 Yeah, spaced out. I see what you did there. Yeah, a little space pun. We just got Matthew Siegler'd I'm gonna go ahead and get us started today my part and this is relatively short except to make sure that the cameras and stuff are working So welcome everybody to the SMU Department of Physics speaker series This is our latest event today and to introduce our speaker. I'm gonna turn things right over to Professor Crystal Lynn Smith So crystal take it away Yeah, hi everyone. So today we're very pleased and privileged to have as our speaker Dr. Ron Beluz Dr. Beluz and I are old friends. We got our PhDs together at the University of Maryland Dr. Beluz is an astrophysicist at the Johns Hopkins University Applied Physics Lab or APL in Laurel, Maryland. His main research areas of interest are asteroid collisional evolution planetary surface processes and planetary defense His current work combines theory computer simulations and observational data to better understand the history of asteroids and moons Ron was born in Davao City, the Philippines and grew up in Beirut, Lebanon He received his BS in astronomy and astrophysics at Villanova University in Pennsylvania and his PhD in astronomy at the University of Maryland in College Park Following his PhD, he worked at the Institute of Space and Astronautical Science in Sagamihara, Japan and the Lunar and Planetary Lab in Tucson, Arizona And in 2021 the asteroid 28594 Ronald Beluz was named in his honor So let's welcome Ron and I go ahead and take it away Okay, let's give him a round of applause. Great Thank you. Thank you for that lovely introduction. Sorry, Steve, go ahead No, I was just going to say that if the audience has questions the room isn't mic'd So just raise your hand and wave and I'll see you and I'll run a microphone over to you And Ron, it's okay if we interrupt you with questions. Is that okay? Yeah, that's totally fine. I prefer that. All right, all yours Okay, so yeah, I'm very happy. Thank you for that lovely introduction And today I'm going to be talking about the strengths of near-earth asteroids, which I'd like to start off by kind of breaking down what I mean by that Okay, there we go. This is the outline of the talk First, I'll try to introduce what is strength and why does strength influence asteroid evolution? Before that, I'll take a step back and sort of describe why we care about asteroids And then sort of the part one or following that is What would it mean by strength of solid asteroids? And part two would be how we derive the surface strength of rubble pile regulates And we'll get to what a rubble pile is in a bit And sort of what implications this has for defending earth's biosphere against the asteroid threat I'll summarize then with open questions for asteroid strengths and interiors if we can get to that So there's really three general themes when we talk about asteroid research The first is obviously the science and why we care about asteroids is mainly because they're tracers of Solar system formation and history. Of course, there's planetary defense both in terms of What they pose as an impact risk and how we might mitigate against an asteroid collision in the future And recently there's also been a sort of interest in using or utilizing asteroids as resources Mining them for rare earth metals as well as volatiles So I'll just want to break this down since I know this is a more general audience So as tracers of solar system formation history sort of this is the present day I guess architecture of the inter solar system Showing the asteroid belt. So each one of these dots white dots here is an asteroid And you can see the orbits of the inter planets Mercury Venus earth and Mars You can see that you can think of asteroids as sort of occupying two Main spaces in the solar system. One is sort of this asteroid belt this region over here And most of the asteroids we've explored so far with spacecraft and And when we think of sort of hazards to earth or the this sort of near earth population So everything that is interior to the Orbit of mars and you can see that it's much less intensely populated And I'll get to to why that is in a bit And so so asteroids really are the remnants of the planet formation, right? So these are sort of chunks of mass that Were never incorporated into a large planet If you took up, you know, if you summed up all if you were able to vacuum up all of this these white dots Even though they look like a lot numerous They really only make up four percent of the mass of our moon So so not much mass, but you know widely distributed So when we we think about the early solar system formation one of the other reasons we really care about asteroids is because um, you can divide up the solar system sort of the early solar system into regions where we think That was sort of desiccated and where water couldn't exist um, sort of vaporized away, um and then past the snow line, which is In our solar system somewhere in the middle of the asteroid belt where water can sort of exist And and is stable And sort of sort of one of the main reasons that I guess people care about asteroids and study meteorites and their parent bodies the asteroids is because we think that they They basically provided the earth with water and organic materials, which are sort of two necessary ingredients to kickstart life And so how this happens we think now is because you know So early in the solar system as the giant planets here in this Diagram of showing the left forms You can see that you had material that is sort of water rich and blue and material that is desiccated in red And it's through the migration of these giant planets In a in a model that that's now referred to as the niece model that You you have this material be sort of jumbled up Due to the gravitational perturbations of migrating giant planets that ends up mixing up the sort of these different reservoirs of materials particularly water ice And allows sort of the that those ice and water bearing bodies to Be distributed into into words the inter solar system colliding with the terrestrial planets Here shown in these open white circles And basically bringing those ingredients of life to earth and perhaps the other terrestrial planets as well And then what you end up with is the asteroid belt as we see it today, which is a jumble of things that are water poor and water rich And so sort of one of the main lines of evidence for this is the fact that meteorites are meteorites with with parent bodies That are asteroids really these carbonaceous chondrites Have hydrogen to deuterium ratios that are very similar to earth's oceans Um, so particularly these sort of material sourced from this these blue regions over here on the left The other theme that I mentioned is Asteroids as a threat to earth. This is a sort of fantastic plot created by alan harris and the lst collaboration showing the cumulative size distribution of Near-earth asteroids so things that Those are hazard or a threat to earth see that Say the kt the asteroid that if you're that led to the kt event or something about 10 kilometers in diameter That is sort of an extinction level impact But sort of a much smaller asteroid which are more numerous Also pose a threat and can cause global damage or regional to global damage And so the efforts today really from a planetary defense perspective are to to characterize This threat by discovering these asteroids. So this red line here Outlines the asteroids we've discovered to date Um, and the blue dotted line is our sort of best guess of what this population actually looks like so for things that can cause Say sub global damage and up to Why about life over a continent? We're thinking about, you know asteroids that are only 100 to 200 meters in size of which we only have characterized maybe 20 percent To 30 percent if or discovered only 20 to 30 percent of these objects And that is now sort of a congressional mandate to discover at least 90 percent of these so The next generation of telescopic surveys that will come along come online over the next decade like the large The lst st Collaboration as well as the nazi roman telescope and the new surveyor mission Their focus is to close that gap Or sorry, some one of their focus is I should say lst is going to do a lot of different science And finally this sort of Final theme that I talked about it was only, you know, it's been around for a while. This is an artist concept for Asteroid mining from the 70s. I believe Isaac Asimov wrote about asteroid mining In one of his short stories Go cast that rabbit And so so it's been in the sort of popular imagination for a while But there have been recently sort of serious concentrated efforts by Public and private companies to try and make this reality. But of course it's a very difficult endeavor Mining on earth is sort of a 20 to 30 year Usually 20 30 year project And fortunately these companies that have already started to go under Like planetary resources, but of course You need to have these failures in order to have sort of any progress, but Sort of this sort of a forward thinking approach To to think about asteroids and maybe someday they'll be utilized to extend human kinds reach into the cosmos So what combines all these sort of themes? The knowledge of asteroid strength is critical So what is meant by strength sort of a formal definition here is it's an object's ability to resist deformation by tensile compressive and or shear stresses And so what is the strength of an asteroid? How do we get to know? How does that even how can we even begin to determine that so? Um on earth if you want to know the strength of something say concrete What is usually done is you get a core sample and you bring it to the lab I'm demonstrating here and You get a specimen and you perform a you know a strength test Which is you try to destroy that sample by compressing it and in in that destruction of that sample you can get an idea of Sort of what amount of pressure or energy is needed to To to achieve this sort of irreversible strain on it But for an asteroid, you know, it can mean a lot of things it can mean so the how does it respond to an impact Either through the formation of a crater on the surface or through You know, if you wanted to deflect it or destroy it, you know, what is the strength against its destruction? um And then also It also means sort of what process is is is dominant in destroying these objects, right? So and that depends on a lot of things for an asteroid that depends on the asteroid size Where it is in the solar system and structure So thinking about size now So this is again a size frequency distribution of asteroids cumulatively And this uh figure over here in blue and red is what I just showed you in the past This is a near earth object population. So the thing was sort of over here Uh and orbits around there As I marked on the right um and the asteroid belt The number of asteroids is larger by three almost three orders of magnitude as we can see visually on the right um But you also have a sort of a larger variety of asteroids there um, so the largest asteroids, you know, we Uh are things like Vesta and Ceres which are so large that they know they can have almost spherical shapes and in this sense, we're maybe thinking more of dwarf planets than what we currently know or understand about asteroids, um, but Uh, but uh, we sort of know the geology of these things and their surface expressions Because we've sent spacecraft there like the dawn mission to Vesta on Ceres. Um, and the uh In the sort of late 90s early 2000s, we we had apl had sent a mission called called near, um to to uh to uh this main belt asteroid matilde Where did the fly by so this is a 50 kilometer asteroid still has sort of this spherical expression As we can see here, uh, but once we get to smaller and smaller sizes, gravity plays a weaker and weaker role um, and uh, this was the real target of the mirror mission eros, which was a 16 kilometer size Near earth asteroid, which was turned out to be potato shaped and we had not expected that at the time And uh, recently the cyrus recs mission By nasa went to a small asteroid to return a rubble pile Called venue So you can see here that you get sort of a lot of variety in asteroid sort of surface expression, which we hypothesize is Reflected in their interiors as well What we don't know yet is sort of what do things below 200 meters? actually look like And uh, sort of what is their interior structure like? And uh, the the interest there is like as I said from a planetary defense perspective These are sort of the most populous object And uh, most likely the object that we would have to deflect in in our near future Uh, so what profit processes effects um, uh, or our influence or Help us understand what asked what the interior structure and strength of asteroids are so in near earth space Near earth asteroids are influenced by uh, sunlight. So sunlight can actually cause uh, torques on the asteroid that spin it up um So you have sort of this re-radiation of sunlight and because asteroids are irregular in shape You end up having a net torque if you had a sphere There would be zero torque and no spin up or spin down but because they're irregular you have a net torque um, and uh, this uh One of the uh consequences of this is that asteroids are spun up so fast in time scales that are very short So for a kilometer size asteroid it only takes about a million years For the asteroid to spin up to the point where centrifugal forces exceed gravity And uh, one of the reasons why we think this is a dominant process in the near earth space is because We've actually tracked the spin periods of uh near earth asteroids by measuring sort of uh, their light curves Um, so so increases and decreases in brightness of these asteroids over time And uh, from there you can figure out how fast it's rotated. And so when you plot this so this is uh, spin versus size with increasing spin uh going Vertically up and increasing size going up to the right We notice that um for things larger than about 200 meters or there there's that kind of um number again These asteroids really don't exceed a spin period of about two hours or so And that two hour spin period is actually sort of the critical spin period for which um a you can think about it as a As a fluid would no longer be able to maintain self-gravity um, and uh, basically disperse and uh, the simulation that I was showing here is a simulation of an asteroid Being spun up by solar radiation causing it to form a top-like shape And as well as that material that it ends up shedding turns into a binary Companion and so 15 percent of the near earth asteroids have a binary companion We think that this is the process that that produces so this is then tells us something Maybe about the interior that maybe that they aren't sort of solid objects Above the size, but they're what we term rubble piles just loosely bound boulders and and cobbles and pebbles That are binded by gravity alone um another process where that has helped inform our understanding of strength is from uh planetary encounters so Near earth asteroids Will sometimes have close encounters with large with terrestrial planets like earth and when they do so they can be sort of tidally stretched And if they get too close they can this can completely sort of disrupt them and they turn into their constituent pieces we famously saw this Two decades ago when the comet the shoemaker levy nine passed close by to jupiter and was shredded and those those little pieces ended up impacting the surface of jupiter and providing sort of a the spectacular show um in in the near future in 2029 at the end of this decade a near earth asteroid called apophis Will have a very close encounter with earth they'll be Basically only five earth radii aware away from earth at its closest approach Um And this is um sort of much closer to the than the moon is to earth. It's actually closer than some of our satellites orbit around the earth so this uh, so asteroid community is very interested in this event And uh, it should tell us a lot a lot about the strength and material properties of these asteroids um from a sort of a more general sense one of the reasons why we care about These close encounters is because we think that They happen frequently enough that they refresh the surfaces of asteroids and it might explain why the The most common asteroids that we see called the s types Have reflectance spectra that don't look anything like their closest Meteoritic counterparts. They're called the ordinary chondrite meteorite. So these are the most common meteorite These are the most common asteroid type, but they their spectra don't look anything like so what we think is a process A process called space weathering is happening Basically, the solar radiation is effectively bleaching the surfaces of these asteroids and whenever a Asteroid gets close enough. Um, this is showing the lowest integrated minimum orbit intersection distance of these A sub type of asteroid called cues that look a lot like these very common meteorites um They whenever they have close encounters, um, essentially the tidal forces allow them to be refreshed and finally, um one other way that Sort of the solar system wrecks havoc on asteroids is through impacts impacts create Craters on the surface of asteroids But we also think that they cause seismic shaking because you have essentially maybe a solid block or a small relatively small asteroid being um, and a sort of being seismically shaken by by a surface uh It's sort of causing an impact that causes surface degradation Um, and we don't know whether this happens for sure Uh, but one of the indications we have that this might be happening is when we take a census of craters On asteroids that we visited there always seems to be um a sort of uh, we expect the craters To follow a size distribution Power law size distribution like the solid curves show over here But whenever we take a census there's always sort of this knee in the distribution Uh showing that there's a depletion of small craters and we think that something like this is causing that depletion So in this talk i'm going to focus on these um impacts, um that we've uh and sort of What we can learn about the strength of asteroids from impacts As well as the touchdown of a spacecraft on the surface of an asteroid So so how do humans and the creations influence Uh influence our understanding of the asteroid strength So i'll talk about how we found some broken things and broke through uh the surface of an asteroid okay So this is a mission that i've been involved with for a while ever since my grad school days It's a nasa mission called the osiris frex mission and it um, it's a sample return mission So one of the primary goal of this was to go to what we call a sea type asteroid This is a carbonaceous asteroid that is in near earth space, but is we think volatile rich or what was it something? And as I said, uh, you know during the early stages of the solar system You had the giant plant planets migrating around and causing huge gravitation of disturbances that likely kick these things into the near earth space um and um And that's why we have such a volatile rich object basically in in the vicinity of earth's orbit um And so we were able to uh obtain a sample About a year and a half ago in october of 2020 and we've already departed the asteroid since and the sample is on its way um prior to obtaining a sample we had to do Sort of like a large census of the surface in order to Understand what where we would be able to safely obtain a sample and that would be scientifically interesting So osiris frex is a spacecraft. It looks like this at solar panels. It's here for to obtain uh Energy to power its subsystems. Um, it has these two main camera systems that were used to image the surface has a Visible to near infrared spectrometer and a thermal imager called otas And uh for me for for my research in particular one of the things that I use quite a lot is something called the laser altimeter which Scanning laser altimeter, which I'll show the data out in a bit which provides a lot with a lot of structural information uh on the surface so So benu from not not from really knowing it's interior, but from its surface. It's really expresses From its surface expression looks like a rubble pie last way. Um, Basically, we don't think it's a monolith, but that it's this gravitate a jumble of pebbles and boulders and and dust that forms a gravitational aggregate And when we got there, we were sort of surprised by by the surface. We expected something much smoother smoother and sandier Beach based on its thermal properties Uh, but we got sort of a very bouldery appear Surface instead so you're highlighting just a few geologic features like these large boulders up here This one's about 50 meters in diameter. This one's an outcrop. That's about 90 meters in size. Um So so there's a and we've observed a lot of craters the largest one being about 100 about really 200 meters This is 120 meter on one on the equator But they're sort of large craters and small craters and we found, you know, the evidence of boulders that look like they're fragmenting perhaps due to Maybe a thermal fatigue process or impacts by micrometer rights But really the surface was was very bouldery Which made it challenging to decide on where to go for a sample As i'll say later As i'll mention in more detail later one of the the big challenges was is to find a place where we could obtain a sample well, um, that was That that sort of a surface part of the asteroid surface where Rocks were two centimeters or smaller because that was sort of the limit In size of Particle that we could sample anything larger would not be able to enter the sample chamber Um, this is where we ended up going. Uh, it's a 20 meter crater that had relative that appeared to have the uh the small sample material that we were looking for And it's also expressed itself, uh with with You know, it's sort of a haven It amidst a very uh dangerous region that full of hazards the boulders So in the process of looking for the sample we image the surface Really high resolution down to a few millimeters in size in order to look for those two centimeters of smaller particles In that process, uh, we found, um Evidence of boulder destruction through What looked like impact craters and Sort of we had not observed craters like this on an asteroid previously, but it wasn't until we went into a um a polar orbit around the uh around the Asteroid like i'm showing in these diagrams over here Where we were imaging a lawn determinator that these sort of very small craters started popping up and we're able to Cattle up them. Um, so Our idea here is that um, it's for the same reason that um craters on the moon are more uh divisible when you're looking at the determinator Because you're casting long shadows, so you're able to see um a shallower features At uh at the distance we were observing and that we're able to finally Make those observations and find uh Evidence of small craters on the surfaces of of boulders basically For the first time So so we had sort of what we call crater candidates like i'm showing in these two images And they ranged in size from three centimeters up to 40 30 centimeters and in this image. I'm showing here And we went back and looked at images that were taken at Um, I guess lower phase angles where the shadows weren't as stark Um, we noticed that there were likely large craters on boulders as well Uh larger craters up to five meters wide um But we only had this photographic evidence and uh photographs are sort of notoriously Um, it's it's it's difficult to to I guess validate some of your findings with images alone um And uh, but but this wasn't the first time we've observed Sort of craters on extra uh on solid rock on extraterrestrial bodies The these were last measured On uh on rocks returned by the apollo astronauts from the moons of lunar rocks, but not quite at this size and scale um So so to the rescue here fortunately We also had this instrument called the osiris Rex laser altimeter or ola for short And what ola was able to do was um, so it functions very similar to I think about it as a radar Um, but instead of using radio waves it uses visible light waves or lasers to get a very good, uh For high precision and high accuracy um Structural information of the surface So it's a sort of uh It was able to obtain more than three billion altometric returns in total That amounted to um a resolution of two centimeters per per pixel On the surface and this is an example of one of these ola scans that we're able to get so this looks like an image, but it's actually a a fully 3d point cloud of data of an asteroid surface um And so this is the ola data for example of that Crater on a boulder that I was showing earlier the image we got Uh, but now we have sort of a full 3d picture of it. Um, thanks to ola And that basically validated, uh our our initial Uh hypothesis I guess that these were were sort of depressions that were had crater form On the on the surface of a solid rock on an asteroid um And uh, and uh, we know that because they have dimensions and morphology that are consistent with Larger craters that we've observed on the moon and craters that we uh form in the laboratory through impact experiments So so big deal. It's just a bunch of holes on a bunch of rocks. So why do we care? um There are two things that we ended up doing with these that sort of we're super useful to To understanding asteroids in general the first was we developed a technique to Use these craters to quantify the strength of solid objects Namely the these asteroid on materials And the second bit was that we use the boulders as a witness plate to understand that uh, the dynamical history of venue um Whenever you have craters on the surface of a planetary body, they can tell you a lot about its impact history And if you know well the sort of flux of impactors or how many impacts you expect per year On the surface of of say the moon or an asteroid Then um by measuring the number of craters and their sizes you can get a good idea Of the age of that surface and we ended up doing just that So overall we measured more than 600 of these craters on But boulders across the surface of these asteroids and we were able to get their dimensional information using that laser altimeter data as I said And from this we're able to get the sort of disruption the threshold of the boulders and derive essentially This factor f that we were going to care about which relates the size of a crater to the size of the impactor It's one And this gives us that number two Point that I mentioned before the dynamical history and we're able to use the boulders as a pernometer essentially So how do you get if you just measure a hole on a solid object, you know, how do you even get a strength uh for that object? um and What we did here was we said, um, there should be sort of a maximum crater size for a given boulder size So you can think of, you know If you have boulders of the same size you can sort of make a crater on this On on those samples of boulders and keep increasing that ratio of the crater size to boulder size Until essentially the boulders stop existing, right? And you stop observing craters of a given size on for boulders Of that same size so there should be sort of a maximum Size hole you can make on a on a boulder before that boulder stops existing Okay, so so within this framework, we're able to equate two formalisms How efficiently you can make craters of a given size to how How much energy you need to disrupt a material of a given Sorry a boulder of a given material composition And with this framework we're able to get the strength of these objects But it first had what we had to do was we basically had to compile A lot of these measurements so that's why we did you know hundreds of these measurements and found sort of The limit of the crater size for a given boulder size as a function of boulder size And over you know, we found these sort of five boulders That were at this limit where we think that you know any crater beyond that limit it would have destroyed that boulder and From this we're able to come up with a relationship about on on essentially you know how quickly the boulders of larger sizes become weaker per unit mass and and essentially this is expected for natural materials and And there's really unnatural materials as well like things like concrete Because the larger an object is of a given material composition then The large then its largest flaw Could be even greater and an object is only as strong as its largest flaw so Well, it's much harder to destroy, you know a 10 meter size boulder than it is to destroy Sorry, 10 centimeter size boulder per unit mass or per unit I guess yeah per unit mass. It's much easier to destroy the the larger boulder in reality um, and this gets us to To uh to the idea of sort of a catastrophic disruption threshold which um Which in the planetary community we use a lot to quantify or parametrize Um, uh, the ease of which collisions can destroy, you know asteroids or planetesimals Which is handy for understanding collision evolution throughout solar system history Um, but what these craters were allowed allowed us to do was formulate our own catastrophic destruction threshold curve this q star d Which is the energy per unit mass needed to destroy a an object of a given size um And what we're seeing here is that small objects per unit mass are harder to destroy because they have smaller flaws um, and you get to a point or where there's a minimum size uh of object or sort of a weakest object which is usually 100 meters to to 200 meters in size where where Which is still dominated by its strength um Anything larger than say 200 meters in size will be dominated by The objects gravity instead and its strength properties no are no longer as important Um, what we essentially did here is compare our findings For the catastrophic disruption threshold in blue to things that had been hypothesized in the past for For the catastrophic disruption threshold by using numerical simulations And uh, what we find is that we think that these objects are weaker by At least an order of magnitude than had previously been assumed um But using these uh, this information we're able to get a strength for the boulder as well as this factor f Which relates an impactor size to the crater size Um, so so benu's meter size boulders then uh, you can say have a have a compressive strength about half a mega pascal Um, if we want to compare to terrestrial Uh materials this would be make them weaker than Than chalk. Uh, so so it would essentially be you'd be able to crumble these things In your hand if you had a piece of benu sample Um, and then we were able to use the craters on these boulders to basically show that During its time in near earth space Benu is only uh exist for about two million years or so What this shows then is that um, it basically tracks when benu became a near-earth asteroid I go to this in more detail, but for the Take of time, I'll just say that you know, it was basically validated these ideas that um, that you need to have a reservoir for near-earth asteroids and that's the main belt and uh, we showed that um That uh, that uh, that you know, this particular near-earth asteroid has only been around for two million years And uh, the reason why near-earth asteroids don't stick around for very long compared to the age of the solar system Four and a half billion years is that they end up either hitting the sun or being ejected from the solar system or hitting the earth So they don't they don't exist for very long the sort of transient objects So overall, you know, our finding is here was that uh, you know, benu material is very weak and And benu itself hasn't been a near-earth object for very long Uh, but there are other considerations. We haven't really explored yet Sort of what are the interplay with other processes that can break down boulders like thermal fatigue? Give me evidence that this is happening on the surface of benu Uh, there's also evidence of different sort of boulder types on the asteroids So can this one strength measurement fit all of them? And uh, we don't quite know yet what the role the The hydration of these boulders play so other asteroids, uh, like benu these hydrated The carbonaceous asteroids Are not quite as hydrated as benu itself. Um, they you know, we visited other Asteroids with spacecraft, uh, japanese spacecraft called Haibusa 2 went to an asteroid called Ryugu and showed that it isn't quite as hydrated as As benu itself experienced some kind of dehydration event, maybe a close encounter with the sun Um, the stat changed its strength properties. We don't quite know yet and so Sort of in the last third of this talk, I'll I'll talk about uh another Way we're able to get an idea on strength of the now the surface of the asteroid and this happened when we we sampled the surface um, so so Not normally that this uh spacecraft benu doesn't have this Long arm articulated. It's usually hidden away. But when we finally found a place that we could sample adequately That was sort of smooth and had small pebbles We articulated this arm um, and it has at the end of that arm is called the Tag sand which is a touch and go sample acquisition mechanism And the way this thing works is what we hope to do is touch down on the surface And that the surface would somehow comply like this diagram shows over here and then uh, once it we were we would um Once a trigger would would be released and that's basically when we felt the sufficient amount of force or uh We would release nitrogen gas which would mobilize the underlying dirt um, and allow it to be captured by the By the sampling mechanism enter the channel sampling chamber so we So benu's a small object small asteroids a half a kilometer in size like I said before which means it has a very low escape velocity so if you were to flig something from the surface at only Um 20 centimeters per second it would likely escape and never come back Um, so we touched down then knowing this we touched down at a very slow speed um, because we're uh in order to not disturb the regular too much So we touched out at 10 centimeters per second um onto the surface And uh and so previously had uh, yeah, I see that might I need to plug into my chart or pause us with that Yeah, but if you need to sort that out go ahead Yeah Yeah, let me do that real quick so that sure no problem Well, let's let me let me use that as a moment to see if anybody in the audience here in the room has any questions so far We can queue up a question for you Yeah, how about online? We'll wait for Ron to reconnect. I have a couple of questions, but um, I can ask a few now once we're done with this or I can wait till the end Yeah, let's let's give him a moment to reconnect But I'd say if you have one that's sort of relevant to the bolder part of the talk Probably a good idea to ask it um Yeah, I I'll Ron, are you able to hear or are you getting a charger? I'm back. Okay Yeah, the question I wanted to ask that Steve has queued up for you is um This this idea of cratering it it's I was wondering if you could talk a little bit about how much that depends on the Rubble pile versus solid body aspect of the objects. I mean does a rubble pile just absorb A cratering event in a way that a solid body would not So it's a great question. Um, it's really uh, not so much absorbing as it's dominated It's a rubble pile or sort of you would hit the Here I think I'll I'll show a diagram here. Um Um Okay, maybe this is the best diagram to show so you can see that the surface is this is a sort of a 100 meter by 100 meter surface and you have this 40 meter boulder in the That's dominating this sort of local area. So if you were to impact This boulder To kind of dislike the asteroid the response would be dominated by the strength of this boulder And we've kind of now figured out what that strength would be And so you would form a crater that you know has an f of uh, like I said something like 20 So if you hit it with a meter size object, you would make a 20 meter crater and likely destroy it but um If you were to hit with that same meter size object Sort of the dirt part of this the regolith And you weren't uh, you were no longer interacting with a solid object that f Uh, or basically the whole cratering process would be dominated instead by the gravity of the asteroid, which is So small it's a 10 micro g's So like seven times 10 to the negative five meters per second squared And your f increases then to a factor of like 100 instead So, uh your meter size Impactor would make a hundred meter size crater Um, which would cover this entire Region and you would eject a lot more material in the process I see. Yeah. Thank you. That's very clear. Yeah Okay, I think you can proceed front Thank you. I'm sorry about that. I apologize. Oh, sorry. Hang on this thing. We have one more question for you Let me run a microphone Uh, can you hear me? Yeah Yeah, I was just wondering, uh, what Property was a strength recording? Is that like a young's modulus or? Well, what is the strength exactly? Yeah, we think it's a, uh compressive strength Um, so the so basically, you know in this case it would be the pressure needed to um Lead to the complete failure of that of that, uh material All right. Thank you. Yep Okay, I don't see any more questions. So go for it run Yep um Right, so as this has been done before, um, so there has been there have been five Missions spacecraft missions to we call small bodies a mixture of asteroids and comets that have interacted with the surface before Um, I show them here as well as their dates updating back to early 2000s up to now and you can see that we've interacted with different impact speeds that is mostly set by the gravity of that asteroid. So these speeds are fairly close to the escape speed of those objects But they've had really different outcomes So we've had soft landings Small bounces about a meter in size. We've had really large bounces if anybody knows about the, uh european space agencies Uh adventures on the comet 67p This filly lander essentially bounds hundreds of meters and Was sort of ended up in a region of permanent shadow Which didn't allow it to uh conduct all of its science operations So this is fraught with difficulty essentially Um at benio, this was the first time we've had a case where we just sort of plunged into the surface itself um Here i'm showing uh snapshots of what happened with the filly lander and this is it's sort of its across the asteroid And again, that is a reminder. This is where we touch down on the surface of benio itself So what happened here? This is these are images taken about two seconds apart when we touch down on the surface And uh what happened here is that at the time of contact we we have you know, we're taking images of this This the sampler doing its thing, but we also have inertial measurement units on the spacecraft itself that are recording the The forces felt by the uh by the spacecraft and we know that at that Call the time of contact c we felt a force of about 10 newtons And uh which suggested that this is uh well shown a bit a very compliant loco fusion material um one second after we fired after contact we fired the The taxam gas bottle nitrogen gas And it seems we really mobilized so all this stuff flying out is caused by that gas being ejected um Six seconds after that we started a back away bird And at that point we were still moving forward at four centimeters per second um and and nine seconds after contact we finally started moving upwards and We had achieved a maximum depth of about 50 centimeters. So we were kind of That pogo stick arm was about a meter or long. So we were about halfway halfway down um And it wasn't until 60 60 seconds after contact that we rose above the original surface. So this is sort of What we had what we were dealing with which was You know quite different than the This animation that the mission produced When we when when this mission was being conceived and sent to space So space exploration is difficult. Um, so what we did here is uh, what we'd like to do Was was to basically get an idea of what the surface regular Properties of this regular surface is this by regular if I mean, you know, the the pebbles cobbles as a As a I guess as a granular material How does that behave in this low gravity environment? Uh, and what we did here is we ran a bunch of simulations dating back really to my grad school days as I think christen will it would Would recognize these um to basically get a sense of you know, what? the sort of one-dimensional penetration into A granular material in that microgravity environment. Um And the key parameters were very here was the sort of geotechnical properties of the of the of the Of the material namely his angle of friction and uh It's packing in this case and here we show two cases where we vary Just the angle of friction and we saw you know when we're doing our edge cases. We saw certainly Scenarios where we would penetrate down and others where we wouldn't and of course the one on the left more approximates reality um the issue here is that we we can get a good sense of What the angle of friction should be on the on this asteroid for the? I guess the granular material, which is That it's composed of Um, and you know when we measure the slopes on the surface they exceed 40 degrees slopes, which means that the For cohesionless material at least that that that that should approximate its angle of friction So um, so there's sort of so there's a problem here, you know If we assimilate something with you know packing that is what we expect so 40 isn't really out of the question for granular materials We would have expected less penetration um, and for the sake of time, um, I'll just say that Uh, what we what we ended up doing was we varied the uh, sorry, I just skipped that slide We ended up varying the packing fraction by quite a bit exploring this other phase space. Um, and um And what we found was that uh, when we do this we're able to get a a better idea Of how the force we felt Um Can be modeled by the geotechnical properties of the regulates. So we built the force of 10 newtons and uh, we think that the Grains have an angle of friction of 40 degrees um, and when we sort of plug that into our our form, uh, or what we expect Sort of our functional behavior These are the results of simulations i'm showing here. Um, we come up with a very low packing fraction for the material the surface of about 0.2 to 0.3. Um, Which means that there is about 70 to 80 that at the top sort of few centimeters or tens of centimeters of this rubble pile is is a 70 to 80 percent void space And you need really a very low amount of cohesion about 0.02 pascal to to support this structure um And what we find is that this sort of high a low packing fraction is consisted with other estimates that we got Based on modeling of other surface features of terraces that we saw on the surface So what this means is that the uh, the penetration depth in We find is that the penetration depth in microgravity regulates is really sensitive to friction properties Packing fraction we've come up with this force law for low speed impactors and the ferocity site of the touchdown site is is really quite quite large So um and uh, so the final point I wanted to touch on as I as I want to wrap this up is that Is the idea of planetary defense so so, you know first this question really hit the nail in the head You know for for one rubble pile asteroid if you were trying to deflect say a Bennu from impacting um earth Where you hit it definitely ends up mattering a lot whether you hit a solid boulder or or the regolith itself and and It matters even more, you know when we start thinking about Whether you know things that are say 200 meters in size Or a smaller are just maybe all boulders and no regolith and not bubble piles themselves as we As we're seeing from telescopic evidence So there's this mission that Is led by apl called the dark mission which will Demonstrate a kinetic impact or technology And what we're doing here is we're going to be hitting the The secondary of a binary system to see how efficiently Kinetic impact so this is just sort of a dumb mass hitting an asteroid At six kilometers per second how well can we deflect it off its Off its orbit and what we're expecting is a shipped in the orbit Orbital period of a few centimeters per second sort of a few yes a few a few seconds that will be detectable From ground-based observatories But there will be a follow-up spacecraft sent by the european space agency as well that will Characterize the orbit and surface properties even better So i'll go back to this diagram again and uh and say that you know these these boulder the Maybe the boulders that we've characterized on venu. Maybe this is their strength are telling us something about the strength of the the most populous and uh Hazardous object that who we would likely need to deflect in the future Um, and we've seen evidence of things this size of 160 meters being solid boulder bouldery objects on the surfaces of other asteroids the other asteroid c type asteroid yugu And if this is the case or what our analysis is showing is that you would need Uh, maybe 30 joules per kilogram, which is which is a lot in order to deflect or disrupt this body Probably a lot less if you wanted to just deflect it if you had it sufficient time to do that And i'll end it on this slide. Thank you very much Okay, let's thank ron. Thank you Let me bring up the participant window so I can see raised hands for the online folks Let's start with questions in the room any questions from anyone in the room. Ah, yeah Okay, that's still working good This question's relating to apathos yes if the path is is Changed drastically by the earth's gravity been on its return in 2036 It's forecast to hit the earth what is being done to both study as well as Deal with it Yeah, yeah, that's a great question. So I think recently the its format has been better characterized so that we think that it's uh the deflection from the tidal forces um Make it not a threat at 2036 as we had uh, maybe Feared originally and so I don't think we have to worry about it hitting in 2036 anymore and the best we can do is forecast the The trajectory of these things only about 100 to 200 years into the future. Otherwise, it gets too chaotic To be able to predict But sort of sort of things that are being done is that Is that there is um Some formulation of missions to go to apathos after its near-earth flyby In the 2029 to better characterize the uh, the asteroid So we might be hearing very soon of a for a mission to apathos a rendezvous mission not a deflection mission in 2029 and I say the thing we don't want to do is try to to disturb it now because then it would be sort of uh If we were to be to deflect it or anything prior to its encounter with earth or soon after then that could lead to sort of a What's that tennis term a dangerous experiment says one of our audience members Yeah Tickling the dragon as an unforced error. Yeah, unforced error. Ah, yes very good But but following up on that question So let's imagine we we get to 2029 we better assess the probability that when it comes back It'll have a much closer pass with earth on the return Um, what's the timeline for the dart mission as a demonstrator? And does it fit within the 20 was a 2036 rough horizon for the return Yeah, so so dart was uh, you know, it was launched last year in november and it's getting to that system that uh dita most um this fall so it would be uh, you know, about a year to to uh, to do this demonstration. I think if uh I think if 20 if apathos was was gonna actually hit the earth what was in a collision course with earth It's a sort of a nine-year And uh, sorry, that would be 29 36 a 60. Yes, you know, six to seven year um Warning would be sufficient for a kinetic impact to to uh, successfully deflect it. Yeah I think it's something like if you had a warning window of I want to say two years or more than you want to use a kinetic impactor to deflect it If it is anything less than that then you have to go with a A literal nuclear option to uh, try and vaporize the asteroid Part part of me is excited and part of me is terrified by the possibilities. Hi, Matthew. Yeah Hi, Ron met met sigler. Good to see you. Hey, Matt um So, yeah, I I guess one of the neat future ideas I've always wondered about is how you could You know figure out if there's big boulders in the center of these objects, you know, like Like let's say we're just seeing the surface and the inside is full of big rocks And I was kind of wondering about ideas you've heard about, you know, do you put a seismometer and then hit it with something or You know, this is a there's a lot of particle physicists here that would want love to image it with muons or something um, like Yeah, or there was the the consort experiment to remember That was trying to look through the comment with radios for rosetta. Is is there? Yeah, yeah So, so yeah, there isn't anything unfortunately in the works, although there's a lot of uh, so there's no official NASA mission to do asteroid interior Exploration, uh, but there I know a lot of things in development to To do an asteroid interior study. Definitely seismometers would be at the top of the list So I guess that was the question with all these strength studies and how weak the surfaces are would would a seismometer Work or would it be? Yeah, that's we bound. I mean, that's a good question. So it would be a matter of Coupling it with the regolith well, right? And bearing it the I think one of the difficulties is to get an instrument on the asteroid and And have it survive, you know, multiple asteroid days, right? It's probably not as precarious on the moon as as you know where you have to survive for 15 days of lunar night But but you still have problems with sort of uh thermal operations of these and and sort of powering them with with with what with solar panels or batteries So so that's a good question You mean you have to be deeply buried on the asteroid and I think we don't know enough about granular mechanics in low gravity environments to to be able to say what the wave speeds would be in systems where the pressure Is is is very low. So, you know, theoretically it should drop down to 10 meters per second, but does that actually happen? Uh, I'm not sure You know, you're extrapolating from earth experiments of granular materials under different pressures and And so so one of the ideas that that's coming out now that's in development is to maybe maybe do What's called laser Doppler vibrometry? from orbit where you would essentially do I guess you're essentially doing a kind of interferometry Within the spacecraft itself in order to measure Vibrations on the asteroid surface either through natural sources like meteorite impact or to bring an explosive device and Launch it onto the surface and you know, you do an active source experiment Uh, I yeah, I mean the muon thing sounds sounds pretty cool As well. Um, oh, they're saying it's too far fetched Okay, that's too bad. I don't I wouldn't say far fetched. It's just that whenever a particle physicist is asked to solve a problem They ask can we throw a subatomic particle at it? So I mean, we're just a hand real good for a nail, right? Yeah, yeah, yeah I mean, yeah, I mean that there's there's sort of uh So so so not so so we can get a good idea of sort of the near surface um, you know composition using uh gamma ray neutron spectrometers. This is sort of a Kind of a popular way now of doing part of particle physics on asteroids as well lying on I guess cosmic ray bombardment of hitting neutrons To do get some kind of elemental abundance of the upper community Few meters of the of asteroid. So this is this was done in on the asteroid eros for example by the near by the near mission and end of life the near mission ended up sort of softly touching down on the surface of Of its asteroid and doing the I sort of a gamma ray neutron Spectrometer experiment and getting pretty good results Thanks for the question. Yeah, uh, christa. Did you you had another question? Didn't I saw you were unmuted? Um, yeah, I did So at the sort of very beginning of the talk you showed that very nice plot that had the cumulative distribution of sizes but also it was color shaded by the Fatality of the impact Um, so I was wondering, you know, those colors must shift Left and right depending on Um Depending on whether the thing is a solid body or a rubble pile, right? Um, so what I'm wondering is if something was to like how how much does that green Part get larger or smaller depending on what that thing is not not whether we can Destroy it somehow, but whether We like what what happens when it is the atmosphere or the ground Yeah, I mean, that's a very good Question So So, you know, I You know, I'll say this I I don't know, you know, so you have a porous body kind of entering earth's atmosphere um And it it was truly sort of strengthless a gravitational aggregate You might have some of it being torn apart so that The atmosphere would filter away some of its smaller parts Consistent pieces, but what we do know is that you know If from, you know, just meteorites and fireballs hitting the earth The entering atmosphere if you're larger than I want to say five, you know Maybe smaller two meters in size then you can survive the passage through the atmosphere and hit the grounder Burst in the atmosphere causing a lot of damage as well So so in that sense, you know say that you are be able to torn apart and most of the mass survives the The atmospheric passage then really it's a matter of comparing the porosities sort of The porosities of a rubble pile, right how much mass per unit, you know per volume Versus sort of a monolithic body and things I mean, it's a really good question, right This is one of the reasons why we need a asteroid interiors exploration thing is Is a we think, you know, if you were to ask me, what is the porosity of Bennet? We know it's mass because we went into orbit around Bennet and were able to measure It's mass and we get an idea of the porosity of this object By comparing the mass to volume so we get a density But then we compare the density to its closest meteoritic counterpart Which we think our carbonaceous contract cms and ci's and that's the only way we can get at Sort of this void space and porosity of the whole asteroid when we do that it comes up to about 50% right So so if this was We think it's something like 25 to 50 percent porosity So in that sense this, you know, this is really should be a mass diagram Right because you know because we're looking at really energy up here So these colors would shift by most maybe a factor of two Or less so so you you would still care about say if you care about a 200 meter asteroid Um, you almost don't care if it's a rubble pile or a monolith that's coming at you you would have to act Does that make sense? Sorry, that was a really long There's a lot of things that affect that i'm sure But yeah, that that's that that was helpful. Thanks Okay, I think we have time for one last question Oh, yeah Christina. Hang on. Let me get the mic over here Hi, okay. So I was wondering is there any part of the sample return process Or even the the collection process in general that could compromise some of the The the return samples And do you see that and you have to account for that kind of thing in your models? Thanks. That's a really good question. So from a chemical perspective no because the gas we use is It's a inert it's not going to react with with the sample itself. Um, but one of the things that we Is planned to do and maybe you know, this is a good question to ask matt Is uh, matt sigler is is a thermal and physical characterization of the sample To get a better sense of things like you know, whether we got the strength right from remote observations and whether you know, what is the porosity of these These asteroids itself. Do we actually have a good meteoritic counterpart? Um, and so we want to measure things like density in that sense Things like the atmospheric is a reentry of the sample could could Could could influence that because you you would end up with a lot of shaking in the chamber. Um That might might end up Say Influencing the physical characteristics that being said the The japanese team Also did a sample return of their asteroid those samples came back And for the most part they still look like things that were on the surface of their asteroid review But we're going to have to see for cyrus rex because we're expecting more sample than them they ended up collecting about five grams and Our best estimates now for the sample size is something closer to 200 grams of sample So, you know fingers crossed that that's the case Yeah, excellent question Thank you Okay, um actually time to that question. Sorry. I'm going to add one more question here Is there a campaign to look for things like amino acids in sample return from these things? I mean People get very excited about the idea of things getting transported from planet to planet to planet Right sort of spreading the base the building blocks of life through the solar system much earlier in the history of the solar system Is is there anyone concentrating on that question? Yeah, that would be if you want to uh, I know their name that definitely they're for the astrobiology aspect of it Folks are definitely very interested. Um, there's a person named danie glavin who works on uh, nasa goddard, I believe And uh, that's I think that's one of the things they're looking at. So, you know, they've discovered Um, you know these building blocks, uh, these amino acids and other Um, I believe meteorites so so they're certainly looking for evidence of that here particularly for the for this class of object which we think uh Delivered a lot of organic matter To earth Okay. Yeah, very good. All right. Well, let's thank ron one more time and then we'll close up. Thank you ron Good all Yeah, thanks for the great questions Okay. Yep. All right. We're all done So we'll we'll close up for today and I'll shut down zoom. Thanks everybody for joining and thanks again ron. That was wonderful. Thank you very much All right guys, we'll get you out here at some point I would love that. Yeah. Yep. See you. All right. All right. Bye everyone