What Are The Chances Of A Planetary Collision? Writing in the journal Nature this week, scientists present scenarios in which Mercury, Venus or Mars could collide with the Earth during the next 5 billion years. Astrophysicist Greg Laughlin discusses these possibilities and why there's no need to panic yet.
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What Are The Chances Of A Planetary Collision?

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What Are The Chances Of A Planetary Collision?

What Are The Chances Of A Planetary Collision?

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JOE PALCA, host:

From NPR News, this is SCIENCE FRIDAY. I'm Joe Palca. You may have heard of the butterfly effect, that the puff of an air - a puff of air from a butterfly's flapping wings in Brazil could set off a tornado in Texas, for example, because that little puff of air causes a minute change in the initial conditions of the atmosphere, and all because of that tiny change, the weather system evolves in a chaotic way that's impossible to predict.

Well, the same effect could apply to the orbiting planets in our solar system. Move the Earth just an inch or so this way or that in its orbit, an almost imperceptible change compared to the massive distance it covers during that orbit, but after that shift, the gravitational effect of the Earth on other planets would be slightly different, and the system evolves, causing a multitude of unpredictable changes down the line.

Well, a new study in Nature this week details thousands of scenarios that could result from tiny changes in our solar system just like that, with some interesting and, you know, because we like catastrophic effects, we say interesting and catastrophic effects.

So here to talk about this work is Greg Laughlin. He's a professor of astronomy and astrophysics at the University of California at Santa Cruz. He joins us from the studios of KUSP in Santa Cruz, and he's the author of an accompanying editorial in Nature this week. Welcome back to SCIENCE FRIDAY, Dr. Laughlin.

Dr. GREG LAUGHLIN (University of California at Santa Cruz): Thank you, Joe.

PALCA: And if you have questions for Dr. Laughlin about this topic, you're welcome to give us a call. Our number is 800-989 - that's 800-989-8255. I'll give you the whole number this time, or 800-989-TALK, and you can send us a tweet @scifri on the Twitter system, or you can call us. That would be fine too.

Anyway, so Dr. Laughlin, you know, we make fun about this because, you know, it's more fun to have a catastrophe, but clearly we're not talking about anything happening, you know, soon.

Dr. LAUGHLIN: No, certainly not. The orbits of the planets are one thing that can be predicted in the near term with extraordinary accuracy. You know, we can pinpoint eclipses across millennia, and for the next 50 million years or so we can chart with great confidence where the planets are going to be.

PALCA: So what is going on here? When did people start getting interested in the question of what could happen billions of years out?

Dr. LAUGHLIN: Well, the question of whether the planetary orbits are stable actually goes all the way back to Newton, and you know, Newton was one of the finest minds of all time, but he explicitly sort of said that the problem of solar system planetary dynamics was too hard for any human mind to solve. And you know, so through the years that's presented a real challenge to astronomers and mathematicians, and in the last 30 years or so, Newton's prediction I guess in a sense came true because it's been computers that have aided human minds in understanding the very detailed motions of the planets as they stretch out into the future.

PALCA: So what is the - why are computers necessary? I mean, if a human mind can't express the formula, was is it that computers are doing that allows them to discern the issues that are involved?

Dr. LAUGHLIN: The human mind can express the formula, and Newton expressed quite accurately…

PALCA: Oh, they just can't do the calculations, okay.

Dr. LAUGHLIN: Exactly, yeah. So we understand the forces, but how the forces evolve over the extremely long term, that's what you need a computer to discern.

PALCA: I see. So what - first of all, these scientists in France, what were they doing, and how did they move the bar forward in terms of this computational question?

Dr. LAUGHLIN: Well, computationally, they used an enormous array of computers, the Jade supercomputer, to integrate - or that is, chart -the motions of the planets in 2,500 separate cases, and in each case they made a tiny, tiny sort of millimetric change to the position of Mercury, and over time - you know, for the first 50 million years or so, all the simulations are basically exactly the same, but then because of chaos, they slowly start to diverge so that when you get to billions of years out into the future, all these simulations that start out almost exactly the same are showing very, very different outcomes.

And the good news, if you're optimist you'd see the glass 99 percent full. The good news is that in 99 percent of these 2,500 simulations the planets continue to orbit more or less as they have been doing for the last 4.5 billion years, but in this sort of one percent of the cases that's generated all this attention, Mercury gets into trouble. Mercury's orbit gets increasingly elongated and eventually crosses the orbit of Venus, and when that happens, then basically all hell can break loose.

PALCA: Whoa. So why is Mercury more vulnerable to these perturbations than, say, you know, a gas giant like Jupiter or Saturn?

Dr. LAUGHLIN: Mercury is the smallest planet in the solar system, and so it's the most easily pushed around, and it also is…

PALCA: Typical, isn't it though, isn't it?


(Soundbite of laughter)

Dr. LAUGHLIN: Jupiter is the bully of the solar system and then Saturn is, if you will, the enabler of Jupiter's bullying. And so what can happen is that Mercury, its procession rate - that is, the rate at which its orbit, not the planet, but the orbit itself circulates around the sun, gets locked into that of Jupiter's, and that is what causes a sort of a steady stream of pushes that makes Mercury's orbit get into trouble.

PALCA: And when you said - when you said I think I'll change your expression to when all heck breaks loose in the solar system, what kind of…

(Soundbite of laughter)

Dr. LAUGHLIN: Family radio.

PALCA: Yeah, family radio. Scientific terms we use here. But what kinds of things are you thinking about? I mean, what would happen if Mercury comes swinging past us?

Dr. LAUGHLIN: Well, in most of the cases, Mercury is simply ejected from the solar system, but if Mercury or another planet were to make a close encounter with the Earth, you might think that it would be sort of a miss-is-a-good-as-a-mile scenario, but it's not. In one of the particularly dramatic cases that they calculated, Mars comes within about 800 kilometers of the Earth, and if that happened, if that were to happen, then both Earth and the Mars would be stretched into kind of a teardrop shape very briefly, and that stretching, the tidal friction inside the Earth, would melt the Earth, and…

PALCA: Melt, that could be bad.

Dr. LAUGHLIN: It would be very bad.

PALCA: That's what we call the bad scenario for life on Earth. Yeah, I see. By the way, I wanted to mention that these French scientists have names. They are Drs. Laskar and Gastineau, and they're at the observatory in Paris, Observatoire de Paris, observatory in Paris.

So I mean, why - this is just - this is an intellectual game, or does this help us understand solar systems beyond the Earth's - you know, the one that the Earth is in?

Dr. LAUGHLIN: Yeah, when we - we know now of more than 300 planetary systems outside our own sun in the local galactic neighborhood, and when we look at those, we see cases where clearly disasters have occurred in the past. We see in sort of the fossil evidence of the current-day orbits, we see evidence that planets have collided or have been ejected, and so it is sort of interesting to see that there are all these unstable systems or systems that were unstable out there, and yet our solar system, so far, has been sort of a model of clockwork predictability.

And so seeing that, you know, we've got sort of a one percent chance of experiencing instability billions of years from now, that puts us more sort of firmly in the context of the whole galactic planetary census.

PALCA: Right, and we do have a bit of an event horizon here because in five billion years it all becomes moot, right?

Dr. LAUGHLIN: That's right.

PALCA: And what happens then?

Dr. LAUGHLIN: Well, starting a few billion years from now, the sun as it evolves gets more and more luminous, and that will cause serious problems for the biosphere here on Earth. As the sun become more luminous, the Earth will be sort of increasingly unable to regulate its climate as it does now, and eventually - it's not quite clear when, the models are differing on this - but eventually will get an environment that's much like the kind of lead-melting surface that Venus has today.

PALCA: Now, if this were more of a, you know, a standard show that was politically based, I'd ask you whether the Republicans or the Democrats would do a better job of controlling the luminosity of the sun over the next billion years. But luckily…

(Soundbite of laughter)

Dr. LAUGHLIN: I think I'll pass on that particular question.

PALCA: Well, let's see if we can take a call now from one of our listeners. Remember, our number is 1-800-989-8255, and let's go to Zach in Fremont. Zach, you're on the air.

ZACH (Caller): Thank you very much for taking the call.

PALCA: Sure.

ZACH: My question relates to the interplay between the previous topic and the one that we're just discussing now, when we'll be slamming things into the moon to see what happens and if that sort of, if that could actually cause this sort of spiral meltdown of interplanetary orbits.

PALCA: Oh, interesting question. In case you didn't hear, Dr. Laughlin, earlier in the hour we were talking about this lunar mission where they're going to crash a Centaur upper stage into the moon and watch what kinds of dust and stuff it throws up. Is that an event that could change these calculations?

Dr. LAUGHLIN: Well, it won't change the calculations. The calculations are done from a statistical perspective. But remarkably, what that will do is that will change the entire future course of the solar system. For the next 50 million years or so, the changes that were imparted by that small hit on the moon will be completely negligible. But what we will do is, the solar system will take a trajectory that's different from the trajectory it would have taken otherwise. We have no ability whatsoever. It's essentially impossible to say whether the change is for the good or for the bad. It's just that it will be different.

And this occurs every time, every time you sort of swirl the air currents in a room. Every time you swatted a fly, the butterfly effect acts to change the dynamics of the Earth's atmosphere so that the weather, several weeks from now, will be different from what it would have been otherwise. It's this remarkable feature of the natural world of sensitive dependence of nonlinear systems on initial conditions that makes this sort of butterfly effect work. And this is, you know, originally sort of thought about by Poincare back in the 1880s in response to a contest on trying to understand the stability of the solar system. So it's a very, very interesting idea and very timely.

PALCA: But it's interesting. As you say, the concepts go back, as you say, to the 1800s. But the ability to realize them through the computational efforts are only coming with the computing power that's coming online now.

Dr. LAUGHLIN: That's right. So, you know, the ability to chart the courses of the planetary orbits over five billion years, and then because the system is chaotic, and because one calculation isn't enough because you need a statistical distribution, the ability to do that thousands of times is really something that is just becoming available to us and is really something that is a product of the extraordinary advances that have been made, both in algorithms and in computation, and in computers.

PALCA: All right. Let's take another call now and go to Willie(ph) in Sacramento. Willie, oh, you're okay? Welcome to the program.

WILLIE (Caller): Hi. Yeah. Thank you. Thanks for taking the call. I'm enjoying the program.

PALCA: Sure.

WILLIE: Going back to the premise of - I guess it was would Venus collapse or collide with the Earth, I was wondering, what would the velocity be? And can we escape by leaving the Earth and traveling to Venus?

PALCA: Well, interesting question. I guess - let me see if I could rephrase that. I mean, would we see this coming? If Venus were actually, or Mars, or Mercury for that matter, were heading toward the Earth, would we know that that was about to happen?

Dr. LAUGHLIN: You know, we would know millions, if not tens of millions of years in advanced that we will do for serious problem. The final approach would take place at a velocity of something like 30 kilometers per second, but it would be something that where the orbits would sort of adjust themselves gradually over millions of years. And so, it would be a highly predictable event.

PALCA: We're talking with - I'm sorry. We're talking with Greg Laughlin. He's a professor of astronomy and astrophysics at the University of California.

This is TALK OF THE NATION from NPR News.

Yeah. So this would be something we would - we'd see coming. And then I guess the second part of the question was, could we get off and go on to the other planet? I don't think that would be such a good idea.

Dr. LAUGHLIN: Venus right now, it's - if you're on the surface of Venus, the temperatures are hot enough to melt lead. So it's not a good backup plan.

PALCA: Hmm. Okay. Well, let's - and maybe we should see if any of our listeners have a better backup plan in line.

Let's take another call now and go to Patrick(ph) in Minneapolis. Patrick, you're on the air.

PATRICK (Caller): Hi. Yeah. I'm just wondering about the simulation and visualizations involved. Like, so, you know, is it - is the visualization at such a fine scale that, you know, you see this teardrop effect happening when Mars gets closed to the Earth or whatever? And also - I'm worrying about that. And then, like, if varying the cosmological constant is taken into, you know, account at all, like, how that would affect the simulations?

PALCA: Interesting. All right.

PATRICK: I'll take my answer off the air. Thank you.

PALCA: Thank you. Nice to talk to you. What about that? Cosmological constant?

Dr. LAUGHLIN: So the cosmological constant is not included in the particular simulations that have been described here. The effect that the cosmological constant would have would be vastly smaller than a number of other effects. Perhaps the most important one would be simply the gravitational tugs from passing stars over the next few billion years.

General relativity, however, Einstein's theory of general relativity, is taken into account in the simulations. And remarkably, the effects of general relativity actually act to significantly stabilize the solar system.

So Mercury is much less prone to getting into trouble than it would be if you don't take the general relativistic corrections into account. So it's a kind of remarkable, although ironic thing.

PALCA: Really. And so - well, Einstein didn't make it happen, but he -his ability to discern that actually keeps us on a more stable footing.

Dr. LAUGHLIN: Well, the way that gravity actually works, that is the Einstein's - the way Einstein's theory describes how gravity works, that makes for a more stable environment for us, even though the effects are extraordinarily subtle.

PALCA: Hmm. I just - I want to follow up, just - the caller had one other question, which was this notion of a teardrop. I guess that's -when you do a simulation, you're just saying what the gravitational forces would happen. You don't have a picture of this exploding or something like that.

Dr. LAUGHLIN: Right.

PALCA: (Unintelligible). I'm sorry.

Dr. LAUGHLIN: So that's right. And that was a good question. The simulations that Laskar and Gastineau did, they just take into account the gravitational forces between the planets. And so, when Mars and Earth make this dramatic close encounter, you would need to resolve the kind of gory details of that encounter. You would need to use a different kind of simulation.

And so, we know from simulations of stars that nearly collide and even galaxies that nearly collide, we understand quite well how these kind of teardrop shapes arise. These are called the Roche lobes of the planets. And so, we can sort of imagine what will happen without getting exactly the details. Although it would be an interesting simulation to do.

PALCA: So, is that a big field - I mean, is that what astronomers do now? They do a lot of simulations? Or I guess there's still obviously a lot of observational work that you need to complement these things.

Dr. LAUGHLIN: Right. So astronomy, you know, like any sciences, is driven in its core by observations - observations from telescopes like Hubble Space Telescope. And you see all of the stuff going on out there in the universe. You see colliding galaxies. You see solar systems forming.

And so, simulations can play a role in understanding these complex and detailed phenomena that the observations are showing us.

PALCA: Excellent. Well, we've run out of time. I'd like to thank my guest. Greg Laughlin is a professor of astronomy and astrophysics at the University of California, Santa Cruz. Thanks for joining us today.

Dr. LAUGHLIN: Thanks, Joe.

PALCA: After the break, two new space telescopes that might give us some clues about the early universe and the birth of stars. Stay with us.

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