Astronomy Cast

We live in a cosmic shooting gallery. It’s not a matter of “if” but “when”! Dinosaurs, blah, blah, blah. You know the drill. But seriously, folks, it’s raining rocks & ice out there! How seriously should we take it? What happens when a variety of different objects hit the Earth? Different kinds of objects affect Earth very differently when they impact. Let’s discuss what makes an impactor more or less dangerous.

Show Notes

• What Is a Quasar?
• Discovery and Redshift Identification
• Supermassive Black Holes at Galactic Centers
• Active vs. Dormant Black Holes
• Accretion Disks and the Eddington Limit
• Why Quasars Turn On and Off
• Intermittent Jets and Radio Evidence
• Microquasars as Scaled Analogues
• Brightness Variability Across Timescales
• The Milky Way’s Past Black Hole Activity
• Galaxy Mergers as Quasar Triggers
• Quasar Feedback: Star Formation vs. Suppression
• Jets and Their Impact on Surrounding Galaxies
• Black Hole Interactions and Periodic Flares
• Human-Timescale Observations Near Black Holes
• Early Universe Quasars with JWST
• The Mystery of the “Little Red Dots”
• Direct Collapse Black Hole Formation Hypothesis

Transcript

Fraser Cain:

AstronomyCast, Episode 781, When Black Holes Awaken. Welcome to AstronomyCast, our weekly facts-based journey through the cosmos, where we help you understand not only what we know, but how we know what we know. I’m Fraser Cain, I’m the publisher of Universe Today.

With me as always is Dr. Pamela Gay, a senior scientist for the Planetary Science Institute and the director of Cosmic Quest. Hey Pamela, how you doing?

Dr. Pamela Gay:

I am cold. This winter?

Fraser Cain:

Yeah.

Dr. Pamela Gay:

Like, it’s so cold that even the squirrels are struggling with the ice. I saw a squirrel fall out of the tree yesterday. It was the funniest thing.

Fraser Cain:

Wow.

Dr. Pamela Gay:

There’s this plop in the snow and then the walk of shame that the squirrel had to make.

Fraser Cain:

Or it jumped. It, you know, it knew this was the fastest way down. There was no jump involved.

Yeah.

Dr. Pamela Gay:

So, yeah. All right. How are you doing with this winter of arcticness?

Fraser Cain:

Warm. Are you kidding? We haven’t, we’ve barely seen below freezing.

We’re having 10 Celsius days right now here in Western Canada.

Dr. Pamela Gay:

Yeah, we’re minus 10 Celsius.

Fraser Cain:

I could be growing oranges. I could have orange trees. We are truly the Florida of Canada at this point.

But I want to shout out and encourage people to check out something that you do, Pamela. And I gave you a shout out in my most recent Space Bites, and I’m going to do it again here now, which is that, you know, for those of you who think that Pamela is merely a PhD astronomer who runs the very successful CosmoQuest and is a wonderful narrator, she is also a very established and skilled news presenter. And that is because she does a weekly show called Escape Velocity Space News on the CosmoQuest channel on Twitch, or I don’t know where you do this.

Dr. Pamela Gay:

It’s also on YouTube.

Fraser Cain:

It’s also on YouTube. Yeah. And it’s so good.

And you’re writing it and you’re presenting it and you’re putting up graphics and you know what you’re talking about. And so as we live in this world with so much AI slop filling the channels and people looking for genuine human voices, don’t forget Pamela. And of course, you know, people tell me all the time, I’ll listen to your voice, I go to sleep.

I think that’s bananas because I speak quickly as I rise. Sometimes I’m yelling at you. Sometimes I’m very quiet.

I’m very manic. But Pamela has this beautiful, dulcet tones that just will help you go to sleep if that’s what you require. And so check out Escape Velocity Space News.

Thank you. One longstanding mystery in astronomy were the quasars, incomprehensible energy blasting out of a point like source billions of light years away. We now know these are actively feeding supermassive black holes, which can turn off and on in a startlingly short period of time.

Today, when black holes awaken, Pamela, let’s talk about quasars first because man, I always enjoy this. If you go and watch Cosmos, the original one with Carl Sagan, there’s right at the very beginning he is saying, there are these things called quasars and we don’t know what they are. And maybe they’re messages being, I’m not going to do a Carl Sagan impression anymore, but that maybe these are communications being sent by an advanced civilization in our direction.

And maybe these are black holes that are consuming material. Turns out it was the latter. So what are quasars?

Just to give people up to speed.

Dr. Pamela Gay:

It has been amazing to watch the evolution information about this. When I was an undergrad, we were still using overhead projectors with like this plasticky stuff that people would hand drawn and people would draw spiral galaxies with monsters in the center, here be dragons. And it was just awesome.

And the way they were discovered is these point sources were found in images that looked like stars. But when spectra were taken of them, they’re like, what the, none of these lines make sense. And I forget who it was specifically, sorry, I was not prepared for this specific question.

There was one guy who looked at it and was like, that’s redshifted a lot and was able to identify these were high redshift objects, not individual stars. These were galaxies, but they looked like stars. How was that?

It was deeply confusing. Right. And over the literally decades, we have gotten better and better equipment.

And the first thing they realized was these are mostly spiral galaxies that have in their core significantly more light being emitted than in the entire rest of the galaxy.

Fraser Cain:

Yes.

Dr. Pamela Gay:

And then we had to figure out how, and over time it was realized, okay, so this is an extremely small region in the center that is massively gravitationally strong. And there is a disk of material around this that is extraordinarily bright because it’s so compact that nuclear reactions are occurring in the disk. And over time, we started to also put together this unified idea that depending on the viewing angle, we are able to see different kinds of lines depending on where we’re looking in the disk.

And then finally, it was actually my graduate advisor, John Cormandy, and the team he was on, they were able to get spectra of the stars around the core of nearby galaxies and see in the redshifts of this material that the only way to explain the motions close in was if it was a supermassive black hole in the center.

Fraser Cain:

And, you know, the process of determining that there is a supermassive black hole at the heart of galaxies has become very mainstream, they’ve gotten very good at it. The best tool really is x-rays. So they will use telescopes like Chandra, they’ll be able to detect the presence of x-rays coming from the center of a galaxy and then use that to locate the position of the supermassive black hole.

Some of them are active, others are quiet. And so the natural question is, what makes them go from, when they’re active, what’s going on? And then what makes them go from active to quiet?

Are there just black holes that are always active and always putting out material? And are there black holes that are always quiet? Was the supermassive black hole at the heart of the Milky Way once active?

We have questions, Pamela. Answer them in whatever order you wish.

Dr. Pamela Gay:

OK, so, yes, our black hole has been active in the past. When we look at the core of the galaxy and the surroundings in x-rays and infrared, we’re able to see bubbles of material that was pushed out by the massive light pressure of some sort of a disk that was around it in the past. We see these bubbles, we see these high energy particles that are relics of past activity.

We don’t see relics of jets associated with our galaxy, so whatever occurred appears to have been short term, wasn’t highly active.

Fraser Cain:

Right.

Dr. Pamela Gay:

But these kinds of activities that we see early in the universe in large numbers and in decreasing numbers as the universe ages appear to be driven by some sort of an interaction between multiple galaxies that drives material, dust, gas, stars into the core where they get gravitationally locked into a death spiral with the supermassive black hole. And because of conservation of angular momentum, which is the enemy of all activities in astrophysics, as near as I can tell, as this material spirals in towards the core of the supermassive black hole, core of the galaxy rather, you end up with a disk building up, things keyed up in the disk, that disk and its light is what causes the quasar. It’s not the supermassive black hole, it’s the stuff trying to drop enough angular momentum to fall into that black hole.

Fraser Cain:

And the magnetic fields that are surrounding the black hole and its interactions. Those channel jets, yeah. That channel the jets.

And when you’re staring down the barrel of the jet, then you see a very bright center to the galaxy called the blazar. So the question I want to ask you is, are they forever? Are actively feeding galaxies always actively feeding or are they not sometimes?

Dr. Pamela Gay:

It turns out that they either run out of food naturally or fling their food away. So so actively feeding black holes, active galactic nuclei, quasars is what we call them when they’re sufficiently bright. That accretion disk generates a whole lot of light, and if it generates enough light, the pressure can start pushing away material that would want to be falling in.

Fraser Cain:

This sounds like the Eddington limit.

Dr. Pamela Gay:

Yeah. So there’s two different ways that black holes stop feeding. One is there’s just not enough stuff.

They run out of stuff to eat. And we’ve all been there. You open the refrigerator door and realize mistakes were made.

So it turns out, yeah, they can just run out of food sometimes. The other issue is they can have a disk that is so thick, so bright, it starts pushing material away, clears the region around it, and it goes hungry until the next time some sort of interaction occurs. And what’s so cool is we are starting to find these intermittent jets where you see dashed lines in the radio jets coming out of these systems.

Oh, that’s cool. Yeah.

Fraser Cain:

So where they turned off and turned on and you can actually see how long they’re off and how long they’re on.

Dr. Pamela Gay:

Exactly. Exactly.

Fraser Cain:

Yeah.

Dr. Pamela Gay:

And and we’ve we’d previously seen examples of quasars turning off. Hannes Vorwerp is an example of that. But the fact that we’re now finding these dashed jets of systems that turn on and off is just kind of awesome.

And I have to say, black holes at every scale are equally likely to do this kind of stuff. Stellar black holes and binary systems will steal material off of their companion, create accretion disks that light up, form jets. And when that companion either evolves to a different stage or just runs out of food in the gravitational Rocheleau limit, that feeding will turn off.

And so there are cataclysmic variables that include black holes out there doing their black hole things at stellar mass sizes.

Fraser Cain:

Yeah. Yeah, exactly. There’s this really interesting correlation that you can actually study relatively close to us, micro quasars, you know, I say tiny black holes, nearly with five times the mass of the sun, right, that are accreting material from some partner and have built an accretion disk and are firing out jets.

And they are in surprisingly similar ways, identical in terms of behavior to the ones where you’ve got a billion times the mass of the sun and can be used to study this for something that is much, much closer to home.

Dr. Pamela Gay:

Yeah. The laws of physics don’t care what scale it’s at.

Fraser Cain:

Yes. Yeah. Which is which is kind of surprising because you’d think there’d be a certain, you know, something would compound as these things get a lot more massive.

But but it all sort of works at different scales perfectly. So then, you know, you mentioned that you see this dotted line. Yeah.

How quickly does it appear that black holes can shut off and can shut on again?

Dr. Pamela Gay:

It’s millions of years. So so we it’s hard to tell exactly how long it takes them to turn off and turn back on. What we see is is gaps.

And and the example that is most recent is J1007 plus 3540. And it turned off as near as we can tell for about 100 million years and then turn back on. And we see that from the gap in its jet.

And we can see evidence in the jet of prior intermittency. And what’s amazing is this is a system that’s in a very thick galaxy cluster. And and so its poor jet is beat to its beat up by interacting with the surrounding intergalactic media or intercluster media, as the case may be.

Yeah, radio is starting to reveal some really cool stuff. The longer we have these high resolution systems like LOFAR and the new system in India online, the more of these kinds of things we’re going to discover.

Fraser Cain:

So I I think you might be wrong about the millions of years. Like, I think the period is faster than that in some cases that people are seeing these things.

Dr. Pamela Gay:

They turn on and off much faster. But the gap in time that this one is turned off.

Fraser Cain:

Yeah, right. Yeah. So that’s the so the turn off and turn on.

Dr. Pamela Gay:

So they turn on and off fairly quickly. We don’t know exactly how quickly, but they can have gaps in time that are hundreds of millions of years.

Fraser Cain:

Yeah. Yeah. And I think, you know, like part of this is that, you know, as you said, you have this accretion disk that’s around the black hole.

But there’s there’s this this idea that it’s actually really hard for material to kind of make that final drop down into the into the black hole. It’s got to shed all that final angular momentum and that it can just happily be spiraling around the black hole for longer periods of time before bits and blobs of it are being thrown in and sometimes requires some external interaction, another a new gas cloud giving a close star that causes turbulence that then throws in a new thing. And so so I think, you know, apparently we’ve seen quasars where they’ve noticeably brightened in the kinds of timescales that we can understand, like within a decade, within a couple of years, within you can see periodicities in their brightness over days.

Yes. Over days.

Dr. Pamela Gay:

Yeah. So it’s quite common to see that.

Fraser Cain:

Yeah. And so I think what was thought was, well, here’s the thing that this black hole is digesting, whatever, a thousand solar masses of material that it is piled up around it. And that’s going to take 10 million years to do.

No, you’re seeing variations that are happening on within human lifetimes.

Dr. Pamela Gay:

And what’s really cool is you can actually map out the accretion disk region by looking at the time scales of variations, because the time scale, the shortest possible time scale is dictated by how big or how small the area that’s emitting light will be, because you have to wait for the light from the entire object to reach you. And so the shorter the variations we see, the smaller the scale of the structure that’s doing that. And so when we see these variations, it’s a variation in the rate of consumption of the black hole.

We haven’t yet caught a black hole going entirely from active to completely turned off. And so we’ve only seen these variations in eating. And we’ve seen cases of like something gets nommed, but it was a quiet black hole that just suddenly ate something.

So it burped. And we’ve even seen that with the supermassive black hole at the Milky Way. In our black hole, yeah.

Fraser Cain:

Yeah, that there are, I mean, there’s a couple of masses worth of the Earth going into the black hole every year.

Dr. Pamela Gay:

Periodically, yeah.

Fraser Cain:

And and so you’ll get sort of a constant blast of X-rays and various radiation coming from the center of the Milky Way. And occasionally you get a little more.

Dr. Pamela Gay:

Yeah.

Fraser Cain:

Right. Yeah. So you hinted at this early on in the episode that that these jets, this activity has consequences for the galaxy that it is inside.

And around it. And around it. Yeah.

Like reaching out to other galaxies potentially within the cluster that it’s forming in. So so how does a actively feeding supermassive black hole and the jets and the material that it produces, how does that cause mayhem around it, both in the galaxy and nearby?

Dr. Pamela Gay:

So when this is occurring in a cluster environment, we have seen one case of a jet spearing a nearby galaxy and deforming it. So these these jets are carrying energy. They are carrying momentum.

They can push stuff around. And what’s amazing is as we look at more and more radio lobes, we can see them essentially forming fountain like ends as they interact with the significantly denser intra cluster media. So they’re pushing out on the media, creating dense places.

And when what they actually hit is another galaxy, they can trigger star formation.

Fraser Cain:

Or snuff it out.

Dr. Pamela Gay:

Or kill it. Yeah, that’s possible, too. They can just push out the material that would form.

Fraser Cain:

Yeah. Yeah. These jets and people need to understand these jets are last are going out tens of thousands, if not hundreds of thousands of light years long.

The material blasting out of these jets is going at relativistic velocity.

Dr. Pamela Gay:

Yeah.

Fraser Cain:

10 percent the speed of light, 20 percent the speed of light like this is serious business. And if you get caught, if your galaxy gets caught in the death beam of another galaxy’s actively feeding black hole, shenanigans ensue.

Dr. Pamela Gay:

And this is all a story of interactions. Quasars are created by having large amounts of material somehow triggered to fall into the center of a galaxy, which is most likely to occur when you look at these. The most common reason this happens is two galaxies interacted.

Two galaxies are merging and then you have the jets are going out and getting stabby with other galaxies, with the surrounding interstellar media. It is all a story of galaxies. I mean, we have all sorts of words.

It’s called galaxy harassment when they pass by each other too close. It’s ram pressure stripping when the inner cluster media decides to push material out of a galaxy. Quasars are often related with massive amounts of star formation that’s going on as dust and gas are getting compressed.

And once all of this chaos comes to an end, you’re often left with a red dead galaxy.

Fraser Cain:

Right. So you essentially are force feeding a galaxy its own gas. You’re causing all of its stars to form at once, burn through its reserves of gas.

And now it’s out. It’s done.

Dr. Pamela Gay:

Now it’s dead. Yeah.

Fraser Cain:

Yeah. And then all of the the red, all the stars just commonly evolve. Yeah.

Dr. Pamela Gay:

The blue ones go away supernova style as they do and tell this galaxy eat something else.

Fraser Cain:

Right. And then, you know, has fresh gas and then it all starts over. Right.

Dr. Pamela Gay:

Yeah.

Fraser Cain:

But but, you know, this is the effect that perhaps it can have on other galaxies. What what even effect can this have on the galaxy that the black hole is is within?

Dr. Pamela Gay:

Well, I mean, it’s deforming it by using its light to push things out. It is interacting with its media by pushing that around. It’s all a story of getting pushy, for lack of a better phrase.

Fraser Cain:

I mean, I think that I that I find kind of interesting is that you get this almost like a fountain in some cases. So the material is being blasted out. But then the gravity of the galaxy is dragging it back down.

And so it’s it’s sort of raining back down into the galaxy.

Dr. Pamela Gay:

That’s the innermost scale. We see this not in radio. This is something you study in other wavelengths.

Yeah. Yeah.

Fraser Cain:

And and that this could be seeding the galaxy with elements, with fresh material, with shock waves that are causing star systems, you know, causing gas clouds to collapse and begin the process of star formation and that that you get nucleosynthesis. You actually have heavier elements being formed in these accretion disks around the black holes. They’re like stars.

And then the material is being siphoned out of the accretion disk, jammed into the jets and then fountaining back down across the galaxy itself, enriching it with carbon, nitrogen, oxygen. Yeah. And so like, could we depend on a supermassive black hole for life?

Dr. Pamela Gay:

Oh, man. I wouldn’t depend on it, but it’s definitely a component, it’s a component. Yeah.

And as we look out across the interstellar media, we find really complex molecules. And these clouds of material have to get enriched somehow. And when you see them enriched, supernovae would deform them, would shock them.

And so this is a gentler way, in a way, to enrich the outer parts of the galaxy with material without causing the clouds of material to collapse.

Fraser Cain:

Right, right. So there’s one last thing that I wanted to talk about. I don’t know whether this needs to be a show on its own.

And I don’t know if this is in your mind, which is these transient, oh, man, transient luminance events.

Dr. Pamela Gay:

Yes. Yes, those those are cool. And yeah, and we need to do a show.

And I’m working on any of you who are on Patreon, drop me your ideas. I’m working on putting together the schedule for the rest of the season. Yes.

There’s a bunch of things that just like go flare in the night that we need to do a roundup of as we’re starting to get more and more understanding. Let’s go into this anyway.

Fraser Cain:

We’ll talk briefly about these because they’re related to the brightenings of galaxies and accretion disk and so on. But essentially, we now know that supermassive black holes can have other black holes in orbit around them. And as those black holes are passing through the accretion disk, plunging through the accretion disk, you get a flash coming out of the center of the galaxy.

Dr. Pamela Gay:

And so this flash is not the central supermassive black hole. It’s some other high density object, whether it be intermediate black holes or whatever. It just got hungry and stole food from its friends.

Right. And eventually that’s going to lead to emerging black hole. We’re just not seeing those yet.

We need more gravity wave detectors.

Fraser Cain:

And it’s really impressive. Like there’s this flash, you get the X-rays, you get the visual confirmation, and there’s this periodicity where the astronomers are able to say, oh, it went up through one part of the disk and now it’s come back down through the other part of the disk. And we’re expecting to see it flash again in precisely 18 months.

Yeah. And then right on schedule flash as this black hole is is passing through the accretion disk of another black hole.

Dr. Pamela Gay:

And what I love is because the supermassive black hole has such huge mass, we can on human timescales see things orbiting it, including other black holes. And we’ve been seeing this with Andrea Goethe’s work, looking at the core of the Milky Way, watching stars and blobs of gas orbit around over decades. This is astronomy on human timescales.

This is the kind of thing that very rarely happens. And it’s really cool to get to see it with high energy events that get all the telescopes involved.

Fraser Cain:

Yeah, yeah, absolutely. And it really is interesting to me how, I mean, you hate to use this sort of anthropomorphization analogy that it’s all about the village, that no black hole is an island, that black holes evolve in galaxies, the galaxies are part of galaxy clusters, and that things that happen in one galaxy can reach out across millions of light years to the other galaxies that are around it and have an influence on it.

And so one bad apple can spoil the pie. Like, I’ll just keep making analogies here. But the point is, is that what you might have in terms of like, say, even the potential for life in your galaxy depends on what was the environment of the galaxies around your galaxy.

Were they all sort of feasting on material, blasting out jets? Was it like you’re trying to exist within all these death rays and you ended up in a red dead galaxy? Or were you far enough away that that didn’t happen or that there was a near miss that just enriched the galaxy with the heavier elements that are required for life, but not the death beam that caused it to die too young, too early.

And this is the kinds of stories that astronomers are still trying to pick apart. It absolutely plays a role, but we don’t know what role exactly yet.

Dr. Pamela Gay:

And what’s so cool is, I was so disenchanted with JWST before it launched because so much money and so many careers had been ended because their money was stolen for it. Do you feel better now, though? I’m still bitter, but it is living up to its potential.

And we are starting to be able to see in the early universe the way galaxies were clearing out the space around them using these jets. And there’s some really cool illustrations that are out there of this that end up looking like cells because you have these galaxies moving through the material around them. And I’m really enjoying watching the evolution in how people are trying to understand the little red dots.

There’s a couple of competing theories that have come out recently. The one that I like the best is that the very first stars, like all sets of stars, had an initial mass function where some of these stars were so big that they collapsed directly into intermediate black holes. And it is those massive, massive stars and their surroundings that led to these little red dots.

Fraser Cain:

My hope is that we’re seeing the formation of globular clusters.

Dr. Pamela Gay:

Okay.

Fraser Cain:

That’s my hope.

Dr. Pamela Gay:

Yeah, that’s another one of the theories.

Fraser Cain:

Because it’s so compact and tight. And yet, globular clusters are as old as the universe. Where do they come from?

So are they the stripped cores of dwarf galaxies or are they a separate thing that formed and then found their way into other galaxies? That’s my hope. That’s, yeah, direct collapse would be incredible.

Like we’re literally seeing the direct collapse of black holes because they’re gone within. But we did a whole episode on little red dots, didn’t we?

Dr. Pamela Gay:

So yeah, yeah, yeah, yeah.

Fraser Cain:

We don’t need to rehash that. Awesome. But that was super fun.

Thanks, Pamela.

Dr. Pamela Gay:

Thank you, Fraser. And thank you to all of you who support us through Patreon. You make it possible for us to do all the things we do and have other people help us.

So, yeah, this week I would like to thank the following people. This week, I’d like to thank the following $10 a month and up patrons. Don Mundus, Ed, Eric Lee, Father Prax, Frederick Salvo, G.

Caleb Sexton, Gerhard Schweitzer, Gold, Greg Vialt, Hannah Tackery, Jacob Houle, Jarvis Earl, Jeanette Wink, Jim McGeehan, Joanne Mulvey, John Muthis. And, uh, thanks, Fraser.

Fraser Cain:

Thanks, Pamela, and we will see you all next week.

Dr. Pamela Gay:

Bye-bye.

Live Show

We live in a cosmic shooting gallery. It’s not a matter of “if” but “when”! Dinosaurs, blah, blah, blah. You know the drill. But seriously, folks, it’s raining rocks & ice out there! How seriously should we take it? What happens when a variety of different objects hit the Earth? Different kinds of objects affect Earth very differently when they impact. Let’s discuss what makes an impactor more or less dangerous.

Show Notes

• Upcoming launches: Artemis II, New Glenn, Starship, and more
• Why asteroid impacts are inevitable, and how often they happen
• How size and composition change impact outcomes (rock vs rubble pile)
• Airbursts, tidal breakup, and “shotgun” impact scenarios
• Tunguska, Chelyabinsk, and lessons from past events
• Comets vs asteroids: speed, volatility, and risk
• Finding the threats: NEO surveys, ATLAS, and Vera Rubin
• Fireball prediction, meteorite recovery, and rapid-response observing

Transcript

Fraser Cain: 

AstronomyCast, Episode 780, When Asteroids and Comets Attack. Welcome to AstronomyCast, our weekly facts-based journey through the cosmos, where we help you understand not only what we know, but how we know what we know.

I’m Fraser Cain, I’m the publisher of Universe Today. With me as always is Dr. Pamela Gay, a senior scientist for the Planetary Science Institute and the director of CosmoQuest. Hey Pamela, how are you doing?

Dr. Pamela Gay: 

I am doing well. I am deeply confused by what February could bring us in terms of rockets. And very, very sad that I went on vacation, which means that I don’t have the ability to go watch rockets.

Fraser Cain: 

By the time people are listening to this, in theory, Artemis 2 could launch within days of when you’re listening.

Dr. Pamela Gay: 

So we’re looking at Artemis 2, we are looking at New Glenn 2, we’re looking at a variety of other rockets, because why not? There’s a whole bunch of stuff around the globe that’s pioneering this month. I’m going to try and stream a bunch of it, and you usually do interviews with Scott Manley, and I forgot the other human’s name.

And those interviews are wonderful. Go check them out, humans. We’re going back, and I’m really feeling like Blue Origin is going to be able to deliver Viper, and that’s what I’m really excited about.

Fraser Cain: 

It feels very bizarre to me to put these words in a sentence, Blue Origin launches rockets.

Dr. Pamela Gay: 

I know.

Fraser Cain: 

I know. And yet, they appear to be doing such a thing.

Dr. Pamela Gay: 

Yeah.

Fraser Cain: 

Slow and steady. Yeah. Well, slow?

I’m not sure about steady or slow. We have been reporting on their launch delays for almost my entire journalism career.

Dr. Pamela Gay: 

That’s fair. Yeah. But look at where Starship is, and it’s feeling kind of like the turtle and the rabbit right now.

Fraser Cain: 

Yeah.

Dr. Pamela Gay: 

I’m hoping both cross the finish.

Fraser Cain: 

Yeah. Yeah. I mean, New Glenn is not a fully reusable two-stage rocket.

It’s only reusing the first stage. SLS is a non-reusable rocket in every way, shape or form, but it is a monster, while Starship is the one that is actually trying to make full reusability function, and they’re having their challenges as well. Yeah.

So we are definitely in the cutting edge across the world of, you know, China is rushing forward.

Dr. Pamela Gay: 

Yeah. They’re trying it too. They’re trying with the reusable.

Fraser Cain: 

Yeah.

Dr. Pamela Gay: 

Yeah.

Fraser Cain: 

With reusable rockets as well. So yeah, everything is going to be changing. But we’re not going to talk at all about rockets today.

Dr. Pamela Gay: 

No.

Fraser Cain: 

No. We live in a cosmic shooting gallery. It’s not a matter of if, but when.

Dinosaurs. Blah, blah, blah. You know the drill.

But seriously, folks, it’s raining rocks and ice out there. How seriously should we take it? What happens when a variety of different objects hit the Earth?

So you know, we think about objects in space, you know, I’m just like, right now everybody’s listening to this. Imagine an asteroid in your mind, like just like picture that asteroid. You know, the worst science fiction has filled your brain with what an asteroid looks like.

Okay, great. Now imagine a comet. Again, that’s a little better.

Dr. Pamela Gay:

Yeah.

Fraser Cain: 

But still possibly not completely accurate. Now we’re going to sort of explode all of those preconceived notions about what these things actually look like and what they might do to our planet if various ones hit, even if we attempt to prevent them. So let’s just like start with your straight up regular asteroid.

Yes. And you know, again, my, you know, the audience is probably, you know, my imagination is, okay, it’s this sort of potato-like rock. And I sort of think about like Eros and I think about a couple of the other asteroids we’ve got images of.

Dr. Pamela Gay: 

Carbonaceous chondrite.

Fraser Cain: 

Itokawa. Right.

Dr. Pamela Gay: 

Yeah. Itokawa is a great example. It’s a few hundred meters across.

It’s cashew shaped. It appears to be a solid object. And yeah, so you send that kind of a several hundred meter asteroid at the planet Earth and you’re looking at a global catastrophe.

It is 20,000 megatons of energy, assuming your typical impact rate of about 20 kilometers per second. Ouch. But those kinds of impacts appear to happen about every 200,000 years.

Fraser Cain: 

Right. That a multi hundred meter object that will have global consequences appears to happen every few hundred thousand years. And like, unless I’m wrong, like we’re all very familiar with the meteor crater impact and that one doesn’t even qualify for those bigger impacts.

Dr. Pamela Gay: 

Yeah. So meteor crater is about a kilometer across. That’s the kind of thing that you get from something that depending on what it’s made of, what the density of the impactor is, could be anywhere from 50 to a hundred meters.

And so yeah, meteor crater was still caused by something fairly large, but in the grand scheme of things, not that large. And it’s wild to consider all the different possibilities. Itokawa is a great one to consider because it appears to be a fairly solid object.

It’s the not solid ones that deeply worry me.

Fraser Cain: 

And that’s where we got new images of asteroids from Hayabusa2 as well as OSIRIS-REx. And that looks nothing like that solid rock object that is, you know, what we see in all of the artwork of asteroids as the dinosaurs are looking up and watching this multi cratered thing blazing in the sky above them.

Dr. Pamela Gay: 

So with Bennu, you have an object that is roughly the density of a ball pit of basically gravel and rock and boulders loosely held together such that the pull of gravity, if you were on the surface, you would feel like how a piece of paper feels on your hand. So this is a very loosely held together object. And as it approaches the earth, it’s going to get loosened up further and further.

And there is the potential for that to end up being a series of objects impacting our planet depending on how much they get slowed down or not, how much it gets disrupted or not. Yeah, yeah.

Fraser Cain: 

So, you know, we think about this idea of the Roche limit, which is how close objects have to be where the tidal forces of the earth will tear it apart into chunks and then those chunks will be torn apart into chunks and then you’ll get, you know, if it becomes really close and you’ll get something like a ring, which is always a bad day. You never want to see a ring around the earth. That is trouble.

But if it does get relatively close and it’s still going to hit, it will get torn apart as it’s getting closer, especially since it has absolutely nothing holding it together beyond the mutual gravity and it’s almost done. And so you get it turning into this smear of gravel and various boulders and rocks.

Dr. Pamela Gay: 

As the world turns, different parts get impacted. Now, on one hand, there’s the good news that smaller things are just going to get completely obliterated in the atmosphere. Anything under 10 meters, it’s not going to hit the surface.

But all that kinetic energy is going to end up in the atmosphere. And all of that kinetic energy is going to heat up the atmosphere. And our atmosphere becomes an easy bake oven.

And this is not a good way for us to combat global warming.

Fraser Cain: 

Right. Of course. Yeah.

Instantaneously, we’ve got ourselves a problem. I think about the impacts of Shoemaker-Levy 9 on Jupiter back in the late 90s. A tidally disrupted comet.

Yeah, a tidally disrupted comet. Sorry, 94. It was 94.

Yeah. You see these bruises across Jupiter. And so imagine if some rock was getting close to the Earth, the tidal forces, as it was getting closer and closer, started to separate it out a bit.

And then you get this string of impacts across some larger area. And so what would have been constrained down to just a single spot, boom, mushroom cloud. Yeah.

Big impact turns into this long stream. And so when we look at the impacts like Meteor Crater, we know now that it actually was a metallic meteorite held together.

Dr. Pamela Gay: 

Yes.

Fraser Cain: 

But that’s not going to be the behavior. When you look at Bennu or Ryugu, they are… What were they?

How many were they? Like a kilometer across?

Dr. Pamela Gay: 

500 and I think 900. They’re almost a factor of two difference in size.

Fraser Cain: 

Yeah.

Dr. Pamela Gay: 

Meters.

Fraser Cain: 

Yeah, so way bigger. Yeah. And it’s a shotgun hit on the Earth.

Dr. Pamela Gay: 

And this is where we have to start worrying about what is the continuum that we have to deal with. So on one end, you have… We learned last week, two weeks ago, time has no meaning, that the Rubin Observatory has discovered some fast rotating asteroids where you have things that are rotating fast enough that if you stood just inside their surface, you’d experience one sixth G.

So it’d be loosely like standing on the surface of the moon if you were at the equator of one of these naturally rotating, holding themselves together quite nicely asteroids. They’re super exciting. So we have these nice solid objects that when they hit can cause catastrophic massive craters and even worse if they hit the ocean, catastrophic tidal waves that can destroy far more area because of how far the water rushes in.

If you want to read a… It shows its age, but it’s still a good book about this. Lucifer’s Hammer is one that I highly recommend.

It gets the science right, even though it’s an older book.

Fraser Cain: 

And a guy tries to surf the tsunami wave.

Dr. Pamela Gay: 

Yeah, which you know would happen. You know that would happen.

Fraser Cain: 

Yeah. All right. So we’ve been kind of rambling around a bit.

So let’s now put it all together. So let’s start with, first, let’s start with asteroids. We’ll start smallest and go to biggest possible impact that we can envision.

Dr. Pamela Gay: 

All right. So assuming solid objects here.

Fraser Cain: 

Yes.

Dr. Pamela Gay: 

Up to 10 meters, it’s just going to be an explosion in the atmosphere that’s pretty. Around 25, 30 meters, it starts to be able to survive close enough to the surface that when it explodes, it’s like Chelyabinsk, where you end up with the flash of light that causes everyone to run to their windows, which is not what you should do. And then the shockwave hits the window, shatters it in your face, and you go to the hospital.

Do not approach the window when there’s a flash of light. Do not. But you will.

Yeah, yeah. So once you start getting over 100 meters, depending on the substance, again, this all depends on the density of the object. This is where you start to get craters of anywhere from house-sized to let’s form the Yucatan Peninsula.

Fraser Cain: 

Right, right, sure. But like Tunguska, people are familiar with the Tunguska impact, although that’s probably an airbursting thing, but you’re getting a five megaton explosion and crater forming.

Dr. Pamela Gay: 

And with Tunguska, this is where we have to start considering composition. If you have something that’s full of volatiles, it’s going to behave differently as it heats up than if you have something that is made of carbon and deteriorates and burns up at that temperature versus something that is iron that just gets slightly melty, encrusted, and makes it to the surface and ruins someone’s day. So as we consider, is it solid or not?

If it’s solid, solid crater. If it’s solid and dirt, it burns up. If it’s solid and ice, it goes boom.

If it’s solid and metal, it meets the surface. Now, as you end up with something like… We keep finding these snowball-shaped contact binary asteroids.

They’re super cool. They’re barely held together. They become two separate objects during this kind of an impact event because the neck breaks.

It’s the reality. And so now you have two incoming rocks that depending on how much they separate during impact, you can either end up with very close together craters, or you can end up just taking out two radically different parts of the planet. And this is one of those things where the movie Deep Impact and the movie Armageddon came out just, I want to say either weeks or months apart.

I don’t remember, but it was the same year.

Fraser Cain: 

It was months apart for sure. Same year.

Dr. Pamela Gay: 

And Deep Impact is like, yeah, it blew up. It’s going to hit a whole lot of places. We’re in trouble.

Whereas Armageddon is like, it blew up. We’re fine. We’re safe.

No, Deep Impact got that one right. And you want your asteroid to stay together and hit land somewhere in the middle of nowhere and just get melted, like hit a big desert, take out a desert. Do not hit a glacier.

Do not hit volcanoes. Do not hit opposite volcanoes. Do not hit water.

This is a take out a desert and turn it to glass. That would be awesome. Thank you very much.

Fraser Cain: 

Yeah. At what point does NASA, you know, NASA has various scales that they look at and there’s these other sort of international collaborations, the Torino scale, the Palermo scale. At what point do we see, like what size of an object gives us?

Dr. Pamela Gay: 

50 meters. Anything over 50 meters, you really start to worry.

Fraser Cain: 

Yeah. But what gives us like a hemispheres amount of damage and what gives us global damage?

Dr. Pamela Gay: 

Okay. So anything over 300 meters, just average carbonaceous chondroit. Anything over 300 meters is the potential for, yeah, Australia would definitely go boom.

Europe would go boom. Right. Probably stretching it for Asia.

It depends on the size of your continent. Pick wisely.

Fraser Cain: 

Yeah.

Dr. Pamela Gay: 

But then once you start to get like one kilometer and above, this is where like global catastrophe.

Fraser Cain: 

Yeah.

Dr. Pamela Gay: 

10 kilometers, mass extinction.

Fraser Cain: 

Right. That’s the dinosaurs where every plant on earth lights on fire because of the rock raining back down, the hot rock raining back down.

Dr. Pamela Gay: 

Yeah.

Fraser Cain: 

It’s a very bad day. It’s interesting. There was a study that we reported on, you know, people think, well, like a kilometer.

Dr. Pamela Gay: 

Was this the fossils with glass in the gills?

Fraser Cain: 

Oh no. I hadn’t heard that, but these fish had gone to space, right?

Dr. Pamela Gay: 

Yeah.

Fraser Cain: 

Yeah.

Dr. Pamela Gay: 

Well, okay. So there’s multiple stories on this. I love this far too much.

I’m sorry. I’m going to geek out and interrupt you. All right.

So it hit with enough force that the shock wave moving through land was able to send at escape velocities, land, trees and dinosaurs at escape velocities. So the first animals to go into space were actually dinosaur era critters.

Fraser Cain: 

Right.

Dr. Pamela Gay: 

That were very much not alive by the time they reached space.

Fraser Cain: 

Yeah.

Dr. Pamela Gay: 

Then the shock wave went around the world multiple times, massive tsunamis heading up rivers. And there are fossils that have been found in the Western part of the United States, somewhere that begins with the letter T. I have forgotten the exact name of the place where these fossils are from the day of the destruction where they like found one that had its leg torn off.

They found fish that had the glass that formed from the hot materials in the atmosphere, in the gills. And the fact that we actually found a field of fossils that are associated with this impact is just not something I think most of us thought would ever happen. And it has been done and it is amazing.

Fraser Cain: 

Okay.

Dr. Pamela Gay: 

I’m done squeeing.

Fraser Cain: 

No, no, no. It’s fine. Your tendency to being a supervillain is just revealed one more time.

That’s true. All right. So we’ve talked about asteroids.

Let’s talk about comets. How are comets different in their potential for destruction?

Dr. Pamela Gay: 

It’s a whole lot of what is the density. And so comets do include organics. They do include gravel.

They do include dirt. All of that is in there, but they are mostly volatiles. They are mostly stuff that is just going to go boom in the atmosphere as it suddenly converts itself from ice to gas, which is terrible in many different ways.

So if you think about it, you have something that is, again, anywhere from tens of meters to tens of kilometers is the range you’re looking at for comets that is approaching the atmosphere and the small ones that sudden increase in volume as solid nitrogen, as solid water suddenly goes to vapor and expands greatly. That’s where your danger is coming from. And there’s a lot of researchers who think that Tunguska was a chunk of comet that just came in over Siberia.

And as it was coming down, it had that phase change. And that sudden pressure wave flattened all of the trees except for the ones in the very center, killed some reindeer, killed a few reindeer hunters, and left nothing behind. This is one of the most enigmatic events in measured history where there are photos of what occurred, and they have searched and searched and searched that landscape, and there’s just no chunks to be found.

And with that much destruction, it would have to be complete obliteration, which you can get with an air-bursting comet.

Fraser Cain: 

Right, right. And I think the other challenge, of course, is the comets can come in a lot faster. So, you know, you always use that 20 kilometers per second for asteroids.

Energy, yeah, the amount of energy that is delivered is proportional to the velocity squared. And so the higher the velocity, the more damage can be done. A smaller comet can do more damage if it’s coming in fast.

And so think about interstellar objects. Like, think about something like 3i Atlas at the speed that it was going. At some point, it was upwards of a 10-kilometer object.

People were thinking, now it’s probably a little smaller than that. But still, going at, oh, 50, 60 kilometers per second?

Dr. Pamela Gay: 

Yikes. And this is where part of the justification for building the Vera Rubin Observatory was to go out and find the unknowns of these things. So it’s estimated that there’s more than 500 million near-Earth objects that are 4 meters and smaller, and we’ve only found a tenth of a percent of them.

So there’s a whole bunch of baby stuff out there that Atlas keeps finding just as it enters the atmosphere and obliterates itself. It’s estimated that there are 900, 1,000-meter or 1-kilometer or larger asteroids out there, and we’re only at the 95 percent. That’s good news.

It’s good, but 95 percent is not 100 percent.

Fraser Cain: 

For sure. But there’s only 50-ish global destructive asteroids in our vicinity that we are not aware of yet.

Dr. Pamela Gay: 

That’s all. And comets are their own rogue thing.

Fraser Cain: 

Yes.

Dr. Pamela Gay: 

So the probability of getting hit by a large comet is exceedingly tiny because there’s not going to be that many. There’s not that many getting sent in. There’s definitely not that many alien ones coming in.

But there’s still the potential, and where we don’t know all the details on what’s out in the Oort cloud waiting to get sent this way, you always have to wonder, is there a 10-kilometer one out there waiting to come in? Is a Sedna going to somehow get jettisoned our direction?

Fraser Cain: 

Thank you.

Dr. Pamela Gay: 

There’s no reason to think that at some point in our orbit around the galaxy, a close pass with a red dwarf won’t send in a Sedna-sized object to wreck the inner solar system.

Fraser Cain: 

Yes. So we have a very long-lived audience. You all take very good care of yourselves.

And so can you give people a sense of, in their lifetimes or in hypothetical lifetimes where they get their robot bodies, how often they should expect to see various sizes of impacts?

Dr. Pamela Gay: 

So we should expect Chelyabinsk or Tunguska-type things about once a century.

Fraser Cain: 

Those are two separate creatures, right?

Dr. Pamela Gay: 

We should expect a large air burst capable of causing havoc to a city-sized area in one form or another, roughly every century. Right.

Fraser Cain: 

So another is a natural Hiroshima. Yeah. Once a century.

Dr. Pamela Gay: 

Yeah. The others, so 25 meters is once every 100 years. 140 meters is every 20,000 years.

Wow. Okay. That 1,000 meters, one kilometer, it depends on what you’re looking at, 200,000 to 500,000 years.

Fraser Cain: 

And the dinosaur killer, that’s like in the tens of millions of years. The 10-kilometer object.

Dr. Pamela Gay: 

10 kilometers, 100 to 200 million years.

Fraser Cain: 

Yeah. Yeah. When you think about it, it was whatever, 65 million years ago that the one that took out the…

And it’s funny because when you think about the movie Armageddon and the sort of classic, like, how big is it? It’s the size of Texas, sir. What?

No.

Dr. Pamela Gay: 

There are none.

Fraser Cain: 

There are no… Like maybe, what, Ceres? Ceres is that size, yeah.

And Vesta are the size of Texas? Maybe a large moon of Saturn? That, as you said, there is nothing that is the size of Texas that could hit us.

So don’t worry about…

Dr. Pamela Gay: 

No.

Fraser Cain: 

And if it did…

Dr. Pamela Gay: 

That would be so bad.

Fraser Cain: 

You would get the moon. It’s true. You would get an impact so catastrophic.

That it would recreate the conditions that formed the moon.

Dr. Pamela Gay: 

Much smaller, much, much smaller. The moon was formed by an impact with something that was roughly Mars-sized.

Fraser Cain: 

Sure, still. But still, you would get a ring. Yeah, you would get…

I mean, the entire surface of the planet would be flipped over. It would be molten. There would be no…

Dr. Pamela Gay: 

It would be bad. There’s nothing left. But you wouldn’t get a moon the size of our moon.

That’s all I’m saying. We’ll destroy everything.

Fraser Cain: 

Right, fine. The point is, did not some astronomer look at… Of course, no astronomers were involved in the making of that movie at all.

Dr. Pamela Gay: 

No, they had astronomers involved in Deep Impact and that shows. And unfortunately, it didn’t get as good a rating.

Fraser Cain: 

Yeah, yeah. So I think it’s funny. We were there and I remember you called me…

Yeah, when Chelyabinsk happened. When Chelyabinsk went off in 2013.

Dr. Pamela Gay: 

Yep.

Fraser Cain: 

When we had been doing this for quite a while back then. And you were like, something just exploded in Russia.

Dr. Pamela Gay: 

Yeah, he called me on the phone.

Fraser Cain: 

Which is a thing you never do.

Dr. Pamela Gay: 

Yeah, maybe once every three years.

Fraser Cain: 

I get a phone call from Pamela like, uh-oh, right? And then, yeah, we got the Chelyabinsk and that was a thing. And…

It was amazing. Yeah, yeah.

Dr. Pamela Gay: 

Also terrible, but also amazing.

Fraser Cain: 

Yeah, yeah. Again, you’re a super villain.

Dr. Pamela Gay: 

Good science, bad humanity.

Fraser Cain: 

Yeah, your super villain side is starting to show. And that was, I think it was whatever, it was about 15 meters across. It was house-sized, right?

Compared to whatever it was that caused Tunguska. And we all experienced it. And it’s funny because there are probably two, say, five meter impacts that happen every year.

Like the airbursts happen randomly.

Dr. Pamela Gay: 

Yeah. And Atlas is really good at finding them and figuring out where they came down. And what I’m loving is we’ll get usually a few hours notice.

And so you’ll see things often out of ESA of be prepared, go watch, report your images, and then they go find the shrapnel from you. There was a team from SETI that was able to take the data from Atlas, security camera footage, and find the meteorite in a Nairobi animal preserve. And it’s just sort of like…

Fraser Cain: 

Yeah.

Dr. Pamela Gay: 

How? How did… I’m…

it’s amazing.

Fraser Cain: 

Yes. I mean, where things are at now, as you said, is we have this defense network where various automated telescopes are scanning the sky. And Atlas has multiple telescopes.

Yes. And when they detect some object that is on course with Earth, then other telescopes jump in, in some cases automatically, make further observations. They then pin down when the object is going to strike the atmosphere and where and what trajectory it’s going to take so that astronomers are prepared and can observe it.

And so we’ve entered this realm where… How many fireballs have you seen in your life? I’ve seen one.

Dr. Pamela Gay: 

Oh, I’ve seen more than that. But I think it’s because I got really lucky with the August meteor shower and the November meteor shower.

Fraser Cain: 

Right. So you can now get a notification as an astronomer to go outside, where to look. You could even drive a bit to reach the point where the impact is going to happen.

Yes. So you can watch it. It’s crazy that we’re at this point now that we can predict these things with enough accuracy that you can go out and watch a fireball on command, which is just amazing to me.

Dr. Pamela Gay: 

On command of us, not on command of the fireball. We can’t yet order up fireballs.

Fraser Cain: 

No, we can’t do that. But the point being that before it was all random. You just happen to be outside.

You happen to be looking up. Now you can know where to go, where to look, what to see. And it just shows up right on schedule.

All right. Notice we didn’t talk about at all about how to stop these things, but we’ve talked about this in the past.

Dr. Pamela Gay: 

It’s true.

Fraser Cain: 

You all know the score. All right.

Dr. Pamela Gay: 

Thanks, Pamela. Hope for the solid objects to impact. Never go with the loose ones.

Fraser Cain: 

Right. Hit the land. Don’t hit the ocean.

Dr. Pamela Gay: 

All right, everyone. Thank you very much, and especially thank you to all of our patrons. This week, we would like to thank the following $10 a month and up patrons.

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Thank you so very much. And if you want your name in the next set of episodes, I will be pulling the names and recording next Monday for February. So go hit us up on patreon.com slash astronomycast. All right. Thanks, everyone. And we’ll see you next week.

Bye-bye, everyone.

Live Show

Humans live short lives, and from our perspective the seasons are something that come and go with perfect regularity. But astronomers know the terrible truth! And that there are cycles that slowly shift over tens of thousands of years, shifting the cycles and the Earth’s climate. Today we’ll talk about the Milankovich Cycles! The Earth’s orbit, tilt, and other physical attributes aren’t quite as constant as you might think! Come learn how long-term changes do and don’t affect our climate.

Show Notes

• Milankovitch Cycles Explained
• Historical Context
• Earth’s Orbital Mechanics
• Axial Tilt and Precession
• Climate and Glaciation
• Earth in a Larger Cosmic Context
• Science in Progress

Transcript

Fraser Cain:

Welcome to AstronomyCast, our weekly facts-based journey through the Cosmos, where we help you understand not only what we know, but how we know what we know. I’m Fraser Cain. I’m the publisher of Universe Today.

With me is Dr. Pamela Gay, a senior scientist for the Planetary Science Institute and the director of CosmoQuest. Hey Pamela, how you doing?

Dr. Pamela Gay:

I am doing well. Next week we will not be recording live because I’m actually going on a vacation.

Fraser Cain:

Right on. So I have been going through old media that I recall loving, but hadn’t watched recently. And so I think I might’ve mentioned to people that I re-watched the original Lord of the Rings trilogy, the ones with the extended editions in the box set with the four discs per, man, they’re so great.

Like obviously the movies were great, but just that we’ve entered this world where nobody wants to see behind the scenes to hear the commentary, to see the additional deleted scenes.

Dr. Pamela Gay:

I don’t understand that.

Fraser Cain:

Yeah. Like, like as a media creator, I want to see how the sausage gets made. And so being able to see all of these little featurettes that they publish in addition to that is so wonderful.

And I’m guessing that Netflix tried this and just found that, that people were watching them and they’re like, who cares? Right? Like we don’t even need any of this stuff anymore.

Let’s just get the movie and get in, get out.

Dr. Pamela Gay:

So Disney Plus does do them for their Star Wars shows. So each of the Star Wars shows has a behind the scenes special. And those are actually really cool.

They make me want to go try there. They have this giant room that they record in that allows them to like, yeah, it’s amazing.

Fraser Cain:

Yeah. So keep doing that. And then I watched the Matrix series and they held up, especially the first one, like the first one.

There’s a few bits in the beginning where I was like, oh, cringe. Right. But then the rest of the movie I thought was absolutely terrific.

I enjoyed the next two movies. I know a lot of people, although I really disliked the ending of the third movie. So, um, and then I rewatched Moon, which is with Sam Rockwell and where he’s a, he’s an astronaut on the moon running a mining operation.

And it’s, it’s such a great movie. Oh, it’s so good. So, uh, and that was a long time since I’ve watched that movie.

And so I’d forgotten most of the details. And so I’m really enjoying realizing that there are movies that I have enjoyed in the past. And now my, my, I don’t remember the details from scene to scene.

And so it’s kind of like I’m rewatching it fresh. And yet I know that this is going to be a guaranteed great movie. So, uh, if you haven’t already go back, find movies that it’s been so long that you probably don’t remember the details, but you know, you’re going to love them.

Television shows.

Dr. Pamela Gay:

Blade Runner.

Fraser Cain:

Yeah.

Dr. Pamela Gay:

So good. Yeah.

Fraser Cain:

I watched, I watched Blade Runner with my wife probably about six years ago with that sort of same idea. And then we watched the 2049, but now I don’t even remember how 2049 went, so it’s time to watch it again. So there you go.

That’s my recommendation. Go back, find some old media that you’re kind of nostalgic about, but you’re holding off because like, I’ve seen that movie. Have you?

Do you remember it? Are those neurons? Are those neurons still present?

Dr. Pamela Gay:

See it in the modern political context.

Fraser Cain:

Hmm. There you go. Humans live short lives and from our perspective, the seasons are something that come and go with perfect regularity.

But astronomers know the terrible truth and that there are cycles that slowly shift over tens of thousands of years, shifting the cycles and the Earth’s climate. Today we’ll talk about the Milankovitch cycles. So I think we’re going to need another disclaimer.

And that is that all of the cycles that we’re going to talk about and their influence on the Earth’s climate happen very slowly and they have absolutely no connection to the climate change that scientists are currently measuring. It’s also true. Right.

And so they are, you know, that we are sort of forcing climate change because of the greenhouse gases that are being emitted by our modern civilization in a way that is very easy to calculate. You can literally do this on the back of an envelope, calculate, you know, take the volume of the mass, the volume of the Earth’s atmosphere, measure the amount of carbon dioxide that’s in it. You can calculate the average global temperature that will be expected at various concentrations and there will be sort of specific differences depending on whether it’s the Arctic.

That’s where the complexity comes in, all the simulation and so on, but you can just do this math. So I know like some of you are going to say, I’m never going to listen to astronomy cast again. That’s fine.

You know, like you’re going to, you got to do what you got to do. I know that some of you are going to be sort of angry that we’re not sort of, or that you’re hoping that we are going to somehow turn this into an explanation for climate change and we are not. No.

We’re just that these unfold on cycles that are much longer. So, so with that, and I don’t want to like dance around the reality of the astronomy and the geology and the, and the ball of that, things can be true at once.

Dr. Pamela Gay:

There can be ongoing orbital kinematic cycles and there can be anthropogenic climate change. Both can be true.

Fraser Cain:

Yes. And so, you know, can the Milankovitch cycles be, uh, there was the, the explanation for climate change? No, people checked.

Yeah. So there you go. And like, we don’t need to go into that.

Like there’s a whole other world where there’s all kinds of papers where people are checking and it has no effect. So let’s get on then to the Milankovitch cycles with this sort of like, Hey, it’s 1971 and we’ve never sort of experienced the modern political diatribe, uh, against, uh, the results of, of climate science work. And people just want to learn about the kinematics and, uh, orbital, uh, interesting things that happen with planets going around stars.

So let’s talk about the cycles.

Dr. Pamela Gay:

So you know, first you have to go all the way back to the 1920s, which is really cool to me. But do you have to go back to like the, before the BCE, like you have to go back to the Greeks? I mean, you could.

I want to. We’ve actually known about precession for a long time because folks like Ptolemy that were looking back through historic calendars of stellar positions were able to track the changing North pole. It was more complicated than that.

I just oversimplified a whole lot of stuff there.

Fraser Cain:

I’m going to rabbit hole for one second, which is that the ancient Greeks suspected that the earth orbits around the sun and that the proof that they, that they would use to confirm that was that they were looking for a stellar parallax. And we know now that that parallax is there, but you know, essentially the shifting of the view of the stars based on the position of the earth, whether it’s on one side of the sun or the other, right? That’s how you tell that the earth is going around the sun.

You know, it’s this background that is sort of shifting back and forth every year. And they knew that that’s what you would expect to see. And they tried to find it, but all they had were these little sighting tubes that they use that they would align with a star at the right time and the right place.

And they couldn’t measure to the level of precision that was required to be able to confirm the stellar parallax. And so they went down the, no, no, no. The earth is the center of the universe.

Yeah. Amazing. The Greeks were amazing what they could, what they were able to do anyway.

So yeah, they had their suspicions. They were already starting to detect the motion of, of the stars in the sky year after year.

Dr. Pamela Gay:

They just underestimated the, the size of the universe. And when you underestimate the size of the universe, you expect the parallax is to be much, much bigger than it actually is. But they did see other motions in the sky over time.

And that’s just super cool. There’s neat stories related to the salinity of the oceans that you can go find. There’s all sorts of cool stuff.

Now it was in the 1920s that folks tried to like mathematically take this on as geophysics began to be an actual field. So the 1920s is when astrophysics was born, when geophysics was born, when we started taking all of these things that we observed and having enough understanding of math to be able to say, well, this is why we see these things we see. And they had started to realize at this point that there had been geological cycles with glacial periods.

Europe has lots and lots of evidence for glaciers. North America has evidence for glaciers. And so a Serbian scientist, Milutin Milankovic, in the 1920s worked through James Kroll’s earlier work, trying to do all the maths by hand to figure out, all right, what are the different motions we need to consider?

And so you have things like our Earth’s orbit is mostly circular, but not perfectly circular. So at the beginning of January is when the Earth is closest to the sun. It’s a fairly small effect, but it adds up.

And over the course of millennia, Jupiter and Saturn’s influence causes the orbit to get slightly more elliptical and slightly rounder and slightly more elliptical and slightly rounder. We’re actually heading towards a rounder phase right now. But the degree of change this causes is several days per season of asymmetry between the seasons.

So here in the northern hemisphere, because you move faster in your orbit when you’re closer to the sun, we have a slightly shorter winter, we are getting slightly more sunlight. So the summer to winter dichotomy in the north is less than it is in the south. And we literally get less winter in the north.

So that is excellent. Someday, the orbit will be much closer to round and it will essentially be equal seasons for everybody. But we’re not there right now.

We’re headed that direction.

Fraser Cain:

And how long does that cycle take?

Dr. Pamela Gay:

So this is one of those cycles that isn’t as periodic as others because you’re dealing with Jupiter’s orbit and you’re dealing with Saturn’s orbit. So when you’re looking up the extremes, the extremes occur on millions of years. So like the highest eccentricity ever was 250 million years ago.

Fraser Cain:

And that’s like the Earth’s orbit was the most elliptical that it could be.

Dr. Pamela Gay:

Yeah. Yeah. And so in general, there’s a 400,000 year coupling and it’s paired up with the 100,000 year cycles that we see from other factors.

Right.

Fraser Cain:

So about every 100,000 years, you get a sort of a full cycle through the Earth moving from what is a more circular orbit to a more elliptical orbit. And we, as you mentioned, we experience these seasons. The north experiences shorter winters, longer summers.

The south experiences longer winters, shorter summers. And partly that’s due to the eccentricity. Essentially, Earth is at its closest point to the sun when the northern hemisphere is having, is dead in the heart of its winter.

It’s so close to solstice. Yeah. Yeah.

Dr. Pamela Gay:

It’s wild.

Fraser Cain:

Yeah. It’s incredible. And so I know winter may feel like it sucks for the northern hemisphere, but it could be worse consider the people in the southern hemisphere.

And one of the things that we see is that the Arctic can have extremely cold temperatures, but Antarctica has ludicrously cold temperatures. So yeah. So weirdly, the southern hemisphere experiences these more severe winters than we experience in the north.

Dr. Pamela Gay:

And there’s other weird stuff on top of this, like the fact that there’s so much land in the north. Yes. That has all sorts of weird effects because oceans are thermal sinks and land is capable of varying much faster.

So we know there’s other complicating factors on top of this, and that’s what makes it awesome.

Fraser Cain:

And so this shift is driven by the interactions of the gravity from Jupiter and Saturn on the Earth’s orbit, tugging it bit by bit by bit, orbit after orbit after orbit, slowly circularizing its orbit, and then slowly making its orbit more eccentric again. So that is the first cycle.

Dr. Pamela Gay:

Yes.

Fraser Cain:

Let’s move on to book two of the Milankovitch cycles.

Dr. Pamela Gay:

So that is describing the overall shape of our orbit. Now, once you have the shape of the orbit to contend with, you have the Earth’s position within that orbit, by which I mean the way we’re tilted. So we’re quite lucky right now that for us here in the northern hemisphere, I mean, we have winter solstice so close to perihelion.

It didn’t have to be that way. And the tilt that we have right now isn’t hugely problematic. It’s 23.4 degrees. And that tilt will vary over time from 22.1 to 24.5. So we have this tilt that’s varying, and we have the whole thing is rotating. So the amount that we’re tilted can get more extreme, which makes the seasons more extreme.

Fraser Cain:

Right.

Dr. Pamela Gay:

And where we’re pointed relative to the stars will change. And the tilt is changing on a 41,000-year cycle.

Fraser Cain:

Okay. All right. So I’ve sort of imagined this imaginary line that’s passing through the poles of the Earth, and the Earth is spinning around this imaginary line.

Yeah. And if you sort of look at the angle of what that line is, it’s sort of slowly drifting down and then slowly drifting up, slowly drifting back and forth over this 41,000 years. And so during that 41,000-year period, you’ll have the point of the maximum thing, and then it will go through the minimum and then return to the maximum.

And that’s sort of your cycle, your 41,000 years. And so we’re at 20… You said 23.4. Yeah. So we’re kind of smack in the middle of the potential axial tilt. Yeah, it’s perfectly reasonable. A more extreme axial tilt or a less extreme axial tilt.

And I guess if we had zero axial tilt, then there would be no seasons. Everything would be the same every day forever.

Dr. Pamela Gay:

And then on top of this, that entire tilt is rotating. It’s precession like the precession of a top, and that is happening on a 25.7,000-year cycle. So every 25,700 years, that is rotating.

And so that change changes when the seasons are occurring throughout the year.

Fraser Cain:

Right. And I think we all learned, hopefully, in elementary school or whatever, how the seasons work. I’m sure a pop quiz, when you ask somebody how the seasons work, but essentially…

Dr. Pamela Gay:

A lot of people have no idea. It’s really kind of amazing.

Fraser Cain:

Yeah, yeah, yeah. But the gist is, again, imagine the Earth. It’s this ball that’s spinning.

It’s tilted at an angle of 23.5 degrees. During the summertime, the Northern Hemisphere is tilted towards the sun. Therefore, it’s experiencing more sunlight, and the Southern Hemisphere is experiencing less sunlight.

And then when the situation is reversed, then it’s the Southern Hemisphere that’s experiencing more sunlight, and it’s the Northern Hemisphere that’s receiving less. But in addition to that kind of angle changing, you also get this wobbling of the top. And as you said, the seasons reverse over the course of about 26,000 years.

I’ll bet you we did the same thing. Think back to young Pamela in high school or elementary school, learning this baffling fact and doing the math to figure out how quickly the days change. Did you do that?

No. I did. Oh, man.

I was like, wait a minute. That means that since the Roman time, the seasons have shifted by some number of days. And I calculated, I forget what, a handful of days, like a couple of weeks have shifted.

That summer has arrived, I don’t know, earlier or later, I forget which way it goes, by a measurable number of days than what they used to experience.

Dr. Pamela Gay:

These are things that have to get taken into account in archaeology. And so when you’re trying to consider historical reports of the seasons that things occurred from enough thousands of years ago, slight changes that when you’re dating things based on what flowers are in bloom, that’s the kind of stuff that kind of matters. And it’s just wild to think about all the different changes that are going on.

And it all comes down to just a little bit of torque here, a little bit of torque there. It’s all about uneven forces over time.

Fraser Cain:

And so what’s driving the wobble?

Dr. Pamela Gay:

So it all comes down to the Earth isn’t a perfect sphere and the sun and the moon’s gravitational force just do a little bit more yanking where there’s a little bit more stuff to yank on and that adds up to create this torque that generates the precession that we see.

Fraser Cain:

Okay, well, we’re going to continue on to chapter three of the Milankovitch cycle. All right, let’s move on to book three. That was my favorite book in the trilogy of the Milankovitch cycle.

No, I have no opinion.

Dr. Pamela Gay:

So on top of this, you have the Earth’s ellipticity means that there’s an axis of the orbit that is longer and an axis of the orbit that is shorter. And where those axes are, are rotating over time. And so you have how much the Earth’s orbit is round changes over time.

You have the tilt changes over time. You have where the tilt is pointed changing over time. And you have where the orbit is pointed changing over time.

And yeah, it all adds up to change. You have to keep track of both. Where is the nearest point of the sun?

Where’s the furthest point of the sun relative to the stars? And also, where is the north and south pole pointed relative to the stars? So that you can figure out how do the solstices and perihelion and apohelion all line up or fail to line up.

Fraser Cain:

And so what impact did this have historically on the climate of the Earth?

Dr. Pamela Gay:

Well, so it was expected that there’d be a order of tens of thousands of years cycle in the glacial period. For reasons we have not figured out yet, there is a hundred thousand cycle in the glaciation period. And again, we’re still trying to figure out what are the additional effects that were missed the first time around?

Like one of the effects that was missed the first time around is the tilt of the orbit relative to the moment of inertia of the solar system has changed over time.

Fraser Cain:

Right. So sorry. Yeah.

So like the Earth is tilted off of the sun’s orbit. Yeah, the Earth’s orbit is tilted.

Dr. Pamela Gay:

Off of Jupiter’s defined plane that is kind of the primary plane of the solar system. Yeah.

Fraser Cain:

And so if you imagine you sort of hold your hand out and you kind of imagine like here’s the orbit of the sun. Here’s the equator of the sun. And then Earth is like slightly tilted off that.

We call that the plane of the ecliptic is where the Earth’s orbit is that that tilt is kind of going up and down as well. And that’s fairly new. Like this would be a fourth chapter that Milankovitch wrote after.

Dr. Pamela Gay:

But he didn’t write it.

Fraser Cain:

But he didn’t write it. No, no. But like if he did, you know, he wanted to make more money because the trilogy had done so well.

He would have written part four, the orbital inclination. But you know, he never got around to that chapter. So, um, but, but like, isn’t it really about that?

Sometimes these forces, these, these positions average each other out that they do that you’ve got. We’re a little bit close. You know, we’re, we’re on average closer to the sun.

Things are kind of warmer, but then we’re on average like a little more tilted away. The northern hemisphere is more extreme, but then, but then it’s pushing in another way and things get bad when they all line up in the worst possible ways. So you get the coldest, the most extreme, all of these things start to compound on each other.

And then you can find those in the geologic record of like, oh yeah, we had a really bad glacier. Well, no, no kidding. We had, you know, we, we, that all those variations are happening cycle after cycle after cycle.

And each one is having this independent experience, you know, cause to the earth. And sometimes they all balance out and there’s no change. And other times each one is hits the extreme and then you get a severe glaciation or a very long warm period.

Right.

Dr. Pamela Gay:

And, and the other issue that we have to deal with is there’s probably stuff that we just haven’t found in the geologic record because for the most part, each new glacial period likes to just scrape away the evidence of the past glaciation period. We periodically get things like asteroid impacts that kill the dinosaurs. We get massive changes to the life forms on the planet that lead to massive anoxic events and stuff like that.

And so each of the things that is tied to massive die offs does its own part to the environment of our world. So yes, human beings are changing our planet. We are not the first life form to do this.

We are just the most recent to do this. And, and so the geologic record is super complicated to couple with, with what caused what, because sometimes it’s the volcanoes got angry and Siberia, all of it erupted because it could. The Siberian traps is the most terrifying thing in the geologic record as near as I can tell.

Just, just saying.

Fraser Cain:

Right.

Dr. Pamela Gay:

Yeah. If you, if you want to stay awake at night, go read about that.

Fraser Cain:

Look at the Deccan traps.

Dr. Pamela Gay:

Yeah. That’s another one that’s terrifying. So trying to understand, is it, what, what have we missed?

What secondary effects are we missing? What additional things do we need to take into consideration? We’re still learning all of this and that’s what’s so amazing and why we keep doing science is because there is so much stuff left to learn and hopefully to find in, in some rock outcrop we haven’t explored yet.

And this is the thing that the folks who work in fossils are constantly discovering is there’s going to be a new cave. There’s going to be a new rock outcrop. There’s going to be a new, something erodes and uncovers something amazing.

And we’ll find the answers over time.

Fraser Cain:

So one interesting cycle that is sort of disconnected is that the, the solar system is moving around the galaxy, around the Milky Way. Yeah. And that it has, like the, like the earth is going sort of up and down in its orbit compared to the sun, the solar system bobs up and down in the Milky Way as it orbits around.

And that it’s thought that maybe that there, that the solar system is protected by the mutual magnetic fields in the interstellar medium. And so there are times when maybe the, as the solar system rises up above the galactic plane, it can experience more cosmic radiation and maybe that could have led to times of greater die-offs on the, on the planet. So, so, you know, although there’s, there’s no sun that we are orbiting in the Milky Way that is, you know, giving us illumination that we need that defines the global temperature.

There can be, you know, particles that are colliding with the planet, whether, depending on our position above and below the galactic plane, which is a totally different cycle. And, you know, not necessarily super confirmed to be a thing, but it’s kind of interesting to think about these even larger cycles. It’s all cycles within cycles.

Dr. Pamela Gay:

And that one is thought to also be tied to the, the sudden, a whole bunch more comets get sent on their way into the inner solar system. They also get sent out of the solar system. They get sent in both directions.

Fraser Cain:

Yeah. Yeah. Or, or like I said, or less protection from cosmic rays by the some magnetic fields of all of the stars.

So yeah, it’s a, it’s a very interesting sort of idea to think about. Cool. Well, there you go.

The Milankovitch cycle.

Dr. Pamela Gay:

And now you know.

Fraser Cain:

And now you know. Thanks Pamela.

Dr. Pamela Gay:

Thank you, Fraser. And thank you so much to everyone out on Patreon that supports us and allows us to keep putting these episodes together. This week.

I’d like to thank the following $10 a month and up patrons. Abraham Cottrell, Alex Cohen, Andrew Allen, Andy Moore, Arno DeGroot, Bore Andro-Levsvall, Benjamin Carrier, Bill Smith, Boogie Nut, Brian Breed, Brian Kilby, Buzz Parsec, Claudia Mastriani, Cooper, Daniel Schechter, David Gates, Diane Philippon, Don Mundus, Ed, Eric Lee, Father Prax, Frederick Salvo, G. Caleb Sexton, Gerhard Schweitzer, Gold, Greg Vylde, Hannah Tackery, Jacob Houle, Jarvis Earl, Jeanette Wink, Jim McGeehan, Joanne Mulvey, John Muthis.

Thank you so very much.

Fraser Cain:

Thanks, Pamela. We’ll see you next week.

Dr. Pamela Gay:

Thank you, Fraser. Move on.

Fraser Cain:

I guess we won’t.

Dr. Pamela Gay:

No, next week we won’t. Next week. I’m I’m going on vacation with friends.

Fraser Cain:

We’ll see you in two weeks.

Dr. Pamela Gay:

Okay, bye-bye.

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