Small Bodies of the Solar System - Professor Carolin Crawford

[Music] [Applause] [Music] okay it's my distinct pleasure to introduce dr. Amy Meinzer she's here I is this the first time you've been to the CFA it's amazing this first time she's visited here Amy finished her PhD from UCLA in 2003 and she's done even before that but but since some very remarkable work involving both Spitzer and wise she is the PI of any o wise which is extension to wise that has done really fantastic work discovering minor planets in our solar system an unveiling not only size distributions but about albedo distributions and I think you know you'll be quite excited to hear some of the results that they've gotten this is a really a very effective team that she's assembled she's also a p.i of any Oakham you'll hear a little bit about that but I'm not going to take any more of Amy's time because she's going to show some very fantastic work so please Amy the court here thanks Matt can everybody hear me okay great okay thanks so very much it's really great to be here and to see a lot of really familiar faces from from the past and from other projects so I'm here to talk a little bit about the small bodies in our solar system the asteroids and the Comets and just you know what we know about them and what we would like to know and how some recent new mission data has has filled in our understanding of these populations now for a lot of you this is going to be completely old hat and I apologize but since there are a lot of the people who don't necessarily think about the solar system bodies all that much I always like to start off with just a very brief guide to the solar system most of the asteroids in our in our solar system live in the main belt between Mars and Jupiter and they're an ancient population they've been there for billions of years and in most cases that's where they stay and that's they just don't really go anywhere and they've been very stable over long periods of time they've been heavily processed through collisions with other asteroids and through non-gravitational forces that sort of move them around a little bit but generally they stay there for long periods of time and as of today we know of roughly 600,000 of these objects now by contrast the near-earth objects refer to the asteroids and comets that that enter the region around the Earth's orbit and unlike the main-belt asteroids which are quite ancient than the NE O's as they're called are extremely young they tend to last for only a few millions to few tens of millions of years once they reach the region near Earth basically one of three things happen eventually they either hit the Sun they get scattered completely out of the region and and in some cases even just go way out into the outer reaches of the solar system or eventually they hit a planet so the question is how often do they hit the earth that's that's one of the first questions and you'll notice also - that there's a there are clusters of asteroids here these green blobs these are the Jovian Trojans these are in gravitational resonance with Jupiter now back to the NE OS so unlike the main-belt asteroids where we know of hundreds of thousands the any OS we only know of about eleven thousand six hundred today roughly and that number is growing by about a thousand per year on average okay so this last year 2013 was was really a remarkable year for asteroids scientists and I'm gonna just use my laptop audio here you just bear with me but this was really a strange day because we knew that there was going to be an asteroid making a very close pass by the earth but it was well known that it would not hit and so I was actually driving home from LAX and listening at about 11 o'clock at night and listening to the radio that's what happens and I thought wait a minute this is this is not this this asteroid is not supposed to hit the earth we know where it's going what is going on here this is really really bizarre and all of these you know subsequent booms that you're hearing are the are the fragments bursting apart so the object left this huge contrail in the sky and it became clear that it was an extremely high altitude air burst because of the time delay from the appearance of the contrail to the sound waves arriving on the earth about 95% of the energy was actually dissipated harmlessly in the upper atmosphere tens of kilometers up so most of it didn't actually reach the ground but what did reach the ground caused a fair amount of chaos mostly mostly in the form of broken glass and most of the injuries were where glass flying people when they saw the contrails they ran to the window and because it took yeah then these guys this it's now they did get out there's a door you go out the door he makes it out the door which is good but then there's these other guys who are still under there so it doesn't go so well but anyway they eventually get out to the the thing so but you know anyway most of the injuries were because people ran to the windows when they saw the contrail it took about a minute or two minutes for the shockwave to reach the earth and of course people who remember the duck-and-cover days knew not to go to the window but rather to stay under the desks and thus they avoided some of the flying glass injuries but a lot of people ran right for the windows and that's what caused most of the injuries so but but nobody died fortunately and it was a very tiny asteroid this turns out to be roughly 17 to 20 meters across as estimated from from various methods but including infrasound stations that are essentially listening for you know nuclear weapons violations or something like that or missiles and it basically had about a half a kilo 500 kilotons of TNT but you know most of that energy dissipated harmlessly in the upper atmosphere so in the end this is actually a piece of it right here this is my very own little piece of the rock I managed to I get a small piece of this and it's a really interesting rock as far as meteorites go this is one of the most common types of meteorites and so it's an ordinary chondrite and what is kind of noticeable about it though is you can see these these veins these dark veins running through it and these are records of previous collisions that the object has undergone tens of millions of years ago so it's a heavily processed object you can see it's got a nice fusion crust on it from the entry into the Earth's atmosphere but basically this is a fragment of an asteroid that was long ago impacted by other asteroids probably many times over the course of its long history and eventually it got in the way of the earth now why was it not noticed beforehand well it came out as it came out us out of the direction of the Sun in the very last minutes before its final approach it basically came out from about it was around ten degrees or so away from the direction of the Sun so during the daytime sky where ground-based observers can't see and it was a very tiny object so is extremely dim and faint for you know basically the minute that it got even a little bit away from the earth it would have been far too faint to be observed so this begs the question of course how often does this thing this is a sort of event happen and we have some very rough estimates but we'd like to refine them and this is what we've been working on in in my group but basically we know that really large mega events happen about every hundred million years there's a little bit of the entropic principle here I mean we wouldn't be here if these sorts of giant collisions happen more frequently so we know that they're very rare the estimate is sort of a one kilometer impactor about every 1/2 million years or so and down at the timescale of or the size scale of 20 to 30 metre objects it's about every few hundred years perhaps give or take but again small number statistics make this somewhat difficult to constrain and of course this is the famous meteor crater in Arizona which was created by an object not that much larger than the Chive insuk impactor that was about 50 meters but the difference is is rather than an ordinary stony meteorite this was thought to have been caused by an iron so considerably higher density and therefore a much bigger impact it wouldn't have just broken glass okay so of course this is the big one the Chicxulub impact er which was a very massive thing was about five to ten kilometers across and you know it was the equivalent of ten times the world's nuclear arsenal all dropped in one place essentially it just it created a huge massive amount of damage and the ring shaped structure here is visible underneath the the Gulf of Mexico and what it really did was it basically wiped out about 70 percent of all land animals and about 50 percent of all marine species you can see here this is a plot of of genera so one rank up from species as a function of time and life is more or less lumping along here becoming more and more and more diverse and here's the event this is this is marine life so things under the water we're more protected than the terrestrial animals in any case it was it was large okay so but again you know I think I think astronomers as a whole kind of thought well these are events of the distant asked and they really don't happen in the present day and and this was one of the events that really changed people's minds in 1994 a comet was discovered on a collision course with with Jupiter and Jupiter of course shredded it into lots of pieces the question was whether or not Jupiter would even noticed the impact of this of this comet Jupiter so massive and of course subsequent imaging revealed that yeah it actually did notice and it left these pretty large scars and of course each one of these is about the size of the earth so that really got people's attention and I think it's safe to say that that helped to give rise to the idea that maybe we should actually begin surveying for near-earth objects in a more systematic way so this is a plot that basically shows the discovery of near-earth asteroids as a function of time the red objects are the class that are generally thought to be a kilometer and larger so the really large planet Buster type asteroids and you can see here in 1994 is where things really started to pick up but it's important to note that most of the surveys that do the lion's share of the work here are really done by very modest sized telescopes they're sort of 1 to 2 meter class observatories so it's not really very big big observatories by any stretch so while the one kilometer objects appear to be flattening out these these smaller objects just keep going up and up and up indicating that we haven't run out of objects any time soon so one of the questions with the one kilometer objects that was that people wondered about is have we really found all of the population or almost all of the population or is there some category of large asteroids that were systematically missing so this is one of the things that that people have wanted to look at so keep that in mind for later ok so this is our current suite of telescopes that do most of the work these days now linear is really no longer active but the Catalina Sky Survey and the pan-starrs telescopes in Arizona and Hawaii find almost all of the new earth objects that we see today and like I said they're discovering new objects at a rate of roughly a thousand per year and now of course we also have our first infrared survey NEOWISE which I'll talk about in a bit so and that time is now okay so why is this is the wide-field Infrared Survey Explorer mission and this is a medium class NASA Explorer so it's a fairly small sized project for a NASA mission it's about 320 million dollars it was a it's a 40 centimeter telescope that was designed to survey the entire sky at four wavelengths imaged simultaneously between about 3 and 20 microns and we had to use a two-stage solid hydrogen cryostat to cool the longest wavelength channels so I won't repeat too much of this seminar from from lunch today but basically the class of detectors we are stuck with using at the 12 and 22 micron channels has to be cooled to around 8 Kelvin and that really necessitated the use of the solid hydrogen so it launched in late 2009 Ned Wright from UCLA is the principal investigator of wise and the missions surveyed the whole sky finally ceasing operations in February of 2011 the primary purpose for the mission had nothing to do with asteroids it really was designed to to carry out an all-sky map at infrared wavelengths for a really general purpose astronomy and also with a couple specific science goals in mind finding ultra cool brown dwarfs as well as ultraluminous infrared galaxies and the asteroids were kind of were kind of an afterthought but it turns out that was actually pretty good at seeing asteroids and that had a lot to do with the combination of a wavelength cadence and the field of regard so the wise mission is in a is in a sun-synchronous polar orbit so it's basically about 525 kilometres and it always observes near 90 degrees solar elongation so this is a region of sky that's you know sort of the Twilight and Dawn sky on earth it's reasonably difficult for ground-based telescopes to spend a lot of time there just because of the diurnal cycle but this is where we observe all the time so the scan coverage results in lots and lots of coverages up at the ecliptic poles and on average about 12 observations in the ecliptic plane so for asteroids this actually turns out to be pretty good and what we do is we use a very small mirror to quickly flip back and forth between exposures the mirror is only about yea big maybe the size of a compact and that is what allows us to switch very quickly between the different fields okay so here's a picture of the observatory just before launch and you can see here it's it's not really that big this is perhaps about half of life size it's about 15 feet tall and it's got the solid hydrogen Christ at up there the star trackers with the red caps solar panels all the usual stuff that a spacecraft has but what about the asteroids well it turns out of course you know this was not a mission that was dedicated to searching for asteroids and the original plan was to take all of the exposures and like I said they were on average about 12 exposures on the ecliptic plane rising to hundreds at the poles the original plan was to co add all of those exposures together and then just serve the the image atlas and catalogs that resulted just from the co ATS and that's great if the targets that you're looking for don't move or change on timescales of hours but it's really bad for asteroids because the co adding just washes them right out so we were actually given some extra resources from the Planetary Science Division of NASA to do a couple of things first to archive all of the individual exposures as well as the extracted sources for each individual image and then also to add some software that would allow us to actually dig asteroids out of the data in basically real time so basically we took some software that is very similar to that used by pan stars and we basically retrofitted it to work with the wise cadence and we call that the moving object pipeline was run twice a week and basically we delivered what we call track let's position time pairs for the asteroids to the Minor Planet Center here and NPC receives all of our chocolates and basically figures out which asteroid does it correspond to so the idea was to do that fast enough that if we saw something that looked like it could be a near-earth object there would be enough time to quickly go get ground-based recovery of the object because we only observe an object for typically about a day day and a half at a time and that's not really enough to see where it's going to go it's enough to say broadly this might be a near-earth object but it didn't say it doesn't allow us to really nail the orbit down or predict impacts or anything like that so basically the chase was on to do this year-long observing run essentially and get follow-up from from places all around the world to Trump try to nail down near Earth object orbits okay so here are some of the team members and we're partnered with with iPAQ for our data processing and we've got a slew of postdocs who've been very helpful and we have also worked with a number of folks like Dave fallin at IFA and Tommy Groff who's a former student here so why would we want to look at asteroids in the infrared well there's a few reasons the basic physics is pretty simple the asteroid fluxes consists of a reflected light peak and emitted thermal thermal peak as well and the thing is is that the reflected light flux is a much stronger function of albedo the reflectivity of the object compared to the emitted flux and this has a couple of different consequences first off the infrared fluxes are basically insensitive to albedo so a dark object is equally bright more or less at infrared wavelengths as a bright object and that's important because we know that quite a lot of the near-earth objects are very dark and thus they're easily missed by the visible light surveys so that's one thing the other thing is since we have this pretty good sensitivity to to diameter rather than albedo we can actually nail down diameters quite well and this breaks the degeneracy between an object that's small but highly reflective and large but very dark so if we have good measurements that span the rotational light curve which we usually do we can almost always get the diameters to within about plus or minus 10% and that's compared to you know that's a that's a considerably better accuracy for the diameter than diameters that you derive from visible light alone where you don't know if the object is bright or dark and then of course one of the nice things about working in space we're always looking at this low elongation region which is kind of tangent to the Earth's orbit if you think about it so we're looking in a volume of space around the earth it's difficult for the ground-based server to see and of course we don't have any weather so that makes it easy to understand the survey biases or easier which helps for extrapolating the population that we saw to to what we think is really out there okay so there's one other thing that's kind of an advantage and that is the infrared sky is mostly blank if you look here the wavelengths are color-coded so that three and four microns are the are the blue and green wavelengths so they look cyan and twelve microns is red and you can see there's really not a whole lot of red in the image and the red things that you are seeing there are asteroids this is what they look like to wise it basically strings of dots these are what we call track let's position time pairs so the nice thing about this is that even though the telescope aperture is small meaning that the spatial resolution is relatively poor compared to a larger visible telescope the sky is relatively uncrowded so there's a lot less chance of landing on a star or galaxy and that helps with with confusion that's why we can get away with using such a small telescope okay so this is what a typical track look looks like and basically you can see we've got a series of exposures at 12 microns and 22 microns and then we basically just look at a series of exposures before and after the same patch of sky and this helps us to figure out that there's not a star galaxy there that we're confusing the the source with and then we also look for latent images the exposures immediately prior to - the ones we just collected we require a minimum of five detections to figure that something is an asteroid and what that allows us to do is basically be very reliable meaning when we pull stuff out we're pretty sure this is really an asteroid and not just a collection of cosmic rays for example so basically like said our average observational arc though is only about a day and a half which means we have to have ground-based follow-up relatively quickly or the objects get lost and so Tim and his company at the MPC do a fabulous job of handling all the observations from the spacecraft and making sure that the word gets out to the ground-based follow-up community okay so this is what we ended up with during the prime mission this is sort of the the SOI system as viewed by the NEOWISE project so you can see where we're starting off here and we're always looking at about 90 degrees from the earth so we're looking in a different direction than the ground-based surveys which are typically surveying the regions closer to opposition and each of the dots is is an asteroid that we've observed and the Reds and the greens are the near-earth object populations the red ones are our discoveries the greens are other people's and then the Comets here are the blue and yellow guys and in total we observed about a hundred and fifty eight thousand asteroids so what this allows us to do that's the big difference from before is now we can get sizes for these objects and reflectivity zal vetoes which has some correspondence with their composition so in the past only a couple thousand objects have had any sort of either taxonomic classification or radar measurements to really nail down sizes so basically this gives us a whole new look at the physical properties of the the small bodies in the inner part of the solar system and you can see a few things here I mean here's our galactic plain passage even in the infrared we are still confused by the Galactic plane the Jovian Trojans are right here and you can see this right there is where we lost the 12 and 22 micron bands that's where the gas ran out so to speak actually the frozen hydrogen not gas but this right here is the formal end of all of the cryogen this red dashed line and we were able to continue the survey for an additional four months just to fill out the inner edge of the of the solar system right here and on February 1st the mission was placed into hibernation and that was the end that was it so what did we learn from all of that well like I said we have a sample now that we can do bias because we can measure its its sensitivity and the properties that it should have seen so the basic idea is you you take the observed sample as a function of its orbital elements and physical properties such as albedo and diameter and then you basically model out the survey sensitivity as a function of all of these parameters so the way we do this is we look for objects that have very well-known orbits and we see how many of them appeared in a frame and whether or not we saw them in other words we look for what things should have been there and did we see it as a function of magnitude and then basically what we do is the other method we do is we take a synthetic population and we basically play that through a survey simulator and we tune the properties of the input population until it matches what we saw and that allows us to tell that ok these are the properties of the underlying population so the basic idea is we are testing a set of orbital elements that we think represents the near-earth objects in particular we assign a series of physical properties like size distributions and albedo distributions and then we basically just keep tuning those and we basically say ok let's calculate the position of each object in our synthetic population let's calculate a model thermal flux for it at the wise wavelengths and then see whether or not we should have found it based on what we know about the sensitivity of the instrument we also have a complete list of all of the pointings in the whole survey so we know exactly where we pointed and when and so for each frame we basically calculate what should have been in the frame and did we see it and then we just keep tuning the parameters until we get an output of the survey simulation that matches what we saw so what did we learn so that's basically I just said in words what's on the chart here so we'll ignore that couple things came out so the first thing is when we looked at reflectivity of asteroids as a function of size there was something that was was immediately kind of obvious and that is that the distribution of albedo is the ratio of bright stuff versus dark stuff which roughly corresponds to the objects that are primitive carbonaceous objects versus stony objects or you know more processed objects is roughly the same as a function of the diameter over a fairly large range of diameters so all the way from you know pretty large sizes a kilometer sizes down to hundreds of meters the ratio of bright to dark is is roughly the same and that's contrary to some previous studies which found that asteroids were supposed to get brighter as you got smaller well we think that that was actually an artifact of and bias in other words if you have a visible light survey it's harder for you to see the small dark stuff so you could mistakenly infer that it's not there we think that it is there yeah it's just that if you select based on the thermal flux which basically doesn't depend on the reflectivity now you see the small dark stuff in its accurately represented proportions so that was one output and then we can try this thing where we we basically say okay here is our 12 micron selected sample so this is the sample and the previous plot selected on the basis purely of its thermal emission compared with the sample that was selected based on its optical fluxes and sure enough now as you get to smaller and smaller sizes there does appear to be a clear trend of the asteroids getting brighter as they get smaller but we think that this isn't real we think this is probably selection effect if you select based on the optical flux of course it's harder to see the small dark stuff but if you select based on the thermal flux well you see the dark stuff just fine so maybe there is a real effect down here at the very smallest of sizes but it's at this point it's tangled up with the selection biases of the visible surveys and so we really can't say for sure what's going on but my bet is survey selection okay so one other thing we were able to show is that by using our model populations we found that yay verily we pretty much are unbiased with respect to albedo as you would expect and because this is an all-sky survey we're actually pretty sensitive to the higher inclination orbits which are harder for the ground-based surveys to see so when we back out the properties of the underlying population from our survey simulation what we found is that there are actually somewhat fewer near-earth asteroids out there larger than about a hundred meters then prior estimates had suggested and we figure that there are about twenty thousand five hundred plus or minus around three thousand near-earth asteroids now I'm not including comets at this point just near-earth asteroids that are larger than 100 meters and of those about maybe 25% of the discovered so the size frequency distribution is a little shallower than private previous predictions now what that means and why that is we don't know but it does set some constraints on the formation of the near-earth object population that need to be satisfied when we look at the matches of our sample population two are the predictions from the orbital element models it's a pretty good match with a couple of exceptions more or less the distributions and semi-major axis inclination and eccentricity look look pretty close to what we would expect but there were a couple things that stuck out and I'll get to that in a sec but basically now we can also use our model of the albedo distribution combined with the known population of objects to get an estimate of how many have actually been discovered to date in other words what fraction of the population remains to be discovered and there's some good news at the one kilometer and larger size remember we said we weren't sure if there was a class of large near-earth asteroids that were being systematically missed and it doesn't look like there is from our predictions we think that basically all of them have been found about 90% or more of a one kilometer one kilometer and larger NTAs have been discovered already so that's good news a less good news is when you get down to about a hundred meters only about a quarter of them or so have been found now overall the population estimate that we come up with is like I said less than the prior so that's also good news for very small stuff though and the size rains of Chelyabinsk the the NEOWISE sample is just really insufficient we really can't say anything about objects that small that's a different different sample you'd have to work with so you know in short I mean basically there are fewer nea s but it's still a lot of any A's it's still pretty crowded so these orange objects represent the the known objects at present the red ones are the old model population these blue ones and white ones are the NEOWISE sample you can see we've discovered a couple retrogrades going backwards here funny little things and so this is this is what we thought and and this is our new estimate of the population so fewer but that's still a lot of asteroids and these are just the ones larger than 100 meters so there's still plenty to find and lots more work to do and one of the other things we found is as we cut our sample down to just consider those that are actually formally classified as potentially hazardous that there were some other interesting things that came up so a PHA is just could also mean perfectly harmless asteroid doesn't mean that the asteroid and the earth will ever be at the same place at the same time it just means that the orbits get close together that's all so it's a population of interest because perturbations can kind of pull things around on timescales of about a hundred years so we want to pay attention to these ones in particular but in general there's a Zippo chance of impact and it's not something you should lose any sleep over we do want to know how many there are so we cut our sample down to just consider the PHAs and of course now the air bars are going up because we have many fewer objects in our sample but we figured that there are about forty seven hundred plus or minus about fifteen hundred out there that are larger than 100 meters and again about a quarter of these have been discovered the remaining 75% is still at large but when we looked at the distribution of potentially hazardous asteroids that occupy the lowest inclination orbits in other words the orbits with inclinations most similar to Earth's we find that there are about twice as many as the model predicted so that's good if you're looking for places to go that take very little energy it's less good from a hazard standpoint but we would like more data phone calls for people think how dare you publish a estimate for the number of near-earth object to pee eyes of surveys call they said no you lowered the hazard yeah it's it's a kind of a nuanced result if you were overall but the average in the a poses absolutely no hazard the ones you want to watch out for these ones and particularly the low inclination orbits so we would like to get a little more data to see whether or not our result holds up over time but that's what we came up with anyway so the data have been used for a wide range of different things not just the study of the Earth object population we can apply these sorts of D biasing studies to to all the different asteroid populations that we saw and to the Comets we can also look at things like the collisional histories of asteroids in particular asteroids families groups of asteroids that are thought to have formed from a single collision and we can also do some tricks to to basically look at the size distribution of dust that's emitted by comets we can also look at some of the volatiles that they contain the co and co2 and we can also find what I like to call our weirdos things like Trojan asteroids that actually share the Earth's orbit not Jupiter's turns out we actually have one I'll tell you about that in a minute and lots of fun stuff like that so when we consider that the distribution of asteroid compositions in taxonomy this is from from Francesca DeMaio and Rick benzyl this is kind of the the classification of asteroids their near infrared and visible spectra and they span a wide range of types basically from things all the way from things that are extremely red objects to things that have flat or even very blue spectra to things that show different spectral features in them that allow them to be classified based on composition a little bit and when we considered the fraction of objects I mean one of the first things you want to do when you have a new data set is you basically take what you have and then you cross-reference it to whatever else is is in the literature so the first thing we did is we basically took all of our diameters and albedo's and started plotting them versus objects that had classifications like this to figure out were there any correspondences and this is what we found so basically as you look verses diameter these are mostly from main-belt asteroids now and compare the albedo's what we find is that you know sure enough as you would expect the Stonier objects the s complex objects tend to have very bright albedo s-- and the more primitive carbonaceous object classes tend to be dark but as you get to small sizes there's this curious uptick again and I would submit that this is not because there are no small dark sea types here it's because when you go to do spectroscopy to classify them you have to see them in visible light they have to be pretty bright and sure enough I I'm sure they're there a large fraction of the asteroid belt it's dark so you have to be careful I guess the moral of the story is selection bias is really matter and before you infer that there's a change in material properties of asteroids at small sizes you have to be pretty sure that you haven't just not observed part of the sample because you couldn't see them so that's one of the things that came out from there Franchesca DeMeo and i'm francesco sorry i'm presenting your chart for you but this is she's done some really interesting work to show by combining Sloan Digital Sky Survey color data with wise to show that there's some interesting things that crop up when you look for different types of asteroids in places where they really shouldn't be it turns out these these very red objects these d-type asteroids are thought to be very primitive objects that should have been formed you know far out in the outer parts of the solar system and sure enough they pop up in some curious places in the inner part of the solar system where they're really not supposed to be from from existing models of how the solar system formed so when and how these objects got there is it's going to be an interesting story and it suggests that perhaps the early solar system was a more chaotic place than perhaps it was initially thought to be and not so orderly in the process of forming these small bodies so that work is really getting underway the other thing we found like I mentioned is just like jupiter has Trojan asteroids that are gravitationally stuck to that planet it turns out earth has a temporarily captured trojan all its own it's known as 2010 tk7 and it's orbiting about the l4 Lagrange point right here and you can see it's it's this is basically a plot showing its vibration cycle which is about every 400 years or so and it seems to be captured therefore roughly 7,000 years and after that time it will eventually get tabled away and go off onto its own and become a regular near-earth object but we were able to see it by virtue of the fact that we observed in near 90 degrees as you can see looking in its orbit it basically never crosses 90 it doesn't really come out past that so you have to be looking here pretty continuously to see things like this my I'm a bird watcher and one of the things I like to think about with bird-watching is if you see one thing it's probably common there's probably more of it somewhere so my guess is maybe there are more earth proteins out there temporarily captured objects like this but we just have to be looking in the right place to see them so we hope we can set some limits on the population at some point and it's a pretty large object it's all closed to 400 meters across and reasonably dark okay so some of the other work that's going on with my colleague Joe massara at JPL he's been using the sizes and albedo is measured by the the wise mission to recalculate the ages of asteroid collisional families and basically trying to figure out you know where are the family members and can be used albedo and color information to identify new ones and a number of people have been doing this work there's been a number of great papers that have come out recently to combine Sloan data with wise data to try to find the smallest members of asteroid collisional families and help refine their ages so you can see here there's some pretty distinct groupings in semi-major axis versus inclination and these these family members all seem to have fairly constant albedo so that suggests they really did come from a common origin at some point in the past when you remove them you can see that there's still some residuals left over but this background gradient may be closer to the original gradient that left over from the formation of the solar system so that's a lot a lot of stuff remaining to do there and of course with comets we have a really large sample of comets now it's close to 200 objects including a couple dozen new discoveries the thing that's useful about this is you know if a comet is inactive and we can see its nucleus of course we can get the sizes which means we can calculate the size distribution and in principle if we can get you know get all of these nucleus sizes figured out we should be able to apply the D biasing formalism to understand the true distribution of orbital elements and sizes among the Comets now for active comets of course it's you can't get any CLIA size but what you can do is figure out the sizes of the particles you can figure out their abundances and you can also even calculate in some cases the particle lifetimes how long does it take for those the particles to get flung off the comet so there's a lot of great work that can be done there and this is like what I like to call the family portrait of the Comets wise we don't get to choose the name that's the IAU rule they're all called wise our neo wise but there are a diverse and kind of motley bunch and you can see they span the gamut from some very very active objects to things that are really barely active at all so there's a lot to be learned about how comets become active as they push the Sun and hopefully these data will contribute to that okay well one of the nicest things about doing a large survey like this is that it really provides the community with a resource for trying a lot of different experiments and this is for me been one of the most exciting things is you know obviously I have the projects I'm interested in but there's been it's it's been really satisfying to see all the great things that everybody else is coming up with and the wise main mission the baseline is now up to something like 1,100 refereed papers and even just the NEOWISE data just the single epic exposures have been given rise to a couple hundred papers so far and then if they really span all kinds of different subjects so like I said that's that's been fantastic and really happy to see that so as it turns out I mentioned that the spacecraft was was put into hibernation in February of 2011 and we did that because that was the end of the prime mission we agreed with NASA that it was done we still however could use the 3.4 and 4.6 micron channels even though they had run out of gas so what happened was is basically in August of 2013 NASA asked us for a proposal to restart the spacecraft just using the two remaining channels and we turned it in and basically what we needed to do was this in the hibernation State the spacecraft was kind of parked just pointing up if you will just sort of pointing just at the North Pole just just up the thing is at this point we're viewing the earth for half an orbit so that warms it up and basically the telescope got to a couple hundred Kelvin in a very short order so the question was would it cool back down and basically all we did was we resumed Zenith point so we just basically stare at deep space at 3 Kelvin and sure enough after about three and a half months the telescope our equilibrated and it's now at about 74 Kelvin and that's cold enough for our two short wavelength detectors to operate with very low noise so sure enough we got the first good images here this is astronomer christmas and hanukkah and years and all that there's a lot of fun and it came right back and six days later we had discovered our first new near-earth asteroid after we switched it back on so it really worked beautifully so the goal of the reactivation mission now is to basically continue surveying for near-earth asteroids and comets and and the goal is not so much discovering large numbers of them because this satellite was never designed to do this and with its two remaining channels it's never going to discover a lot of objects but it does contribute a couple of unique things it's possible to get sizes and albedo's which is very difficult to do if you have visible light alone so that's good it also lets us discover the large objects we're particularly sensitive to large very dark any owes and this is a segment of the population that's harder for the visible light surveys to see and what we're hoping to do of course is with the extra survey data we'd like to be able to set some constraints even if they're rough on some of the more exotic populations like the earth Trojans and some of the core Battelle's and that just takes a lot of imaging and of course we see a lot of really fun objects this is one of the comets I think this is comet pan-starrs up here zipping by and so of course we can still get size distributions the first thing we did was check whether or not the telescope had been affected by the hibernation it hasn't the image quality looks pretty much the same the photometric zero points are unchanged sensitivities that you know pretty much unaffected so that was good and there's our first Ennio discovered six days after we restarted and basically just to show the the face space that NEOWISE is really good at is basically again here's that familiar diameter versus albedo plot the black points are the new discoveries and this particular plot sab it out of date but you get the idea big dark stuff that's what we find so that's the unique contribution of NEOWISE and contributing lots and lots of albedo some diameters for these objects so we're observing roughly about 0.7 any oohs per day so we're trucking along so after three years or so we should have observed a couple thousand objects if you include stacking by the time the mission ends so here's one of our fun finds little weird stuff like this comes up all the time we discovered this asteroid and it turned out it was gonna make a very close approach about a month later after we discovered it and sure enough about three lunar distances well this was close enough for such a large object that it was actually a beautiful target for radar and the thing is this would not have been observable otherwise by any of anybody else because we founded it - 72 declination and it stayed there basically right up until the day that it made the closest approach so it would have been unobservable to folks on the ground and sure enough when we got the radar image it was really kind of gratifying to see this because the light curve of this thing was was really quite large very large amplitude light curves suggesting that it was a very elongated body and the effect of spherical diameter computer thermofit was around 330 meters and this was the the little world that was revealed to the radar looks like a bowling pin in China so sure enough elongated and about 370 meters along the long axis so the neat thing is if you can take a radar shape model and combine it with infrared observations you can get out a unique property you can actually get the thermal inertia of the surface there's really not a good way to get that except by doing this technique otherwise you have to go to the asteroid and what this tells you is whether or not the surface is covered with bare rock or fine sand so it gives you an important clue as to the nature of what's on the surface so in any case like I said Neal wise was really never designed to do this kind of work but we've learned an awful lot from it and it's been an important prototype so what we really like to do is design a new infrared telescope that actually is optimized for finding new earth objects and we very creatively call it the near-earth object camera neo cam like I said not not a particularly spectacular acronym but you get the idea and the idea is basically to to solve a couple of logistical problems with wise to optimize this this mission for really going after the near-earth object population and not just observing a couple thousand but observing almost you know all of them larger than a certain size basically observing most of the really big stuff that's out there so the idea is basically to make something that looks kind of like Spitzer with a couple of important differences no cryo stab the idea is to go to the earth-sun l1 Lagrangian where we can basically get cold enough that we can passively cool to about 35 to 40 Kelvin and just put on a whole boatload of modern detectors megapixel detectors and from there we can just stay there passively cooled we can last for quite a while and just survey the whole region near to the earth one of the other key changes is to be able to look inward from 90 degrees to about 45 degrees solar elongation which is that's the region of space around the Earth's orbit where the most hazardous objects spend a lot of their time and the idea is to basically just sit out there and then repeat the survey over and over and over again and look for most of the near-earth asteroids so we were actually awarded technology development funding in last discovery rounds in the last discovery round and that allowed us to mature our infrared detectors so we've been able to make some new infrared detectors that actually work at ten microns at 35 Kelvin these are work ads which ordinarily cut off at 5 microns and we've been able to extend that out to about ten now so hopefully we'll we'll be repurposing this in the next discovery call whenever the AO is out which is hopefully very soon and we'll see what happens so thanks very much for listening and especially thank you to the Minor Planet Center for hosting all of our observations we really appreciate it thank you [Applause] yeah let's see here sorry so one of the things I should not speak for Francesca here but basically one of the things like I can't speak for there's basically she's found that there is a lot more mixing of different types of moment types that previous studies have suggested basically things are in places where they knew you shouldn't be or where they're not inspected so now as far as the metallicity goes it's really hard even for a mental compensation for an object it takes a lot of different parameters to be able to constrain that but there has been some work recently where people have tried to look for things that have anomalously high being around her it's big via a kind of catch-all parameter the thermal models if that number is very high it is possible that an object could have high thermal conductivity could also mean some other things but it's kind of a place to look and that works because by Al Harrison you make your day but with all that after this one back from the right oh we have families this one yeah yeah so basically if you remove the collisional families this is what's what they'll work at and that shows the gradient from high albedo to low albedo so the idea is to try to pull out the family so you really can get a good look at what that gradient looks like that was one of the reasons for finding a family members yes is it possible to look for the children's impact on in the new wise archivist or so we actually tried that and it turns out we just didn't cross over that region it would have been vanishing anything this is such a tiny object now the smallest thing we've done out of our are doing this of ours is roughly eight years but it had to be about three hundred distances away for us to see it it's just very thin here so you make the point that you don't think that what you're seeing is a collection if there is a selection effective sorry what is the what is the uptick of the orange points there so you're just following along why does it turn out right we actually think that this is scattering so as you get the smaller sizes your error bars are getting larger and what's happening is you're just it's like if you clip off half of the Gaussian so you're actually not seeing the little bits noodles can't see them there or you shouldn't be there but yeah what's happening is though that the error bars are actually getting it's smaller paper early on you mentioned but the so called mirror that's those have only been there couple of orders of magnitude less and they young now I'm not sure what you meant by you mean they were created over they put into the absorbers on the back door that's right they're dynamically young that's right and you know I kind of glossed over that whole process but exactly how the near-earth object population originates from the medial aspects of the whole other story and and there's a whole lot of work that's being done to basically try and confirm you've ever improved models exactly how the Indians got from the different source regions within the main belt in the comments and that's one of the things we'd like to be able to work on so we think that there's certainly preferred regions and residences that that you know basically get rise to be a knows you would suspect that the size and I'll meet of distributions of those source regions should look similar if you convolve them all together to the any else so that's one of the things make sure you cut down the estimate of a number what was the error made to Nicholas Rogers yeah so the error is you know in the previous estimates are fairly large I think it's thirty six thousand about one hundred thousand now as more survey data are collected for his numbers are being refunded but at the time that this paper was published that was that was sort of the leading number right there so we have fifty thousand eight thousand roughly and they don't put as spin gin and arrow bar that estimate so one of the things we've been able to do is because we're sampling the size distribution directly we don't have to convert from each that gives us an ability to really try to hone in on what the real error bar actually is yeah I think the the issue with the original model was they were using a bad albedo mean albedo calculation right and that was probably why there were they were coming up with more objects as possibly so neo cam will let start to fill in the smaller objects for the hundred meters yeah it's actually going to find quite a large number than the mean so you know there's there are a lot more objects that the small sizes very large sizes of course so even if you find a smaller fraction of them you're still finding a larger number an absolute sense I'm surprised because the curve goes way up just of a hundred regions and it's like terribly unknown how many variety 150 yeah that's one of the things we like to sell so one of the limitations I saw just because it was a one-year mission was that while you saw a lot of near-earth asteroids they can both cracked around you yeah that's and so you must really have a subsample is the tremendous observational bias because everything that happened be doing along with you right so how were you able to go from that which would sort of identified everything that was right around you to say we've discovered ninety-five percent of all the large objects all right so one of the things you do is you basically look at a lot of different trial populations and you know that the thing about this is is it's true that there you're observing a spatially local clump but they represent the broad diversity of orbitals and that's that's one of the things the other thing is are there classes of objects that we missed there might be a few but I mean we surveyed the whole sky so we weren't missing the high inclination population for example we were sensitive to regions in orbit along the face face that are harder to get on the ground plane behind collection objects for example we think it's a pretty representative sample of each just when you look at that you see although all the new the new Ennio is near the earth are clumped right because as you going in look you worry about you know biases and mean an omelet for example yeah but we got an observed anything that suggests that there were there's something unique about this particular group that we happen to see this is why we would like to do a larger project though because there's certain ease you just can't answer with this sample size beyond a certain point you can't cut the sample down anymore what we're in orbit with this government right at the tip of it there yeah because they don't get out to 90 degrees very long this was really lucky we just happen to catch this object it has about a three or four hundred year library cycle but that's kind of one of the interesting news about these is I think a lot of these things actually do library they don't sit right after the marketplace a lot of them are gonna pop out originally but they just won't because of they can't get too close to the earth and still be charging so like below 500 meters or so you really start losing respect to the population how hazardous are the objects that like 300 meters are on there so you don't think it sucks in your regional marriage that you would not want one of those things I mean one of the things we tried to do this is my colleague Don Yeomans likes to say that you know the three best things you can do to mitigate asteroid impacts are find them early find them early and find early basically if you designed a survey that looks for objects when they're you know decades away from impact and you can actually so that's sort of what you're trying to get us so if finding them early is so great then that implies that you could maybe even predict sort of how they would come in and where they would land is that really true or do things get pretty chaotic right at the end depending you know very sensitively on an angle in the atmosphere all right so so one of the things is I mean when you first find an object you know very little about it we you know in one day Tim is the world's expert on this right here I mean you could speak to it way better than I can but basically with one day of observations like what we get you can broadly say so then you know it's not enough to say more than that you obviously have to keep watching it on the appropriate cadence so that you keep extending it archival and you really want to have you know about a month of observations of it if something ever popped up though that's a particular interest the whole community not miles on to it the Apophis object is there isn't that looked like it had some potential impacts and of course the community immediately started to observe it and that almost always refines the chances of a drum drastically downward villains because it's a better BIA that size of the uncertainty lives you have a large inserting in the universe and as you get more outfit observations that it's narrower and narrower and so of course it's it's almost written as the earth but basically you know this is this whole network here at this point what we have is a I would say a very good network of warning in other words if you input observations into the network that there's an asteroid it immediately goes to the Minor Planet Center goes to JPL and they have a really fast response system for figuring out if something is an actual hazard but what's missing there's more observations so that's kind of how actually I'm following up on that if you get the Neal cam and you find an object that's in this arc of you know less than 90 degrees elongation well that's the thing the survey has to do its own problem the cadence has to be designed so that it doesn't rely on others to follow up so what you have to do is you you basically are kind of building a sort of a geometrical progression in time to extend the arc out from one day for a few hours through Danny for a few days to a month and at that point once you have about roughly a month's worth of observations you have enough to go on that you can pick it up and again when it makes it the next apparition so I mean for most of the objects that's more than enough right because you know oh no not hazardous throw that one back you know interesting for science purposes but not for hazard calculation in other words we might want to know something that it's its orbit and it's evolution well its characteristics but you're really only looking for the ones that are actually an impact hazard and those are the ones that you're going to spend extra effort trying to observe so is it better to be a 1 or would it be better to be so we've actually looked at that pretty extensively and it turns out it's not better just because if you're a tall one you can survey so much of their volume around the earth that it does just as well then the problem is is the minute you go into an on nearby orbit you have a huge data rate problem and we just get killed because it's no good having 30 megapixels worth of pixels if you can't get the data back darkest bodies if you guys detect where sensitivity device was extremely visible what sort of a magnitudes are our packages four new lies the famous things that we typically find the absolutely no certainty of 23 when you discover them and it does actually make some of the follow up pretty challenging because sometimes we find stuff is you know the typical follow up observe your network is sort of running from your class telescopes so every now and again we've actually begged and borrowed balloon I would just been great to follow some of the really faint stuff I really don't object so is this something that we wear Alice's gene you were yeah I mean they deliver go absolutely I'm having a very deep sensitive ground a survey would be very complimentary for a couple reasons it would help to extend the arts politics but also that's how we get our vetoes you might not believe us because that tell you something about whether it's part of a research study in a very general sort of way so yeah the survey has to do some follow-up and one thing I forgot to point out all the data from the prime mission of public they're available at Ursa 3d and thread science archive so these go there you can access all of it and our next data release from the restart [Music] where there are hazards now but we will want to check on them in 50 years and they'll be like yeah that's the definition of the PHA actually I think that gets back to the definition of what a potentially hazardous asteroids is it's basically something that gets within 2005 a year of the Earth's orbit the object's orbit not necessarily that week at that point in time but the reason that they chose that particular number is if you look at perturbations from the other planets that's enough one about a hundred year timescale to say that you can't the unequivocally rule out an impact so these are the ones you really want to pay attention to and again it's all a question of how many in the order 100 years the kind of time scale for yeah [Applause] [Music] this right there is where we lost the 12 and 22 micron bands that's where the gas ran out so to speak actually the frozen hydrogen not gas but this right here is the formal end of all of the cryogen this red dashed line and we were able to continue the survey for an additional four months just to fill out the inner edge of the of the solar system right here and on February 1st emission was placed into hibernation and that was the end that was it so what did we learn from all of that well like I said we have a sample now that we can do by us because we can measure its its sensitivity and the properties that it should have seen so the basic idea is you you take the observed sample as a function of its orbital elements and physical properties such as albedo and diameter and then you basically model out the survey sensitivity as a function of all of these parameters so the way we do this is we look for objects that have very well known orbits and we see how many of them appeared in a frame and whether or not we saw them in other words we look for what things should have been there and did we see it as a function of magnitude and then basically what we do is the other method we do is we take a synthetic population and we basically play that through a survey simulator and we tune the properties of the input population until it matches what we saw and that allows us to tell that ok these are the properties of the underlying population so the basic idea is we are testing a set of orbital elements that we think represents the near-earth objects in particular we assign a series of physical properties like size distributions and albedo distributions and then we basically just keep tuning those and we basically say ok let's calculate the position of each object in our synthetic population let's calculate a model thermal flux for it at the wise wavelengths and then see whether or not we should have found it based on what we know about the sensitivity the instrument we also have a complete list of all of the pointings in the whole survey so we know exactly where we pointed and when and so for each frame we basically calculate what should have been in the frame and we see it and then we just keep tuning the parameters until we get an output of the survey simulation that matches what we saw so what did we learn so that's basically I just said in words what's on the chart here so we'll ignore that couple things came out so the first thing is when we looked at reflectivity of asteroids as a function of size there was something that was was immediately kind of obvious and that is that the distribution of albedo is the ratio of bright stuff versus dark stuff which roughly corresponds to the objects that are primitive carbonaceous objects versus stony objects or you know more processed objects is roughly the same as a function of the diameter over a fairly large range of diameters so all the way from you know pretty large sizes a kilometer sizes down to hundreds of meters the ratio of bright to dark is is roughly the same and that's contrary to some previous studies which found that asteroids were supposed to get brighter as you got smaller well we think that that was actually an artifact of selection bias in other words if you have a visible light survey it's harder for you to see the small dark stuff so you could mistakenly infer that it's not there we think that it is there yeah it's just that if you select based on the thermal flux which basically doesn't depend on the reflectivity now you see the small dark stuff in its accurately represented proportions so that was one output and then we can try this thing where we we basically say okay here is our 12 micron selected sample so this is the sample and the previous plot selected on the basis purely of its thermal emission compared with the sample that was selected based on its optical fluxes and sure enough now as you get to smaller and smaller sizes there does appear to be a clear trend of the asteroids getting brighter as they get smaller but we think that this isn't real we think this is probably selection effect if you select based on the optical flux of course it's harder to see the small dark stuff but if you select based on the thermal flux well you see the dark stuff just fine so maybe there is a real effect down here at the very smallest of sizes but it's at this point its tangled up with the selection biases of the visible surveys and so we really can't say for sure what's going on but my bet is survey selection okay so one other thing we were able to show is that by using our model populations we found that yay verily we pretty much are unbiased with respect to albedo as you would expect and because this is an all-sky survey we're actually pretty sensitive to the higher inclination orbits which are harder for the ground-based surveys to see so when we back out the properties of the underlying population from our survey simulation what we found is that there are actually somewhat fewer near-earth asteroids out there larger than about a hundred meters then prior estimates had suggested and we figured that there are about twenty thousand five hundred plus or minus around three thousand near-earth asteroids now I'm not including comets at this point just near-earth asteroids that are larger than 100 meters and of those about maybe 25% of the discovered so the size frequency distribution is a little shallower than private previous predictions now what that means and why that is we don't know but it does set some constraints on the formation of the near-earth object population that need to be satisfied when we look at the matches of our sample population to R and that's not really enough to see where it's going to go it's enough to say broadly this might be a near-earth object but it didn't say it doesn't allow us to really nail the orbit down or predict impacts or anything like that so basically the chase was on to do this year long observing run essentially and get follow-up from from places all around the world to trump try to nail down near Earth object orbits okay so here are some of the team members and we're partnered with with iPAQ for our data processing and we've got a slew of postdocs who've been very helpful and we have also worked with a number of folks like Dave phone at and Tommy Groff who's a former student here so why would we want to look at asteroids in the infrared well there's a few reasons the basic physics is pretty simple the asteroid fluxes consists of a reflected light peak and emitted thermal thermal peak as well and the thing is is that the reflected light flux is a much stronger function of albedo the reflectivity of the object compared to the emitted flux and this has a couple of different consequences first off the infrared fluxes are basically insensitive to albedo so a dark object is equally bright more or less at infrared wavelengths as a bright object and that's important because we know that quite a lot of the near-earth objects are very dark and thus they're easily missed by the visible light surveys so that's one thing the other thing is since we have this pretty good sensitivity to to diameter rather than albedo we can actually nail down diameters quite well and this breaks the degeneracy between an object that's small but highly reflective and large but very dark so if we have good measurements that span the rotational light curve which we usually do we can almost always get the diameters to within about plus or minus 10% and that's compared to you know that's that's a considerably better accuracy for the diameter than diameters that you derive from visible light alone where you don't know if the object is bright or dark and then of course one of the nice things about working in space we're always looking at this low elongation region which is kind of tangent to the Earth's orbit if you think about it so we're looking in a volume of space around the Earth that's difficult for the ground-based surveys to see and of course we don't have any weather so that makes it easy to understand the survey biases or easier which helps for extrapolating the population that we saw to to what we think is really out there okay so there's one other thing that's kind of an advantage and that is the infrared sky is mostly blank if you look here the wavelengths are color-coded so that 3 & 4 microns are though are the blue and green wavelengths so they look cyan and 12 microns is red and you can see there's really not a whole lot of red in the image and the red things that you are seeing there are asteroids this is what they look like to wise it basically strings of dots these are what we call track let's position Paris so the nice thing about this is that even though the telescope aperture is small meaning that the spatial resolution is relatively poor compared to a larger visible telescope the sky is relatively uncrowded so there's a lot less chance of landing on a star or galaxy and that helps with with confusion that's why we can get away with using such a small telescope okay so this is what a typical track look looks like and basically you can see we've got a series of exposures at 12 microns and 22 microns and then we basically just look at a series of exposures before and after the same patch of sky and this helps us to figure out that there's not a star galaxy there that we're confusing the the source with and then we also look for latent images the exposures immediately prior to - the ones we just collected we require a minimum of five detections to figure that something is an asteroid and what that allows us to do is basically be very reliable meaning when we pull stuff out we're pretty sure this is really an asteroid and not just a collection of cosmic rays for example so basically like said our average observational arc though is only about a day and a half which means we have to have ground-based follow-up relatively quickly or the objects get lost and so Tim and his company at the MPC do a fabulous job of handling all the observations from the spacecraft and making sure that the word gets out to the ground-based follow-up community okay so this is what we ended up with during the prime mission this is sort of the the solar system as viewed by the NEOWISE project so you can see where we're starting off here and we're always looking at about 90 degrees from the earth so we're looking in a different direction than the ground-based surveys which are typically surveying the regions closer to opposition and each of the dots is an asteroid that we've observed and the Reds and the greens are the near-earth object populations the red ones are our discoveries the greens are other people's and then the Comets here are the blue and yellow guys and in total we observed about a hundred and fifty eight thousand asteroids so what this allows us to do that's the big difference from before is now we can get sizes for these objects and reflectivity zal vetoes which has some correspondence with their composition so in the past only a couple thousand objects have had any sort of either taxonomic classification or radar measurements to really nail down sizes so basically this gives us a whole new look at the physical properties of the the small bodies in the inner part of the solar system and you can see a few things here I mean here's our Galactic plane passage even in the infrared we are still confused by the Galactic plane the Jovian Trojans are right here and you can see this then these guys this it's now they did get out there's a door he makes it out the door which is good but then there's these other guys who are still under there so it doesn't go so well but anyway they eventually get out to the the thing so but you know anyway most of the injuries were because people ran to the windows when they saw the contrail it took about a minute or two minutes for the shockwave to reach the earth and of course people who remember the duck-and-cover days knew not to go to the window but rather to stay under the desks and thus they avoided some of the flying glass injuries but a lot of people ran right for the windows and and that's what caused most of the injuries so but but nobody died fortunately and it was a very tiny asteroid this turned out to be roughly 17 to 20 meters across as estimated from from various methods but including infrasound stations that are essentially listening for you know nuclear weapons violations or something like that or missiles and it basically had about a half a kilo 500 kilotons of TNT but you know most of that energy dissipated harmlessly in the upper atmosphere so in the end this is actually a piece of it right here this is my very own little piece of the rock I managed to get a small piece of this and it's a really interesting rock as far as meteorites go this is one of the most common types of meteorites and so it's an ordinary chondrite and what is kind of noticeable about it though is you can see these these veins these dark veins running through it and these are records of previous collisions that the object has undergone tens of millions of years ago so it's a heavily processed object you can see it's a nice fusion crust on it from the entry into the Earth's atmosphere but basically this is a fragment of an asteroid that was long ago impacted by other asteroids probably many times over the course of its long history and eventually it got in the way of the earth now why was it not noticed beforehand well it came out as it came out us out of the direction of the Sun in the very last minutes before its final approach it basically came out from about it was around 10 degrees or so away from the direction of the Sun so you know during the daytime sky where ground-based observers can't see and it was a very tiny object so is extremely dim and faint for you know basically the minute that it got even a little bit away from the earth it would have been far too faint to be observed so this begs the question of course how often does this thing this is this sort of event happen and we have some very rough estimates but we'd like to refine them and this is what we've been working on in my group but basically we know that really large mega events happen about every 100 million years there's a little bit of the entropic principle here I mean we wouldn't be here if these sorts of giant collisions happen more frequently so we know that they're very rare the estimate is sort of a one kilometer impactor about every 1/2 million years or so and down at the timescale of or the size scale of 20 to 30 meter objects it's about every few hundred years perhaps give or take but again small number statistics make this somewhat difficult to constrain and of course this is the famous meteor crater in Arizona which was created by an object not that much larger than the Chive insuk impactor that was about 50 meters but the difference is is rather than an ordinary stony meteorite this was thought to have been caused by an iron so considerably higher density and therefore a much bigger impact it wouldn't have just broken glass okay so of course this is the big one the Chicxulub impact er which was a very massive thing was about five to ten kilometers across and you know it was the equivalent of 10 times the world's nuclear arsenal all dropped in one place essentially it just it created a huge massive amount of damage and the ring shaped structure here is visible underneath the the Gulf of Mexico and what it really did was it basically wiped out about 70% of all land animals and about 50% of all marine species you can see here this is a plot of of genera so one rank up from species as a function of time and life is more or less lumping along here becoming more and more and more diverse and here's the event this is this is marine life so things under the water were more protected than the terrestrial animals in any case it was it was large okay so but again you know I think I think astronomers as a whole kind of thought well these are events of the distant past and they really don't happen in the present day and and this was one of the events that really changed people's minds in 1994 a comet was discovered on a collision course with with Jupiter and Jupiter of course shredded it into lots of pieces the question was whether or not Jupiter would even notice the impact of this of this comet Jupiter so massive and of course subsequent imaging revealed that yeah it actually did notice and it left these pretty large scars and of course each one of these is about the size of the earth so that really got people's attention and I think it's safe to say that that helped to give rise to the idea that maybe we should actually begin surveying for near-earth objects in a more systematic way so this is a plot that basically shows the discovery of near-earth asteroids as a function of time the red objects are the class that are generally thought to be a kilometer and larger so the really large planet Buster type asteroids and you can see here in 1994 is where things really started to pick up but it's important to note that most of the surveys that do the lion's share of the work here are really done by very modest sized telescopes they're sort of 1 to 2 meter class observatories they're not really very big big observatories by any stretch so while the one kilometer objects appear to be flattening out these these smaller objects just keep going up and up and up indicating that we haven't run out of objects any time soon so one of the questions with the one kilometer objects that was that people wondered about is have we really found all of the population or almost all of the population or is there some category of large asteroids that were systematically so this is one of the things that that people have wanted to look at so keep that in mind for later okay so this is our current suite of telescopes that do most of the work these days now linear is really no longer active but the Catalina Sky Survey and the pan-starrs telescopes in Arizona and Hawaii find almost all of the new earth objects that we see today and like I said they're discovering new objects at a rate of roughly a thousand per year and now of course we also have our first infrared survey NEOWISE which I'll talk about in a bit so and that time is now okay so why is this is the wide-field Infrared Survey Explorer mission and this is a medium class NASA Explorer so it's a fairly small sized project for a NASA mission it's about 320 million dollars it was a it's a 40 centimeter telescope that was designed to survey the entire sky at four wavelengths imaged simultaneously between about 3 and 20 microns and we had to use a two-stage solid hydrogen cryostat to cool the longest wavelength channels so I won't repeat too much of the seminar from from lunch today but basically the class of detectors we are stuck with using at the 12 and 22 micron channels has to be cooled to around 8 Kelvin and that really necessitated the use of the solid hydrogen so it launched in late 2009 Ned Wright from UCLA is the principal investigator of wise and the mission surveyed the whole sky finally ceasing operations in February of 2011 the primary purpose for the mission had nothing to do with asteroids it really was designed to to carry out an all-sky map at infrared wavelengths for a really general purpose astronomy and also with a couple specific science goals in mind finding ultra cool brown dwarfs as well as ultraluminous infrared galaxies and the asteroids were kind of we're kind of an afterthought but it turns out that was actually pretty good at seeing asteroids and that had a lot to do with the combination of a wavelength cadence and the field of regard so the wise mission is in is in a sun-synchronous polar orbit so it's basically about 525 kilometres and it always observes near 90 degrees solar elongation so this is a region of sky that's you know sort of the Twilight and Dawn sky on earth it's reasonably difficult for ground-based telescopes to spend a lot of time there just because of the diurnal cycle but this is where we observe all the time so the scan coverage results in lots and lots of coverages up at the ecliptic hulls and on average about 12 observations in the ecliptic plane so for asteroids this actually turns out to be pretty good and what we do is we use a very small mirror to quickly flip back and forth between exposures the mirror is only about yea big maybe the size of a compact and that is what allows us to switch very quickly between the different fields okay so here's a picture of the observatory just before launch and you can see here it's it's not really that big this is perhaps about half of life size it's about 15 feet tall and it's got the solid hydrogen Christ add up there the star trackers with the red caps solar panels all the usual stuff that the spacecraft has but what about the asteroids well it turns out of course you know this was not a mission that was dedicated to searching for asteroids and the original plan was to take all of the exposures and like I said they were on average about 12 exposures on the ecliptic plane rising to hundreds at the poles the original plan was to co add all of those exposures together and then just serve the the image atlas and catalogs that resulted just from the co ATS and that's great if the targets that you're looking for don't move or change on timescales of hours but it's really bad for asteroids because the co adding just washes them right out so we were actually given some extra resources from the Planetary Science Division of NASA to do a couple of things first to archive all of the individual exposures as well as the extracted sources for each individual image and then also to add some software that would allow us to actually dig asteroids out of the data in basically real time so basically we took some software that is very similar to that used by pan stars and we basically retrofitted it to work with the Y's cadence and we call that the moving object pipeline which run twice a week and basically we delivered what we call track let's position time pairs for the asteroids to the Minor Planet Center here and NPC receives all of our chocolates and basically figures out which asteroid does it correspond to so the idea was to do that fast enough that if we saw something that looked like it could be a near-earth object there would be enough time to quickly go get ground-based recovery of the object because we only observe an object for typically about a day or a day and a half at a time [Music] [Applause] [Music] okay it's my distinct pleasure to introduce dr. Amy Meinzer she's here I is this the first time you've been to the CFA it's amazing the first time she's visited here Amy finished her PhD from UCLA in 2003 and she's done even before that but but since some very remarkable work involving both Spitzer and wise she is the PI of any o wise which is his extension to wise that has done really fantastic work discovering minor planets in our solar system an unveiling not only size distributions but about albedo distributions and I think you know you'll be quite excited to hear some of the results that they've gotten this is a really a very effective team that she's assembled she's also a p.i of a neo cam you hear a little bit about that but I'm not going to take any more of Amy's time because she's going to show some very fantastic work so please thanks Matt can everybody hear me okay great okay thanks so very much it's really great to be here and to see a lot of really familiar faces from from the past and from other projects so I'm here to talk a little bit about the small bodies in our solar system the asteroids and the Comets and just you know what we know about them and what we would like to know and how some some recent new mission data has has filled in our understanding of these populations now for a lot of you this is going to be completely old hat and I apologize but since there are a lot of the people who don't necessarily think about the solar system bodies all that much I always like to start off with just a very brief guide to the solar system most of the asteroids in our in our solar system live in the main belt between Mars and Jupiter and they're an ancient population they've been there for billions of years and in most cases that's where they stay in that's they just don't really go anywhere and they've been very stable over long periods of time they've been heavily processed through collisions with other asteroids and through non-gravitational forces that sort of move them around a little bit but generally they stay there for long periods of time and as of today we know of roughly 600,000 of these objects now by contrast the near-earth objects refer to the asteroids and comets that that enter the region around the Earth's orbit and unlike the main-belt asteroids which are quite ancient than any O's as they're called are extremely young they tend to last for only a few millions to few tens of millions of years once they reach the region near Earth basically one of three things happen eventually they either hit the Sun they get scattered completely out of the region and and in some cases even just go way out into the outer reaches of the solar system or eventually they hit a planet so the question is how often do they hit the earth that's that's one of the first questions and you'll notice also too that there's a there are clusters of asteroids here these green blobs these are the Jovian Trojans these are in gravitational resonance with Jupiter now back to the NE OS so unlike the main-belt asteroids where we know of hundreds of thousands the any OS we only know of about eleven thousand six hundred today roughly and that number is growing by about a thousand per year on average okay so this last year 2013 was really a remarkable year for asteroids scientists and I'm gonna just use my laptop audio here you just bear with me but this was really a strange day because we knew that there was going to be an asteroid making a very close pass by the earth but it was well known that it would not hit and so I was actually driving home from LAX and listening at about 11 o'clock at night and listening to the radio that's what happens and I thought wait a minute this is this is not this this asteroid is not supposed to hit the earth we know where it's going what is going on here this is really really bizarre and all of these you know subsequent booms that you're hearing are the are the fragments bursting apart so the object left this huge contrail in the sky and it became clear that it was an extremely high altitude air burst because of the time delay from the appearance of the contrail to the sound waves arriving on the earth about 95% of the energy was actually dissipated harmlessly in the upper atmosphere tens of kilometers up so most of it didn't actually reach the ground but what did reach the ground caused a fair amount of chaos mostly mostly in the form of broken glass and most of the injuries were were glass flying people when they saw the contrails they ran to the window and because it took yeah

Channel: Phillip Herring

Views: 2,511

Keywords: Astrophysics (Field Study), Astronomy (Field Study), Solar System (Star System), Small Solar System Body, gresham, gresham collge, gresham lecture, gresham talk

Id: 5CNXr6g9yZM

Length: 87min 5sec (5225 seconds)

Published: Tue Jan 16 2018