Comets: Visitors from the Frozen Edge of the Solar System - Professor Carolin Crawford

good afternoon everyone aha great response today's talk is about comets and these are members of our solar system and fresh comets arrived in our skies every year now some of them are complete newcomers to the inner part of the solar system some are old friends doing their repeat visit as they swing past the Sun but most of them are only of interest to astronomers both professionals and amateur astronomers the public only tend to get whiff of how many comments there are when there's a really great comment in the skies that comes to the point where you can see it with the unaided eye well part of the inspiration for today's talk on comets is that we might be on line for the visitation of a great comet sometime in the next month comet Ison however you see a you go to the word might we also might not be on line for a great comet there's a wonderful quote from the very famous comet hunter called David Levy and it is that comments are like cats they have tails and they do exactly what they want what I hope you'll get a sense of from this lecture is why predicting what comets are going to do to some level is still fairly inexact science and give you a flavor of all the kind of unpredictability that comets bring with them now comets of course are not just recent phenomena they've been in our skies over the centuries and were well known to ancient cultures but you have to realize that they were quite a startling thing to see in the sky unlike the stars which are in the same relative fixed patterns which come round predictably from one night to the next from one year to the next even the planets which wandered a fairly stately pace relative to these stars comments appear first of all without warning they move rapidly relative to the background of star constellations they change from one day to the next sometimes they get brighter they grow a tail tell grows longer then they start to fade away and they get dimmer and then they disappear and all of this within the period of a few weeks a few months and this was scary in a way it was unexpected they were unpredictable and not surprisingly many cultures around the world comets were seen as omens messages from the gods here's one of the early depictions of a comet especially the comet as a a portent here is a good omen it's depicted as the Star of Bethlehem in giotto's adoration of the Magi and it's thought that this you know wave portraying star Bethlem as a comet was inspired by visitation of Halley's Comet in 1301 and that maybe that inspired Giotto to to make this move of course this is this is a nice omen traditionally comets were seen as a bad thing they were scary they would presage fear their presage famine war death of rulers bad things and here's just a series of a few seventeen sort of 16th century woodcuts and here you see a comet I think this is a depiction of a fourth century comet sweeping across leaving devastation in its wake here are the three great comics of 1680 1682 in 1683 the three Horsemen of apocalypse riding out in front of them throws bones and instruments of torture making up the clock numerals and here we have comets depicted as far resoures in the sky so basically not a good thing if you saw them they are linked or you know great comets because of this connection with bad things happening great comets have been recorded through time when they have either preceded or been associated with some great event so for example one of the best comets recorded in history one that was completely visible in daylight happened around the time of the assassination of Julius Caesar in 44 BC it happened just a few months later and here it is depicted on Ono Guster's see coin and it's on Augusta Caesar coin here's the comet and it's got a whole whole load of Comet tail's and it was saying he used this this appearance of the comet as leverage to say look you know there sees a soul ascending to heaven you know it's it's disapproving of what you've done and use it as leverage for himself to become the next Caesar there other instances you know it's not just Western cultures and the Incas saw a great comet just before they were sort of cruelly conquered by Pizarro and his men and there are other examples so the great comet of 1811 is mentioned Tolstoy's war and peace as an enormous and brilliant comet which was said to portend all kinds of woes in the end of the world and of course it just preceded Napoleon's invasion of Russia in this particular case so all along their associations of comets with bad things that have happened throughout history Halley's Comet in particular gets quite a bad rap here's a panel from the Bayeux Tapestry these chaps on the left they're saying all esteem around Stella which means they marvel at the star and this guy's scurrying across and whispering the bad news to herald and this again is because there was an apparition of Halley's Comet between June April and June in 1066 which obviously presage bad news for Herald perhaps better news for the William there are other examples of Halley's comet it appeared three years after the fall of Constantinople to the Turks at that point it was even excommunicated by the Pope of the time as an instrument of the devil allegedly even two relatively recent times comets are seen as not good things here we have the great comet of 1857 ripping the world apart as it comes past and even comparatively recently the 1910 apparition of Halley's Comet is again breathing do and destruction in its wake he have a skeleton riding the comet and very convenient the Eiffel Tower is being felled in its path and this was a very scary apparition and was deemed to be one because he it was coming so close to the earth that the earth had passed through the comet's tail and it was known at the time that comet's tail does contain amongst other elements cyanogen and some of the sensationalist media stoked up fear of mass cyanide poisoning and some very clever entrepreneurs did a brisk line and selling gas masks and comet pills of course the comet's tail is so diffused that even any elements that do you know that the earth does go through don't affect us in any sense we were not all mass poisoned but again it's interesting that how the media whipped up this storm about this cometary apparition and despite some of the astronomers trying to do a sort of counter motion fears of the comet of foolish and ungrounded we are very sensible information to take them so there have been numerous fantastic comes all of these are what I'm calling a great comet and a great comet is one that's visible to the unaided eye sometimes in daylight massive tail across the sky bright colors maybe difference of colors may be more than one tale well-known and they're depicted in all these wonderful drawings and paintings that I'm showing you and if you stack them up over there recorded you know last couple of millennia on average they're about three truly great comets a century I don't know about you I'm beginning to feel a bit shortchanged now even though they were recorded in these paintings I mean partly because they were seen as such omens there are good records of their appearances say particularly from the Chinese astrologers astronomers who again saw these stars as a bad omen and they believed recording the Comets form so here's an old Japanese silk painting where you have different forms of Comet that presaged different kinds of disasters and they also record the route the comet takes across the sky excellent records which go back to like 1500 BC but astronomers still use now for mapping out the orbits of comets and investigating them but they still didn't record their thoughts on how the gods manifested this apparition and in fact that came to Aristotle in the ancient Greeks and he is the line that ever seen to the ancient Greeks the whole of the sky was perfect and unchanging it was the heavenly realm something as unpredictable and weird as a comet did not belong in the skies it had to be something that happens at the earth of the top of Earth's atmosphere as some kind of exhalation of gases I think was a phrase and that notion persisted for many centuries and it was only challenged when the Danish astronomer Tycho Brahe he was inspired by another great comet this is the great comet of 1577 and he thought he could test Aristotle's idea and he measured what is known as the parallax of the comet and how you do this is you have two different observers at two different locations on the earth they each observe the comet at the same time relative to which background stars appears now the nearer the comet the more shift there is relative to the background between the two observers the further away the comet the last shift there is compared to the background stars so the two observers observed the comet and observed the moon and what they found is neither showed any parallax and he concluded that the comet had to be at least six times further away from the earth and the moon was and that is your first point where people realize that comets are part of the solar system then nothing to do with the realm of the earth there's something distant and like you know in the realm of the planets and the stars the true nature of comets though had to await a little bit longer until you have the English astronomer Edmund Halley and he was the person who underlined their fundamental nature he had just observed the great comet of 1682 it's one of these three comets in the woodcutter showed you earlier now it Newton had just come up with wonderful new calculus and Hallie was playing with it he was looking at the records of 24 different comets over that had been observed over the previous four centuries and trying to calculate their orbits and what he noticed was that there was a striking similarity in the orbit of the comet of 1682 with the one that had been observed by Kepler in 1607 and then also one in 1531 the orchard characteristics and the orbit was very similar it went the different way around the Sun if the planets go round that way the comet was going on that way we call that retrograde so it's going the wrong way around the Sun not just that but the orbital parameters you know how inclined it was compared to the the orbit of thee that the planet was the same and in fact he also knew the comet from 1556 was also went retrograde and his lightning leap of intuition was to realize this was the same object it came back every 76 years and we saw it only when it came close to the Earth and the Sun on an orbit that took it seven to six years to go around the Sun and he made a prediction he said the comet will return and he predicted it will turn in 1758 now as you see he didn't live to to see the comet arrive back on schedule but nonetheless when it did its return was heralded as a great scientific success and it now of course bears his name it's Halley's Comet it appeared in it appeared every time live December six years since here's a photograph from a 1910 apparition there was another great comet in 1910 but this is Halley's Comet and I asked visited us in 1986 now this wasn't a terribly good apparition because it came round the Sun when it was on the other side from the earth the sun's there there's the comet and Earth so it wasn't that brilliant in our skies it's been since gone out into the deep you know freezer space and it's going to return to us necks in 2061 so mark the newark new diaries everyone of course when we observe Halley's Comet now we don't have to stay on earth we can send spacecraft out to meet comets we can do this in 1910 we sure could do it 1986 and a half the tiller of spacecraft went to rendezvous with Halley's Comet in 1986 and the most successful property was the European Space Agency's Giotto mission which flew it was in about 600 kilometers of the comet and snapped some of these pictures before it kind of got splattered by all the stuff that's being evaporated off the comet and it cease to work and here is our first photo of the cause of all the activity in a comet this is called the comet's nucleus and it is the only permanent part of a comet and everything else stems from this nucleus it is a solid chunk of ice and rock and in the case of Halley's Comet is about sixteen by eight kilometers in diameter and it's composed of ice with little grit of dust in so the ice is not just water ice you've got ice of ammonia you've got a carbon monoxide carbon dioxide various other ices and embedded within that a small grains of grit and by grit I mean silicates and carbonate so things like certain sand small solid particles so think of it it's much like a giant iceberg a giant iceberg somewhere between five and ten kilometers across now most of the time this nucleus sits a long way from the Sun and it's really cold and it's dormant and it stays frozen and nothing happens but if it falls on an orbit towards the Sun it begins to heat up and when things heat up they no longer ice and you're beginning to see the start of this process on this picture of Halley's nucleus here the ice sublimates so as it heats up it turns directly not to water but to vapour and it comes off the cometary surface as Jets of Tyrael Jets of ice spraying all out of the place and turning into gas from the surface not just that as it sprays out it takes out some of that grit and that sand that's embedded with the ice and also sprays that out so what you're seeing here is all the onset of this activity on the surface of the cometary nucleus and this only happens when the nucleus comes and it's all bit round past the Sun so all of this material jets out here's another picture and you can see clear Jets of material coming off and it sort of Sderot cracks that open up in the eyes and let huge you know if you've got fissures then more water ices exposed and you get bursts of activity and altogether this material expands away from the nucleus because it's expelled at great speed and it forms a kind of halo around the nucleus and here you have Halley's Comet and you can begin to see the halo just forming here's a better example in comet Holmes the nucleus is right in the middle there the stuff sprays out and it forms this big envelope around the head around the nucleus and that can be as wide as a million kilometres okay that's way bigger than the earth it's approaching the size of the Sun so you have this enormous extended envelope of gas and material it sprayed out from that tiny nucleus and it goes so far from the comet it's only weakly held by gravity because the comet doesn't have an awful lot of gravity because it's so small and mass and as the gas gets far out from the comet it gets acted upon by the light from the Sun it gets ionized changed into charged particles and after that it gets swept back to form the tail and this is when you comment resembles what we generally think of a comet so all this this envelope and this tail all originates from that nucleus and once the material has left the nucleus it never it's never rip never goes back to nucleus it's left forever and you generate this coma and the tail as long as the nucleus is warm and as long as that nucleus is active you tend to get two tails in any respect or commenter it at least two tails I should say and there's a blue straight tail and occur more curvy fan-shaped yellow tail and these are composed of different elements of different types of stuff that's come from the cometary nucleus first of all you have the gas ions these are charged particles you also have the grains of dust and they're each acted upon by different thing so if that's your cometary nucleus and the Sun's up there somewhere above the ceiling you have first of all you have the solar wind this is a wind of charged particles it's streaming from the Sun all the time that acts on the ions that have been expelled from the cometary nucleus and it pushes them directly radially away from the Sun they fluoresce with that characteristic blue color and so the blue tail is made up of the gas ions it's kind of a plasma tail you also get the tiny particles of solid matter they get pushed out of the nucleus these get acted upon by the radiation so the push of the sunlight the enter of photons of sunlight and that pushes them a little way out of the nucleus and then it doesn't push them so hard that they form a straight tail they kind of get left behind as the comet carries on and that's why they have a more curved orbit and they glow yellow just purely because they reflect the sunshine so you have a tail made of the dust then you have a tail made of the gas ions and so any comment now you're going to begin seeing these two tails in the pictures the iron tail and the dust tail of course there are many different you know structures within tails as there are as many different comets the first thing though if this means that the tail always points away from the Sun this was known even about the 16th century that if the Sun is even below the horizon the comet tail always points away from it it doesn't stream behind the comet and that means that as the comet falls into the solar system the tail follows it but as it swings around the Sun and travels out so here is falling in travels out the kind chases its own tail so the tail just tells you more or less the direction of the Sun to explain just the effect of the solar wind on the dust tail and the the iron tail this you may not believe it is a picture from spacecraft pointing at the Sun you can't see the Sun because it's too bright now this spacecraft sits and it watches the Sun all the time it's looking for those big plasma ejections that the Sun gives the big explosions in the solar wind the Sun is about the size of this white circle in the middle it's actually blocked out by disk you can see that the sort of handle that holds this disc in place and the satellite is watching all this stuff that's streaming from the Sun that's a very bright planet these are stars it's interesting the stuff coming on off from the Sun and it's blue because it's not a normal light this would be an ultraviolet light however what you will see is if you're looking towards the Sun you will see comets as they swing around the Sun so these so much in satellites are incredibly good for watching comets as well so just watch the tail of this comet as it swings past the Sun and its orbit it's coming in here and then there's a big burst from the Sun and it just kind of sways and pushes the tail right round and it starts chasing its tail as it heads out of the solar system and so this is how the duck lid tails are affected by what's happening with the Sun and the two tails as they have very different structures this is the blue plasma tail now because it's charged particles that will also interact with magnetic fields contained in Neath the solar wind and sometimes of the sun's very active it will strip away completely the iron tail it'll get it'll have kinks and twists and you know according to what magnetic field is doing sometimes as I say it will completely disappear but it would just grow again because it's being replenished by material from the nucleus and the discount ends - fern out especially if the you know the comet's moving quite fast on a curve so here you can see a very wide dust tail and there's a greenish tinge to many of these comets and that comes from diatomic carbon it comes from cyanogen that's contained within the gasses coming from the cometary nucleus they tend to grow green glow green when exposed to sunlight in the vacuum of space here's another example blue I entail great big fan of dust tail now here it's very clear the comet has two different tails imagine now you were facing that comet head-on and looking at it from this angle when you see comets from that angle you will get appearance of two tails so from there this all fans out on this side and you have the iron tail on that side so comets can look quite strange with two tails apparently going completely opposite direction and completely contrary to what you would expect so these are that's the structure of how comets are formed because what happens to comets when they come into the Sun they don't all swing around the Sun here we are looking back at the Sun a few planets and stars look at these two little comets many comets on their first big adventure into the solar system fall splat into the Sun Suns got so much gravity they have no hope of escape and so there are many many comets which come fresh in to the fresh tourists to the inner solar system and they end up as part of the Sun and between the Sun and a comet the Sun will win so again we'll stop what she knows to watch this next comic coming now because you'll see what happens to a comet as it comes too close to the Sun so resume again there's the Sun the comet is going to come in from down here and it's going to disappear and then we're going to zoom in to the surface of the Sun on a very high-energy thing and you're looking for the tiniest trace it comes in across here you see it oh just replay it in a minute oh sorry sorry that wasn't so exciting oh yeah yeah so even though it looks like a comet should do something really spectacular it doesn't it reaches enormous temperatures remember the surface of the Sun 6000 degrees it just goes splat it evaporates as soon as it gets near and that happens to many many comets other comets get too close to the Sun they get affected by its gravity or they get heated up so much that those fissures that are letting out the the water ice from the surface open up too far and the comet begins to crack and disintegrate sometimes that happens again first time through comet comes too close to the Sun the heating the gravity's too much for it and it just rips itself apart it does depend on the size of the nucleus so if it's less than about a half a kilometer across it could be vulnerable to this happening or as you see comets get smaller with time maybe this is the adventure demise of many comets here's one photographed by the Hubble Space Telescope within this process of shedding bits breaking about large fragments breaking up that nucleus and forming kind of mini comets in its wake and this happens quite gradually here's a succession of images through April 2006 to showing how they separate off and those will all eventually evaporate way and that's the demise of many comets so you can see it's relatively few that as you make it round the Sun unscathed and become repeat visitors to the inner solar system but even then why say unscathed they get modified and changed by this close passage to the Sun now here are two repeat visitors obviously Halley's Comet on the right comet Bora Lee another comet on the left and they're actually look quite dark and if a comet makes repeated do you know goes round the Sun and out into the Deep Freeze of space what did happen what happens is that it gets baked and then frozen and then baked and then frozen and after this happens a few times it develops quite a solid crust first of all a lot of the water ice will evaporate away and the first year passes past the Sun most of that gets bored off some of the dust will get removed some of it may that does may actually fall through gravity back and compart the surface again but garages the ice gets preferentially removed and the dust remains and then what ice and material is left gets frozen baked frozen baked it forms this thick crust that then begins to insulate and protect the nucleus a bit more and these this means that the comics gradually become less and less active as they repeatedly repeatedly come past the Sun you need sort of cracks to grow in the surface to expose freshwater ice to grow the tail so again repeat visitors tend not to be as dramatic as fresh arrivals which are nice and full of water ice and their first time past the Sun and so you get this repeated baking the material that is ejected from the comet doesn't is lost from the comet forever and in the case of those very lightweight dust grains that get pushed out by the radiation pressure of the Sun now those are the things that maybe microns you know much less than the width of a human hair across they get acted upon by the sunlight and they form this wonderful tail you can see along the passage of the comet's orbit slightly bigger grains to things like the size of sugar cubes or small pebbles tend not to get pushed out they're heavier they don't get acted on by the radiation pressure they tend to just get left behind and so the orbit of every comet is marked by just a trail of debris of matter it leaves behind this means that there are places in our solar system where you just have random piles of cometary debris now if bat piled debris actually intersects with the passage of the earth around the Sun so for example maybe here's a comet maybe long since gone but we still got all the debris that is shared during its many passages round and if it intersects with the Earth's orbit that means once a year we go through a whole pile of junk and at that point that's when we get our annual meteor showers now not all meteorites are predictable there are certain times in the you get we here astronomers talk about the Leonid so they each required saw the Geminids or the person's each of those are generated from one of these cometary streams from a long time ago and it just become the same point every year because it's the same point in the Earth's orbit when we cross through the debris and how spectacular is showing is will just depend you know exactly where in that pile of debris we cross or whether we're going through it as it happens to be nighttime so there's a little bit of variation except we know it's going to have the same time every year and of course that's when we get the most spectacular meteor showers as if I'm talking about comets coming on orbits and the question is where do they come from and in fact why do we care even and comments are interesting because we think they're absolutely pristine material left over from when the solar system formed now I've talked about comets coming past the Sun each time a comet come past the Sun it maybe loses somewhere between a thousandth and a hundredth of its material when Giotto went past Halley's Comet you remember that initial picture I showed you for common tree nucleus it estimated the scientists rather estimated from the data but that nucleus is losing some of the order ten tons of dust and about 56 tons of water every second and so it's a quite a lot of material being lost and it's still about you know to say 1% 0.1% of the whole of the Comets mass every time it goes past the Sun gets lost so that means once a comet is started going past the Sun it doesn't necessarily live very long but the fact that we get continual new visits the fact we still get comments nowadays good I only live hundreds of thousands or tens of thousands of years which is short compared to the whole solar system means that they must be being replenished from somewhere and the question not only is where but also why that's important in terms of the history of our solar system now we think the comets must have form same time as the were just after the Sun just the rest of the solar system the asteroids the planets the moons and we all formed from a solar nebula it's depicted here there's the protosun a collapsing cloud of dusty gas around the Sun that eventually goes on and forms the planet there's a lot of material that doesn't go into forming the planets now closer into the Sun is warmer further out is cooler if you're forming something I see it's only going to form out in the cooler regions we have a region called the snow line it's about four times the earth-sun distance and you've got to be out beyond there before Isis can start to condense out of this this cloud of gas and dust around the Sun so comments have to form way out and the outer edges of the solar system and there are a number of places they can come from and where comet those dirty icebergs come from you can almost like divide them into two classes there are those that we call the short period comets these are the periodic comets that return about and say their period to go once around the Sun is the last than 200 years and then you've got the long period comet and these are the ones where their orbit if they orbit at all some just come past the Sun and carry on going out into space they're much longer than 200 years and they each have very different dynamics in terms of their orbits which suggest they come from different parts of the solar system the short period comets these are things like Halley's Comet with periods less than 200 years are thought to originate beyond all the gas giants beyond Neptune okay obviously this is a drawing we haven't got a nice picture of the solar system like this if that's the CERN this is the orbit of Neptune there's a whole Bell to trans-neptunian objects called the Kuiper belt here's the orbit of Pluto that dips in and out of it and just to superimpose the orbit of Halley's Comet a short period comet again it looks like it dips into bottom that belt and all these short period comets have orbits which are comparatively flat compared to the plane of the plan they they tend I know I said Halley's Comet doesn't but they tend to go the same way around the Sun as the planets and so they had they've been influenced by the gravity of the giant planets as well and what is thought is that out beyond Neptune this is just what all this stuff that's formed from that original solar nebula it's nice and cold and icy it's out beyond Neptune so it's formed I see little chunks but they're too sparsely distributed to clump together to form another planet and so it's just debris that never made it into being a planet those are the short period comets come from the Kuiper belt it's a slightly different matter for the other comet because they have to come from much further out the long period comets they have all bits up to tens of thousands of years I mean if you can call that an orbit at all not just that they arrive not from the plane of our solar system they come from every single direction and they are hypothesized to come from a huge cloud of you know these iceberg comets this you know debris that lies way out beyond the Kuiper belt way out beyond the whole solar system in what's known as the Oort cloud here's a schematic so all of this Kuiper belt near the planets fits in the center there and so if this stretches out to something like 50 to ten thousand sorry fifty to maybe a hundred thousand times the earth-sun distance huge distances from the original solar system and these can't have formed out there because there wouldn't have been enough matter in the solar nebula okay would have been icy but this is way out beyond where the solar nebula is they need to have been tossed out there in various ways they need have been formed within the nebula and then thrown out to form this huge Oort cloud we think it's very cool because comets arrived from every set every direction out there they're very weakly attached to the Sun and they can stay dormant you know those those icebergs for millions if not billions of years we think you know there's a reservoir some trillions of cometary icebergs sitting out and it's all cloud you know again the fact that we're still seeing new comets arriving from there every every year suggests that there's a vast storehouse that they come from and they can sit out there and disturb most of the time but they're also vulnerable to other gravitational forces so you know maybe from a cloud passing or nearby stars and every so often they'll just get stirred up and that'll send one of these commentary nuclei tumbling towards the Sun so here's the inner solar system I get obviously not to scale right fifty thousand times here's some distance out here but one can just get perturbed slightly and that'll send it falling in and it'll make its first passage in through the inner solar system sometimes they get kind of captured by the gravity of Jupiter and kind of tamed to make more of a short-period comet and so these are true visitors from the outer parts of the solar system which makes them exciting for astronomers because they are relics that have not been acted upon they've not had the same thing happen to them but the planets have if you like their little pristine samples of that original solar nebula that went on to form our solar system they've been out of the deep freezer space all that time they haven't been chemically altered they're small enough that they haven't undergone to the gravitational compression which would heat them change their nature would start some chemical reactions so they really are thought to be little pristine samples and that's why strana MERS are so curious about comets and how they got out there well again not to scale but you've got the CERN asteroids Jupiter Kuiper belt and then the Oort cometcloud around here they've been thrown there by the gravity of those enormous gas giants imagine you formed your planets but they're still embedded so this is perhaps you know four billion years ago they're still embedded within this solar nebula rocky material sort of within Mars icy material out in the realm of the gas jars and therefore susceptible to the gravity of those gas giants and giant planets have lots of gravity and they tend to do something which we call clearing their path which means they get rid of anything that's in their way and they do this in a number of ways obviously it's gravity and they'll just pull them around and accelerate the mound and throw in every which direction some will fall onto the Sun there's too little comments we saw they'll just end up being part of the Sun some maybe don't quite make it to the Sun they get captured or they get thrown onto the inner planets onto earth and Earth's moon Mercury Venus Mars and it's this that's known as the late heavy bombardment there is responsible for all the cratering activity there's so mark the surface of here we have mercury or the moon or even some of the the moons of the the gas giants so this is appeared where you know billions of little chunks of debris rained down and all the inner planets not just that but sometimes the gas giants capture the debris this sort of nuclei themselves some fall unto the gas giants to get swallowed up by them some get captured into being moons and if you look at some of the moons of Jupiter and Saturn they look a lot like we expect in a dead cometary nuclei or you know fragments from the early solar system to look like and then finally you can also instead of throwing them into the solar system you can throw them right out and this is what we think happens to form that all cloud material is just thrown out every single which way until the planet the giant gas giants have cleared their path and you form this reservoir of all cloud comets so you've got all the Comets in the Oort cloud and then you've got the ones that form out way beyond Neptune but haven't really been that acted upon by the gravity they formed in sits you and they've just settled down to form the Kuiper belt and aside on this is that we think comets are really important for delivering Earth's oceans because when you have formed it's in that part of the solar nebula that's close to the Sun it's in the warm part of the nebula where ices are not going to condense certainly not water ice not just that but the early Earth is going to be hot and molten so any water on it is not going to survive we think this heavy bombardment which finished about 3.8 billion years ago is responsible for you know delivering water to us by all these water rich bodies such as comets and that's our best way of actually supplying the Earth's oceans with water any snag with this scenario is that scientists have measured to cut the water comet water content of comets and particularly looking at the balance of normal water against what's known as heavy water which it uses an isotope of hydrogen that's slightly heavier it's got two neutrons and they look at the ratio of this you find it there's more heavy water in comets than you find in the oceans on earth it's a bit of a problem but who says that the the Comets that brought the water the earth necessarily came from the same part of the solar nebula as the Comets we're observing now four billion years later so we kind of have to get around that but nonetheless there's nowhere else we think that the water could have been delivered to earth from and so all this cometary ice later became our oceans and not just that they're exciting results that suggest that comets also bring some complex hydrocarbons with them and who knows what role they may have had in kick-starting some of the geological of the biological cycles on our planet now impacts or collisions of comets with plant circles didn't just happen in the dim and distant past it is still going on not all of them make it even anywhere near the Sun and here's one very famous example of an impact with the giant planet Jupiter that we were all able to observe in 1994 nineteen years ago and there was a comet discovered the year before and it was announced in April 1993 always remember the announcement because it was made to a meeting I was out in first of April and we were all sure it was an April Fool's the the joke was on us when we saw the pictures because it wasn't just an ordinary comet it was called a comet train we have lots of mini comets out by Jupiter this was discovered by Carolyn and Jean Schumacher and David Levy hence the name Schumacher Levy and what by backtracking it's all but they found this was those short period comets that actually swung around Jupiter in 92 it got too close to Jupiter got fragmented to form this comet train and then you could predict that it was going to fall onto Jupiter in July 94 so everybody was watching for this we had prior notice this of course is an atom the comet train fell onto Jupiter unfortunately just beyond what they decided Jupiter that was visible from Earth this is just a little animation showing you they fell on Jupiter just on the nighttime side so we had to wait 15 minutes till the impact site would rotate into view but here's a sequence of images from some of the first impacts there about 20 fragments of this comet or 20 big fragments the biggest ones were about two kilometers across and realized they're impacting on Jupiter at 60 kilometers a second and when they impact of course you're not hitting a solid surface it's just cloud tops but as they fall through you're getting they suddenly get compressed very quickly and you almost like get a complete vaporizing of a comet that forms an enormous plasma bubble of hot gas that rises up through the cloud tops so what you see when the comet impacts on Jupiter is almost like scars holes in the cloud is this gas has bubbled up and broken at the surface so here's the first shot you can see a plume as the impact fragment there's a fragment impacts just around the side of Jupiter and then it swings into rotates into our view and then you can see the next fragment scar as well and astronomers were able to watch over the next few weeks as these bubbles these scars dispersed from something very neat and very symmetric and gradually getting dispersed by the weather on Jupiter the winds and the cloud tops and so the scars don't last very long we still see this happening there two years ago there were a couple of impacts as an amateur astronomer caught that must have happened on Jupiter again by seeing these dark scars on the cloud tops of Jupiter and of course you do still get impacts on earth the asked big comet impact but we think happened was the Tunguska event which occurred in Siberia so this is not the one earlier this year by the way you didn't miss a vital bit information about that this is about hundred years ago again Siberia where there was an enormous explosion it happened here that happened a few hours later it might have impacted somewhat near Moscow and who knows how different our history would have been but it was an enormous explosion it felled 80 million trees flat out over and over about 2,000 square kilometers but there's no crater there are no neat right impacts which is why it's thought to have been a comet rather and master oyd and it didn't even reach the ground it exploded about five kilometres up in the air and there were two explosions or two supersonic shockwaves one as the comet came into the atmosphere and then one is it vaporized above the air and it fell Dahl these trees in the process I mean we don't have to wait for comets to come to us and fat people you don't want them to come to us if they look like that we do have the option of actually going out to visit them and so I wanted to show you some of the missions and what we've learned about comets by flying close to them now here is the nucleus of Comet wild as observed from a spacecraft called Stardust which flew close to it in 2004 what close it means within back 250 kilometers and the whole iceberg is about five kilometers across now this is a nice fresh comet hasn't gone round the Sun very often and the spacecraft went and flew past it and looked it's got unlike some of the other nuclei Shoney's kind of got a slightly rough surface it had it's got these sort of flat craters with steep walls and cliffs around the outside it's still relatively pristine relatively fresh and the comet just under the mission didn't just go and look at the cometary nucleus it was beginning to jet material off it flew through the tail of the comet and as it did so it exposed a tray of canisters filled with aerogel here's the betray and aerogel is nice because you can capture particles of the comet in it so this material has been given off by the comet to make the tail exposed this this trace it flew through the tail and captured thousands of particles have been given off by the comet which was then returned to the earth two years later for close examination in the lab now here's a microscopic view of some of those particles within the aerogel this was covered with a thin aluminium foil and so this is a side view here's the aluminium foil here's the hole the little fragment of dust from the comet has punched through the foil and then on impact it's kind of disintegrated and then bits have shot in every which direction each of these trails has got a little chunk or cometary dust at the end how long the trail is depends on how massive the little particle is and how fast it was going at the time and so you're able to analyze bits of grit from a comet in the lab they've done that and they've found some interesting results I don't have time to go into the great detail but the biggest surprise was that there were grains in there that had been made at very high temperature and this was strange because the comet had not gone past the Sun often enough for it to have really been baked hard enough to get this kind of chemistry that we'd expect from repeated exposure to hot temperature and this suggests that the comet is actually made up not just of ice but also stuff from the solar nebula that originates from close into the Sun and so somehow the two have got mixed up so you can see perhaps the formation of comets is a bit more complicated than I've alluded to earlier on my talk how do you expect of the hot and the cold part the nebula into one comet and not just this there was exciting evidence for you know for example amino acid glycine within the grit of the comet so again very complex chemistry within these cometary particles that was a very simple mission there was one a little bit ambitious a year later when a dual mission called deep impact rendezvous with comet Tempel 1 now this is a slightly bigger nucleus is fairly inactive at this stage is about 14 kilometers across and this composed of two missions there was an observer craft that flew - with a few kilometers of the nucleus and observed it kind of rendezvous that and observed it and it released a smaller probe which crashed into the nucleus at about that point and it had cameras on both to monitor the event the idea was to release ejecta from the comet and then from close at hand you could observe the way the sunlight was reflected of this eject to find out what it was composed of and also how much was released would tell you how porous or fluffy how brittle the cometary nucleus earth's tell us something about its composition and its structure now the small probe that fell you know that was impacted onto the nucleus it was about the size of a washing machine and here's the movie and just speed it up from that little probe now I say it impacted on the nucleus to be honest it's more like it's that there and it got run over you would feel very scared if you saw this coming towards you yeah and after that it gives up so that's the that's the do from the probe and now from the observer craft here's the outgassing of material from that impact there was so much material released in that impact is equivalent to several months of a comet going around the Sun far more material than they expected which suggested that perhaps the crust the comet had a crust that was quite thin and that the this little probe impacted and punched a hole through it and a lot of material was released suggesting the comets a bit more porous and fluffy and a little bit more brittle that's more like you know I think it's a light breeze block that kind of structure than we'd expect now we're great believes in recycling spacecraft of there's fuel left in the tanks now there was so much stuff released that you couldn't actually see the crater that had been made so the Stardust mission that had gone and looked at Comet wild was then recycled to go and revisit comet Tempel 1 in 2010 so it's 5 years later and look for the crater now everything is also looking the rest of the nucleus because it had gone round the Sun five times they wanted to see what you know this repeated baking had done to the surface but they also looked for the crater left by that little probe and to be honest if they hadn't put the arrows here you wouldn't necessarily know they mean they claimed there's a structure about 150 metres across left by this probe and it suggests a lot of the material that was excavated actually fell back down and filled in this crater which is why there isn't so much structure to be seen the observer craft from that impact was also recycled and rendezvous with comet Hartley 2 this is just a two kilometre long nucleus and again just looking at the activity onset onto it and it's you can see it's not uniform it's happening more from the rough ends and not from the smooth saddle shape of the nucleus so all very interesting but nothing compared to the next mission you're going to hear about rosetta mission next year taking 9 or 12 years in fact to travel to rendezvous is this an animation of course to come and cheer me off Gerasimenko or something on those lines it's taken 12 years to get there another minute it's in hibernation because it's kind of going through a boring part of its journey they're going to wake it up next January and when it's going to rendezvous this icy nucleus this is a short-period comet so we know it's going to be it's going to rendezvous with this nucleus when it's out beyond the orbit of Jupiter and it's going to sit there and observe it for a few months it's going to approach and look at it and study it and map the surface and then come November or maybe a little bit earlier it's going to release the lander called fili here's a drawing it's going to release this but is going to go down and it's going to grab Hawk it's got grappling hooks on the end of its landing feet and it's going to harpoon the surface and it's going to grip onto the surface of the nucleus and ride the comet nucleus in through the inner part of the solar system going to do that next November and then sit in the comet and ride it in as it goes past the Sun and witness firsthand all that generation of activity from the surface of the comet and what can possibly go wrong right I mean there's a lot of unknown like what was the suit you don't want it to bounce off the cometary surface you don't know whether it's just going to go do a deep impact site kind of crash into the surface whether you know you don't want one of these cracks to open up underneath Fillie lander hugely ambitious hugely exciting mission it's going to be in the news next summer really looking forward to seeing that so thank you very much that theater today's lecture and just to remind we're going to have a break for the next couple of months and I'm going to return in February and tell you about the lives of stars thank you

Channel: Gresham College

Views: 355,928

Keywords: astronomy, astronomy lecture, astronomy talk, astrophysics, astrophysics talk, comets, great comets, ison, halley's comet, edmond halley, giotto, tycho brahe, brahe, copernicus, rosetta mission, kuiper belt, oort cloud, jupiter, solar system, deep impact, space, space lecture, space talk

Id: gBjnlfLxhEI

Length: 53min 51sec (3231 seconds)

Published: Thu Dec 12 2013