 Greetings and welcome to the Introduction to Astronomy. In this lecture we are going to talk about the asteroids. And that's the beginning of our discussion of some of the smaller debris in the solar system in this unit. So we will look at asteroids and comets as well as other small objects. So, asteroids, when were they discovered? Well, it was thought because of the spacing that there might be a missing planet between Mars and Jupiter. There was a large gap there relative to what we saw elsewhere, and there seemed to be various numerical patterns that fit. So, people searched that area, and in 1801 Piazzi discovered Ceres in this location in between Mars and Jupiter. After that, several more objects were discovered. So, first Ceres was thought it might be a new planet. It was then reclassified as an asteroid, and as we've previously discussed, is now considered one of the five dwarf planets in the solar system. Now we know of thousands of asteroids, Ceres being the largest of them, but many, many smaller ones exist as well. Where are these asteroids? Well, they're in different places. We have the main-belt asteroids in white here, and those are, that's most of the asteroids, those are located generally between the orbits of Mars and Jupiter, and they orbit around just pretty much as the planets do. They're a little bit more irregular than the planets, a little more elliptical orbits on the average, but typically they orbit in the same plane in the same direction as the planets. Now, the Trojan asteroids share the orbit with Jupiter, and we noticed that there are some on one side of its orbit, and some on the other side of its orbit, and they orbit with Jupiter. Jupiter never crashes into them, and just remember, if they're at the same distance as Jupiter, they orbit with the same period. So, as Jupiter continues through its orbit, they will orbit as well with the same roughly 12-year period, and they are caught in a gravity well with Jupiter and the Sun, so it's sort of a stable point there, and when asteroids fall into that area, they end up being captured there. And then finally, there are the Apollo asteroids, which cross the orbit of Earth, and you can see a few of them scattered in here that go in closer than Earth, and some even closer than Mercury. Those are the ones that are the most danger to us, because they do cross Earth's orbit and means there is the chance for an impact that we will look at later on. Now, how do we classify these asteroids? Well, we have three general types. We have the C type, which are carbonaceous, and that's silicate rock, so rocks and organic compounds. These are important because they are primitive asteroids, which are unchanged from the early solar system. So, these are the things that made up the planets, types like this or types that built up planets like Earth. We also have the S type that are present, and these are stony. These have fewer carbon compounds and just stony. Now, if you look at them, you kind of note that the carbonaceous ones tend to be further away from the Sun, and the stony ones on average tend to be closer, so there is an interesting distribution there as well. And then we see the M type. The M type are metallic meteorites. These are definitely not primitive. These are ones that formed inside something larger. They had to form from metallic materials that differentiated inside a larger body. Now, when we look at the asteroids and how they're distributed, we note that they're not uniform. If we look at them and we looked at this with the rings of Saturn as well, but there are regions called the Kirkwood gaps where there are fewer asteroids. These are resonances with the orbit of Jupiter. Now, we saw this with Saturn, but here we see, for example, a 3 to 1 resonance, meaning that an asteroid at that location would orbit three times every time Jupiter orbits once. We also see resonances of 5 to 2, 7 to 3, and 2 to 1 here. And there's more detail here. We're not going into all of the detail, but those are gaps. Those are areas within the orbits where an asteroid orbiting there would constantly feel an extra tug from Jupiter because of that resonance, and they would slowly clear out that region over billions of years. Now, asteroids are essentially invisible from Earth, except as dots of light. So, how do we study them? Well, the only way to study them is to travel there. And the first study of asteroids were Gaspra and Ida by the Galileo spacecraft. So, the Galileo spacecraft was heading out to Jupiter, and it was select two asteroids were selected that were roughly on the route to Jupiter to be passed by, to be studied. These are relatively small asteroids. We see that they are not spherical by any means. They're very irregularly shaped, but they're very small, irregularly shaped, and heavily cratered. Ida is actually known to have a small moon called Dactyl, and you can see that in the image here, that it is off to the side. A moon probably from an impact, something that didn't quite get thrown away from it in the impact, and ended up being in orbit. Now, since then we've done more detailed explorations of asteroids, and then we had the Dawn spacecraft that studied two asteroids, Vesta and Ceres. Now, in 2011 it entered orbit around Vesta, which we see here, and orbited that for about a year. We saw many large impacts and surface rocks were found to be basaltic in many cases, and that means, if you recall that term from the moon, that those are formed from volcanic activity, so some kind of molten material that was once present on this asteroid. Now, of course, it's heavily cratered, so there's been nothing there for a long time. Then Ceres was studied. After it left orbit of Vesta, it traveled to Ceres, arriving there in 2015, and it saw some light spots there, which were possible salt deposits, which were very interesting. While the craft was traveling there, we saw these bright areas, and the question was what these could possibly be. But it means that there's a possible previous or current ocean below the surface. So still a lot of things we do not know about these objects. The Dawn mission ended in 2018 when the craft ran out of fuel and is no longer able to keep its antenna pointed toward Earth. So it's still there, still orbiting Ceres, but is unable to communicate with Earth. Now, the best way to study anything is to actually get a sample of it. So we've had several sample return missions. The first was the Hayabusa mission in 2005, which returned a very tiny amount, less than one gram of material in 2010, from the asteroid Itakawa. Now, that was one exploration, and then the Hayabusa-2 mission in 2018 was able to return about five grams of material in 2020 from the asteroid Ryugu. And that material will now be studied on Earth. It is samples of these asteroids here on Earth that we can now study. Another big mission was the Osiris-Rex mission in 2021, which is going to be returning a relatively large sample. We don't know exactly how much, between 60 and 2,000 grams, so that's a big range there, and that is due to return to Earth in 2023, September. So that will be a larger amount and we'll be able to share more material and study about these asteroids. Now, Bennu was an interesting asteroid here, and here we see the asteroid as it rotates from various images. It's not a super-large asteroid and is really covered in a lot of different material there. A lot of different debris, and the idea was to collect this debris, and that was done by touching down to this very gently and kind of giving a little bit of a puff there. You'll see this and then puffing out the material, and then a collector that could then collect some of that material that was thrown up in the impact. So that was then collected. The sample container has now been closed and sealed and is on its way back to Earth as of this recording and is due to arrive in September of 2023. How about impacts of asteroids? Now, we looked at some of the large impacts before and we talked about the Apollo asteroids. These are ones that cross Earth's orbit and are the potential collision hazard. Sometimes we call these near-Earth asteroids. Now, one of these was explored, that was the asteroid Eros, which was explored by the near-shoemaker spacecraft. But there are many objects, even just a kilometer in size that remain undetected. And remember that even something a kilometer in size could do very significant local damage to the Earth if it were to strike. Now, one example of that is, and we've looked at this previously as well, was the Tunguska impact. And that was in 1908, where an object exploded in the atmosphere over Siberia and flattened an area equivalent to a major city. More recently, we had the impact in Chelebinsk and that was in 2013. And that was about a 20-meter object that we'll see here as it comes into the atmosphere, again breaking up in the upper atmosphere. 20 meters, not too large, but did cause a little bit of damage to various buildings and broke apart there. So this is well beyond any shooting star that you would see. This is a much, much larger object, and if you think about it, the explosion was 30 times larger than the warheads that were used in World War II. Now, 30 times larger in terms of energy released, of course, this was not nuclear, so there was no nuclear fallout from it, simply that much energy released from just a 20-meter object. So a 20-meter object where it had not broken up in the atmosphere and striking the ground could cause significant damage. Now, how often do these impacts occur? Well, daily we get hit by various objects. Some will do damage, others fall harmlessly in the ocean. The frequency will decrease as the size of objects increase. That's fortunate for us. We get hit a lot by very small objects and not very much by the very large objects. So the size of the object, looking at the diameter here, we're going up to an extinction-level event, and we're looking at something that is 9 kilometers. That's like the one that caused the extinction of the dinosaurs. Smaller events like Tunguska were probably in the 60 or 70-meter range, so a little bit bigger than the Chelebins was considered to be about a 20-meter. And this shows you about how often these occur, so something that's about the 20 meters occurs every few decades. Tunguska is a century, or maybe a couple centuries. Something like the one that caused the extinction of the dinosaurs, we're talking every 100 million years. But then those are the ones that can have significant global events, those are what we call the extinction-level events. And these have occurred multiple times in the past. So what can we do about this? What can we do to try to protect ourselves from these asteroids? There is a lot of debris in the solar system, and we've seen that many of them do cross Earth's orbit. Well, one thing we look at is the Space Guard Survey. That is an attempt to try to detect over 90% of near-Earth asteroids that are at least a kilometer. An attempt to give us some advance warning of it. Advanced warning of an impact. However, it's very difficult to determine the orbit accurately. Why? Because the orbits can be very easily deviated by a minor interaction with one of the planets, or even with other asteroids. So it's hard to predict what it will do 50 or 100 years from now, unlike the planets which are not going to be deviated by the interactions with an asteroid. The asteroid will be easily changed, its orbit. And it is also difficult to detect and track all of these objects. It's not something easy to just collect all of these objects. There are so many of them. So even ones of decent size, they're very dark and hard to detect. So what if we find something on a collision course? Well, you can't destroy it. It would be very difficult to destroy something like an asteroid. So the best thing that you could do is to try to deflect it, just to nudge it off course. We don't need to destroy it. We just need to make sure that it doesn't strike Earth. And one example of this is what we called the DART mission from 2022, which was the double asteroid redirection task. And we're seeing here in the animation the impact there, and a spacecraft was sent to crash into an asteroid. Now it's actually two asteroids, one orbiting the other. And the idea was to see if we could change the orbit. And results have shown that we were able to shorten the orbit by about 32 minutes. Now that itself was not to change anything or protect us here on Earth, but to see how much of an effect a small satellite can have on an asteroid. And if we could detect something far enough in advance, we could do something like this to be able to deviate it. And again, the Earth is a very small cross section in space, so things have to be lined up just right. And a quick little nudge can make me the difference between getting struck and missing by hundreds of thousands of miles. The difficulty again is time. Will we know of it long enough in advance? So as long as we know about it long enough in advance, something like this may be able to help. Once the closer it gets, the harder it is to redirect. You need more and more energy to try to change that asteroid's orbit the closer it gets to Earth. So let's go ahead and finish up with our summary. And what we've looked at this time is that we looked at the asteroids that exist in different parts of the solar system. We talked about the three classes of asteroid based on the composition. And we talked about several missions to explore the asteroids up close. We also looked at how asteroid impacts have occurred in the past and will occur again in the future and some of the things that we are trying to do to help protect Earth from these. So that concludes this lecture on 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.