 Greetings, and welcome to the Introduction to Astronomy. In this video, we are going to talk about the asteroids, or the small objects that exist between Mars and Jupiter. And these are primarily rocky objects, similar in some ways to the terrestrial planets, but in many cases more primitive, some of them being the very early building blocks that made up eventually, that originally built up the terrestrial planets. So we'll want to look at these in a little bit more detail. So let's start off with their discovery. How were these discovered? Well, for a while it was thought that maybe there was a missing planet between Mars and Jupiter that had not been seen. And that was because of the large gap between Mars and Jupiter. Within the inner solar system, the planets are all spaced very closely, all within about one-and-a-half astronomical units, or one-and-a-half Earth-Sun distances between the Sun. And then all of a sudden you went out to Jupiter at about five astronomical units. And that meant that there was this big gap between Mars at about one-and-a-half AUs and Jupiter at five AUs. So it was thought that maybe there was some kind of missing planet there. And in 1801 Piazzi discovered Ceres, an object that orbited in between Mars and Jupiter. And it was thought maybe this was that missing planet. Maybe this was what had been missing for a long time. So maybe this was the missing planet that people had thought existed. However, very shortly thereafter several more objects were discovered that had similar properties. So Ceres, which was originally perhaps considered a new planet, had to having been discovered, and in fact not that long after the planet Uranus had been discovered. So thought maybe there was a new planet here. As we found more and more objects, they then reclassified this as the largest asteroid. And of course Ceres is now known as one of the dwarf planets in the solar system. Now we know of many thousands of objects here, and the vast majority of them orbit between Mars and Jupiter. So let's take a look at those orbits. And what we see is that there are different types of asteroids in different locations. We have the main belt asteroids, where we find most of them, and that is between Mars and Jupiter. So that would be those highlighted in the white here. So they are orbiting between Mars and Jupiter, and those are what we call the main belt asteroids. Most of them orbit in circular or slightly elliptical orbits, and travel in between those two planets. Now there are other asteroids as well. We have the Trojan asteroids, which are related to Jupiter's orbit. They're here in the green. And those are Trojan asteroids, or ones that share Jupiter's orbit. So they orbit at Jupiter's distance from the sun, five astronomical units, and with the period of Jupiter of about 12 years. So they follow Jupiter's orbit, but Jupiter is here, and they are 60 degrees either in front of or behind Jupiter. So they orbit with exactly Jupiter's period, so that as Jupiter moves, they move and stay right ahead of Jupiter, or follow right behind Jupiter. So those are a group that happen to be in this stable position. It's a stable gravity well at 60 degrees between Jupiter's gravity pulling on them, and the sun's gravity pulling on them, so it's a very stable area, and asteroids that find their way into that section will end up staying. And then finally there are the Apollo asteroids. Apollo asteroids are those that cross the orbit of the Earth. So you see, while the vast majority of them are out here by Jupiter, or between Mars and Jupiter, that there are a few in the inner part of the solar system. Only a couple shown here. But anything that crosses the orbit of the Earth is an Apollo asteroid, and that's one of the ones that we would actually have to watch out for. Those are the ones that have potential to impact us here on Earth. Now how can we classify asteroids? So we have the C-type asteroids, which are what we call carbonaceous. These are made up of silicates, or rocky material, and organic compounds. And recall that organic compounds does not mean life. That simply means that they are carbon compounds. Carbon compounds tend to be sooty, and therefore these are relatively dark objects and harder to find. So they're not reflecting a lot of light. These are important because they are primitive. And that simply means that they have not changed since they formed billions of years ago when the solar system was forming. They are essentially unchanged. And that is important because it gives us a chance to now study those objects that would have one point four and a half billion years ago been colliding together to form the Earth. So on the Earth they all got melted together and separated by densities. Here we're actually seeing those original objects, so it's a chance to be able to study those things that billions of years ago made up the Earth. Now we also see S-type asteroids. These are stony, and they are brighter, so they're easier to see. These are dark, these are bright, and they have fewer carbon compounds, so they're less sooty, they reflect more light, and they're a lot easier to see. When we look at distances, the S-type are closer to the sun. So on average when we look at the asteroid belt, the S-type asteroids are closer to the sun, and the C-type asteroids, the carbonaceous ones, are further away. Now finally we also see the M-type asteroids. These are metallic, so they are not primitive asteroids. They are things that have been differentiated because they are solid chunks of metal, which would not have normally just formed. They must have been inside a larger object, which was able to be heated up and differentiated forming a metallic core, and then likely was broken apart in collisions. So C-type, S-type, and N-type is the three classifications of asteroids. Now when we look at the orbits of the asteroids, they're not random, they're not just spread out throughout the asteroid belt, they have certain areas where they occur. So there are some regions with fewer asteroids, and some with more, and we see those gaps here and a couple others here and here where there are regions where there are no asteroids, or hardly any, and these are resonances with Jupiter's orbit. That means that if we look at what's occurring here, for example right here, we have the asteroid orbits three times every time Jupiter orbits once, and that gives us a resonance with them and that every time Jupiter is back in that same spot, then the asteroid is in the same spot and they give a tug, an extra little tug, so that over billions of years this area has been cleared out by Jupiter's gravity. There are other resonances that occur as well, and regions where there are far fewer asteroids because of Jupiter's gravity. And this is similar to the gaps that we see in the rings of Saturn, they are caused by Saturn's moons, well Jupiter does the same thing to the asteroid belt, and perhaps the asteroid belt formed because of Jupiter's gravity, keeping a planet from actually being able to become dominant there. So, how can we explore these asteroids and what do we see? Well, first of all they cannot be studied from the Earth. They are far too small to be able to see any detail. So we can detect them, we can see their light being reflected, but we can't actually study them in any detail. So two of those that were explored were Gasper and Ida, these were some of the earliest asteroids explored, and here we can see they look just like small rocks with lots of little craters on them. These were the first asteroids to be observed by a spacecraft. So the Galileo spacecraft flying on its way out to Jupiter had to go through the asteroid belt and was planned to go by a couple of these asteroids to give us some images and our first close-up images and looks at these types of objects. And we can see again that they're very irregularly shaped so that they don't look like moons or planets that we've looked at before. Everything was always spherical because they were large enough. Now we're actually looking at small objects that are not big enough to be able to pull themselves into a sphere. In fact, Ida here, down here, actually has a moon of its own, a very small object, from an impact material that was impacted off of it and then never was able to break orbit. So it remains orbiting around dactyl. So they're small, they're irregularly shaped, and they are heavily cratered. They have not had any kind of geological activity going on on these. Now we've continued to explore asteroids and we've looked at a few others and we see here the Vesta, Vesta as seen by the Dawn spacecraft, which orbit, it was an orbit of Vesta for a year. So for a year it went into orbit, orbited around Vesta and explored it. We see lots of large impacts, so several very good-sized impacts. Not bad considering the size of this object. There are a lot of good-sized impacts that have occurred. So we've now had some up-close looks at not only Vesta, but also one of the dwarf planets and that would be Ceres. Ceres has also been explored by the Dawn spacecraft, so it observed Vesta. Then it went to Ceres, it had enough fuel to break orbit of Vesta and head to Ceres. So it went into orbit there in 2015 and studied again. This object, looking at craters and the different structures and one of the things that was interesting that we found as it was heading there was noted some of these lighter colored areas and it was wondered what kind of things these might be and it looks to be that they are possible deposits of salts, maybe possibly from an impact or churned up from an impact or things that formed long ago. So something different. You can see they very much stand out from what we see as the rest of the surface of this dwarf planet. Now one of the other things that we want to look at here is the possibility of asteroid impacts. So what kind of impacts can we get for an asteroid? Well, the Apollo asteroids are the ones to look at, especially for the Earth. They cross the Earth's orbit and that makes them a collision hazard, that they have the possibility of colliding into Earth. Sometimes we call these, also call them near-Earth asteroids. So a few of these have been explored and in fact Eros was explored by the near-shoemaker spacecraft and that was looking at one of these near-Earth asteroids. However, there are many, many of these, we've catalogued many of them, but there are far more including many that are just a kilometer or so in size and these still remain undetected. So there are many of them that could cause significant damage on the Earth that have yet to be detected. So what kind of impacts and results can we get? Well, here's an example of one of these. This is the Tunguska event from 1908 and this is an object exploded in the atmosphere over Siberia and laid down the trees. So the massive explosion laid down trees flat over an area similar to the size of a good sized city. So a major city would essentially have been flattened by this impact. Were that to have occurred over a city? Fortunately, it occurred over a relatively deserted area and did not cause much damage. But these impacts do occur and have occurred here in the early 20th century and have also occurred more recently in Shalabinsk and there in that case a 20 meter object exploded in the atmosphere and that was essentially 30 times larger, 30 times more energy than the nuclear warheads that were used on Japan in World War II. The difference would be of course that there was no nuclear radiation or fallout but the amount of energy released was much larger than even those. So these impacts have occurred, major ones even within 100, 150 years, smaller ones that can still cause some significant damage even within a decade or so. Now one of the major impacts that we look at is the extinction of the dinosaurs. So what is the evidence that an impact caused the dinosaurs to become extinct? And we've put this back to an impact 65 million years ago and that changed the climate of the earth. When you crash something into the earth you can throw so much material into the atmosphere that you can significantly cool off the earth. And this caused an extinction of 75% of the species that existed at the time. So we see some evidence for this and in fact here we see off the Yucatan Peninsula in Mexico. We can see the crater remnants of this through a gravity map where we can see where the gravitational field is a little bit different where there is material still stuck below that. So we're seeing evidence of the crater remnant still present there and you can see the coastline here sketched in white of Mexico there and then the impact partially on land, partially under the sea in this case. The other thing that we see is a layer of an element called iridium. And when we look at that, iridium is something that is very rare on the surface of the earth but is more common in meteorites. And when we look at that layer we see that here there is that layer right in between. Below this layer are the dinosaur fossils. Above that we see no dinosaur fossils. So there's a big difference in what we see above and below this layer. This layer dates to about 65 million years ago. So those are a couple of the pieces of evidence that really convince us that it was a large impact that was responsible for causing the dinosaur extinction. Now how often did these impacts occur? Well it really depends on the size of the asteroid. So a bigger object is going to hit us much less often than a smaller object. So things like annual events which can still be pretty big can actually cause pretty big damage that do happen on an annual yearly basis, some place over earth. Something several meters in size will strike the earth on an annual basis. But as you get larger and larger things like the Tunguska event here occur only every couple hundred years. So doesn't mean we're safe since one occurred in 1908 but does mean that they only occur every few hundred years on average. So doesn't mean we could get another one but on average that's when they would form. Now when you get to the catastrophic events those are over in this level things that are a couple kilometers in size and those occur on the million year time scale. Those are where you could actually get global events that would cause significant damage. So when you get something that large they only occur on the million year or so stage. So not as common but those are the ones that could cause significant damage and it doesn't matter where on earth they hit there you're getting actual things that are global. So if they occur in one continent still they will have global effects maybe not directly from the impact itself damage but from areas that where material is thrown up into the atmosphere and making significant changes to the climate. On the other hand the extinction events the most largest those happen on average every couple of hundred million years. They've occurred in the past it's not that this one here where we talk about that that caused the extinction of the dinosaurs is the only one that ever occurred and caused extinctions. They have occurred a number of times in the past and there have been some that have been more intense than the dinosaur extinction that caused an extinction of ninety percent of the species that existed at the time. So there have been some that have occurred even just over the last few hundred of millions of years we've seen a number of these massive impacts and that does mean of course that they will happen again in the future we just don't know we may not even know that that object that's coming before it actually gets close. So what can we do about this? Well not necessarily a whole lot there are some surveys the space guard survey is one of these that is trying to detect ninety percent of the near earth asteroids that are at least a kilometer in size so things that could cause some very significant damage and an idea is to give some advanced warning to what is going on. Of course if we look at that that is ninety percent of these objects that means there's about ten percent of them that would not have been detected by this. So what are the problems with this? Will we sketch in a lot of the orbits are sketched in here and the problem is that it's difficult to determine the orbit accurately because these are small objects when they pass close to another object whether it be earth or Mars or Venus or Jupiter their orbits can be deviated so you have to try to take everything into account and they can even pass close to each other and change their orbits a little bit so it's very difficult to know exactly where they're going to be coming and it is difficult to detect and track all of these objects because of the constant changes and because of the existence of other objects that we do not even know about. So what happens if we find something you know what can we actually do about it if we detect some object on a collision course with earth well the one thing you really can't do it's unlikely to be able to destroy the object even a large enough one a nuclear warhead is unlikely to destroy it completely and really would just split it into a number of objects so instead of a single impact you'd end up getting a lot of slightly smaller impacts which could be just as devastating to life on the earth. The best bet is what we call deflection which just gently nudges the object doesn't take a lot of energy but the problem is you have to know about it in advance so as long as you have years of notice that this object is coming you can plan a mission to change the orbit of this asteroid and you don't have to change it very much it has to be precisely aligned to strike the earth and if you straight change it off course even by a very small amount then you can have it avoid hitting the earth now it does depend on how far away it is the closer it gets to earth the harder it is to change that orbit the further away it is the easier it is to change but of course the question is will we know of it long enough in advance to be able to plan anything and to be able to launch something and will we be able to change the orbit will we know the orbit accurately enough to know how we should change that so let's finish up as we do with our summary here and what we found is first of all asteroids exist in different parts of the inner solar system that we saw that there were three classes of asteroids C M and S type based on their compositions what they're made up of we've looked at several missions that have actually looked at these asteroids up close and studied them giving us our first good images of asteroids in the last few decades and finally we talked about asteroid impacts these have occurred in the past and will occur in the future and there are a few things that we can do to try to minimize the effects of those impacts so that concludes our lecture on the asteroids 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