Today we're going to turn to the outer solar system and rather than focus on Jupiter, Saturn, Uranus, and Neptune, the four gas giants that make up the outer solar system, I'm going to focus on the little guys. The many moons that orbit these planets and the comets and asteroids. That make up an important part of our outer solar system. I'm going to begin by talking about comets and asteroids. Historically, comets were thought as harbingers of disaster. Possibly of death. The fall of Jerusalem was heralded by a comet, the eruption of Vesuvius. Was preceded by a comet stretching across the sky. The fall of King Harald the King of England who was over thrown in the Norman conquest. Here's the wonderful Bayeux Tapestry it's a place I absolutely recommend visiting some day is by you to see this incredible tapestry. And embedded in this tapestry is a comet. And here are the people staring up seeing the comet, knowing that it's telling them that bad things are about to happen. They also associate a comet with the Black Plague. Breaking out in London, and for the Texans, the fall of the Alamo, a central event in the Texan mythology, was also linked to a comet. Today our pictures of comets are a bit different. So, what are comets? Comets are mostly dirty ice balls. Made mostly of water but significant amounts of organic materials and silicates. We think that comets formed in the outer portions of our solar system, outside the snow line, so out of the orbits of Saturn, Uranus, Neptune, in the protoplanetary disc. And the protoplanetary disc and its outer portions much of the water and some of the heavier elements were condensed into these dirty snowballs. Some of them merged to form Saturn, Uranus, Neptune. But most of that material that didn't end up in the middle of those planets were ejected by the solar system, by interactions with the planets. So that the comet was on an orbit in the outer solar system that was dynamically unstable. And was being perturbed every time it passed around in its orbit by say Uranus or Neptune. Those perturbations drove the comets out of the outer solar system. And many of the comets were probably ejected from our solar system entirely. And this commentary material is wandering outer space, but some of the comments were rejected out to large distances to make up what we call the Oort cloud. The Oort cloud is a nearly spherically symmetric distribution. Of these dirty snowballs, out well beyond the orbit of Pluto. What happens with these dirty snowballs out there is every now and then, a passing star comes close enough to our solar system to perturb the orbit. Of some of these dirty snowballs and send them crashing down into the inner solar system. Thus, while planets move on nearly circular orbits, whether we're looking at the inner planets of Mars, Earth, Venus. And Mercury. Or the outer planets like Jupiter, Saturn, Uranus and Neptune. They all move on orbits that are close to circles. Just mildly elliptical. Comets, which fall all the way in from thousands of Au, thousands of times the earth's distance from the sun. Into the inner solar system. Move on orbits that are very elliptical. As they move on these orbits and get close to the sun, the comic is heated up by the solar radiation and the evaporated material interacts with the solar wind thus forming this dramatic tail that extends outward leaving to the famous cometary appearance that we see when these remnants of our early solar system plunge into the inner portions. Of our solar system come within the orbit of Jupiter and every now and then, come close to the Earth. Most comets are not in the inner solar system but out in what we call the Oort Cloud, the Oort Cloud's quite far away so it's a distance of about 50,000 astronomical units. That's far enough that it's a significant distance towards the nearest star. So this comet cloud that surrounds us is as I've mentioned effected and perturbed by the stars and every now and then a comet rains in on the inner solar system. And you'll notice that the distribution of comets around us. Is close to spherical. Unlike the plants, which arrange themselves in a disc, the comets roughly form a thick sphere, centered around the sun. In addition to the Oort Belt. There's something we call the Kuiper Belt, which is also made up of remnant material from the early phases, the formation of our solar system. And Pluto is one of many Kuiper Belt objects. One of the things that astronomers have discovered in the past decade or so, is that Pluto is but one of many objects at the same time. And if we were to call Pluto a planet, there really is at least another dozen objects like there out there that are really ought to be grouped together with it and then it makes more sense to think of it as one of many objects that ran, that orbit the sun in this Kuiper Belt. Comets sometimes have very dramatic effects on planets in the solar system. One of the most dramatic ones that we've seen in recent years is when comet Shoemaker-Levy split apart. And we see the comet first being torn apart into many pieces by the gravitational field. Of Jupiter, and then the different pieces of the comet rained in on Jupiter. And created these huge portubations on Jupiter's surface. And remember Jupiter is much larger than the Earth. So each one of these explosions. That you see on Jupiter's surface from when a comet rained in and hit it. These are larger than the size of the Earth. So comets can have quite dramatic effects. We've turned the tables on comets recently, we think maybe comets, we're certain comets have hit the Earth. We've started hitting comets. And by doing so we've started to learn more about their properties and back in 2005, NASA had a mission called Deep Impact. That actually sent a massive copper ball, into a comet. And by doing so was able to study some of the composition of the comet. So what you see here is, the copper ball went in. Some ejector flew out and then strong numbers pointed many different telescopes towards the comment and temple one and they were able to learn much more about the properties of the comets. One of the things that these observations revealed is that comets. Have actually a long term presence of surface water. So if water is one of the ingredients for life, perhaps even comets themselves are places where life could thrive. One of the people I'll be talking to in our interview part of the course, is Freeman Dyson and one of the things that Dyson has speculated on. Is that life could originate perhaps and maybe, and certainly thrive on the surface of comets. So perhaps comets are associated not only with, as harbingers of death and perhaps if they hit the Earth, they would cause lots of death and destruction. But also is sources of life. And water. And here is this notion of, that life might persist and survive and thrive on comets. In addition to comets, the outer solar system has other small objects. And the other one we want to talk about are asteroids. Asteroids you should also think about as remnants of the material that made up the early solar system. But think of them as rocky rather than icy, so these rocky asteroids are classified into three main groups depending on their composition. Carbon rich, stony, or metallic. The metallic ones contain lots of material like iron. Most of the asteroids are located in what's called the asteroid belt, here's the orbit of mars, the orbit of Jupiter. And most of the asteroids lie between the orbit of Mars and Jupiter. And some of the asteroids are quite large, some larger even than some of the moons that we'll talk about in a few moments. In addition to the main asteroid belt there's several other very interesting groups of asteroids. There are those that are trapped in an orbital resonance so they actually follow Jupiter as it orbits around the sun, these groups here and then there's a very dangerous group of asteroids, the near earth objects. Asteroids that are far enough in that on their orbits they cross earth. These are of a lot of interest and potentially worry because when those asteroids strike the earth and once an asteroid comes into our atmosphere, becomes a shooting star. We call it a meteor. These killer asteroids can produce a great deal of damage. One of the things that you did. In the problem on the last lecture as you work out the energy deposited by a relatively modest asteroid hitting the earth. And you've seen that an asteroid can carry much more energy than the most powerful nuclear bomb. This shows the rate at which asteroids hit. The Earth, today. So, we can look at, on this plot, there's a lot of information. So, I want to talk about this. So, this is a function of how big the asteroid is, what its diameter is in kilometers, ranging from one that's 10 meters, or 0.01 kilometers, to a kilometer to 10 meters. On this axis, we see a plot of how much energy the asteroid produces in units of megatons. All right, so megaton is a unit that we usually use when we talk about nuclear explosions, and a 100 megaton would be a bomb. Is much larger than the bomb that was dropped on Hiroshima. And you can see that a kilometer asteroid is a thousand times stronger. 10 kilometer asteroid a million times stronger. The K.T. impact and this is something we'll talk about when we talk about the Earth's history. This is when we believe a major. You know, ten kilometers size asteroid struck the Earth, near the Yucatan Peninsula produced an enormous explosion, which wiped out much of life on Earth, including the dinosaur population. This happened not so long ago, and this plot here shows the rate. So roughly once every 100 million years or so, we have a major impact that affects all life on the planet. And more often, perhaps, once every 1,000 years or so, we can anticipate an event that's pretty significant one that will have a very large local impact, the biggest event in recent history is the Tunguska event that took place fortunately in Siberia but was heard hundreds thousands of miles away, hundred and thousand miles away. Leveled a large area of forest. And basically carried as much energy as a nuclear explosion. One of the things astronomers are trying to do is to track the near Earth asteroids to know what's coming, this plots a little out of date, but this shows what range of asteroid's been tracked up to now. We can see that we actually know about all the very biggest ones nearby. So we don't anticipate a major hit that will wipe out all civilization coming very soon, but there still could be fairly large asteroids coming towards the Earth. And there's a goal that the scientific community's studying asteroids of set of pushing this limit this way, detecting smaller objects so that we know what's coming and potentially can devise strategies for avoiding this disaster. So let's Leave this subject of killer asteroids for a moment, I'd like you to go make some estimates of properties of life on comets and then we'll come back and talk about moons.