planets. They're very similar to the planets in that way. The long period comets are in random directions. So let's take a look at what we mean by this, and it gives us two points of origin for these comets. The short period comets come from the Kuiper Belt. Kuiper Belt out beyond the orbit of Neptune, so there are a lot of debris out beyond Neptune, but it orbits in a disk, much like the asteroid's orbit in the asteroid belt. So it is a disk-like distribution, and Neptune, being right at the edge of these, can cause the orbits to deviate, sometimes bringing them into the inner solar system. Now to scale to see the Oort cloud, as we go here, the Oort cloud is the origin of long period comets. It is a spherical distribution of these nuclei, so a great sphere around the solar system, and in this case a passing star could change the orbits, bringing comets into the inner solar system. So every once in a while, stars would pass by, affecting these orbits, and causing those comets to come in toward the inner solar system. So two places for comets, and this explains the distribution, since these are in a disk already orbiting like the planets, they will continue to orbit that way, and they're all orbiting in the same direction. This random spherical Oort cloud will then have comets that can come in from any direction, and any orientation, they can come in at high angles, and they can even be orbiting backwards relative to the rest of the solar system. So what is the ultimate fate of a comet? What is going to happen to these at the end of their lives? Well, a comet is just a ball of icy material, and will continue to orbit the Sun. So what can happen is that they will continue to orbit, but after thousands of years, there will no longer be enough material there to create the coma and tail that we associate with the comet to make them visible. So they will just be dark pieces of material orbiting through the solar system. Some of them may pass too close to the Sun, so they might get too close to the Sun and be broken apart, ripped into little pieces by the gravitational force of the Sun. And then those little pieces will continue to orbit, but they will not have the ability to look like a comet. They'll be far too small to give us a great coma and tail. Some may pass too close to a planet and end up being destroyed, and we see here comet shoemaker Levy, which collided into Jupiter. It broke apart and collided into Jupiter, and you can see the scars of those impact, which lasted for several weeks when this impact occurred back in 1994. We did not actually watch the impact. It occurred when that side of Jupiter was not facing us. However, Jupiter rotates very fast, and we quickly were able to see the scars left behind as those pieces of comet impacted into Jupiter with very intense energies. And just like asteroids, comets could strike Earth and cause damage as well. So let's go ahead and finish up with our summary, and we've looked at comets. We talked about how they have very elliptical orbits. They pass close to the Sun, they'll pass close to the Sun, but spend most of their time in the depths of the solar system. The nuclei of comets have now been studied up close by spacecraft, and we have samples of cometary material that have been returned to Earth. Also, comets, orbits can be deviated by more massive objects, and that can be the Sun that can include the planets and sometimes causing collisions releasing immense amounts of energy. So that concludes this 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.