Planetary Defense is a Public Good

Today is Asteroid Day, the anniversary of the largest asteroid impact in recent history, the June 30, 1908 Siberian Tunguska asteroid strike. The Tunguska asteroid was only about 40 meters in size but the impact was 1000 times more powerful than the Hiroshima nuclear bomb.

Large asteroid strikes are low-probability, high-death events–so high-death that by some estimates the probability of dying from an asteroid strike is on the same order as dying in an airplane crash. To mark asteroid day, events around the world, including here at the observatory at George Mason University, will discuss asteroids and how we can protect our civilization.

Tyler and I are signatories to the 100X Declaration which reads in part:

There are a million asteroids in our solar system that have the potential to strike Earth and destroy a city, yet we have discovered less than 10,000….

Therefore, we, the undersigned, call for…A rapid hundred-fold acceleration of the discovery and tracking of Near-Earth Asteroids to 100,000 per year within the next ten years.

I am also a contributor to an Indiegogo campaign to develop a planetary defense system–yes, seriously! I don’t expect the campaign to succeed because, as our principles of economics textbook explains, too many people will try to free ride. But perhaps the campaign will generate some needed attention. In the meantime, check out this video on public goods and asteroid defense from our MRU course (as always the videos are free for anyone to use in the classroom.)

By the way, can you identify the easter egg to growing up in the 1980s?


In the case of the smaller big asteroids, if the impact site can be predicted with sufficient accuracy the protection may still be excludable though not rival.

"There are a million asteroids in our solar system that have the potential to strike Earth and destroy a city": how do you know, given that "we have discovered less than 10,000…."?

Probably the same way that a doctor can know your white blood cell count or HIV viral load without draining ALL of your blood.

by weighing out the mass carefully in millions of solar systems?

And what way is that?

They don't use samples and statistical estimation in the UK? Explains quite a bit.

How could anyone know that a sample of asteroids is representative? "Estimation" suggests to me that another interpretation is "scientists lie to gullible laymen": the usual story.

Dunno, dearie, how do any scientists that have to judge an amount of something without actually counting each bit do so? I guess they could always be lying to us every time they do that, but that seems a bit paranoid.

do you realize that when you speak proudly about things you have no comprehension that you are doing evil?

IFLS on facebook! Intro to statistics! Science is magic!

My belief in the infallibility and genuine goodness of *Science* (tm) is not a religion.

Astronomers would make this estimate by surveying a portion of the sky and dividing the number of asteroids they found by the fraction of the sky surveyed. They would assign greater weight to the parts of the sky with more asteroids (i.e. on the orbital plane).

Sounds suspiciously like what evil-doing msgkings suggests.

That’s all well and good, you may say, but how can we possibly know how many are out there? The answer to this critical question is that we don’t… precisely. But we have excellent statistical approximations, and we think they’re pretty good.

How do we make such approximations? Several ways, actually, and since they are independent of each other and come up with approximately the same numbers we’re pretty confident that we’ve got a good handle on the issue.

For example, the easiest method to grasp is to recognize that we have a neighbor called the Moon which has no atmosphere (nor has it ever) and therefore serves as a kind of tabula rasa for us here on Earth. All of those impact craters on the Moon provide a graphic record of what’s flown through the Earth-Moon system since the Moon was formed back 4.5 billion years ago. So knowing the sizes of craters that would be caused by the impact of various sizes of asteroids crashing into the Moon, we can count the craters and get a pretty good approximation of the frequency with which various sized objects come flying by.

Another method is illustrated by thinking of a farm (say 247 acres, or one square kilometer) that sits in mushroom country. How would one estimate the total number of mushrooms on the farm? Well if we counted the mushrooms in one typical square meter of the farm and multiplied by 1 million then we’d have a good approximation, right? Obviously the devil is in the details since the soils vary across the farm, as does moisture, sunlight etc. But you get the idea. And shifting this up into space we know the equivalent volume of space we’ve looked at and not looked at… and knowing what we’ve found we can thereby approximate the total out there. Devil in the details, but again, you get the idea.

A third, and interesting way to get a good approximation is to think of a big jar of full of beans. How many beans? That’s what we’re interested in. So we pick out a handful, count them, make a little mark on them and throw them back in. Now we shake the jar and randomize the contents. Again we pick out a handful count the marked ones (re-discoveries), mark the unmarked ones, throw them back in and shake the jar. And again, and again. Each time the percentage of re-discoveries is higher and in fact ultimately approaches 100%. The plotted curve of re-discoveries over time is well understood and from it we can approximate the total bean count quite accurately. So it is with NEOs.



How do you determine "typical", without having the data?

You can't.


Not for astronauts at the International Space Station - we would all be free riding on their taxes

If we discover tomorrow that a 40 m asteroid is on target for a 2017 impact what are our damage mitigation options as of current technology?

If you can land something on it while it is 1 year away, applying a ∆V of 0.2 meter / second in the proper direction would cause it to miss the planet if the asteroid were pointed directly at the center of Earth.

Tunguska had a mass of about 560,000 metric tons so imparting that kind of ∆V would take a lot of propellant. Drilling into it and dropping a nuke in the hole would probably be required. I forget the math for how far you would have to drill; my units aren't lining up.

Ah Armageddon! That was a fine summer flick. Far better than the competing Deep Impact.

You don't need to "split it in two." Simply being a few meters in -- or finding an existing cave or hollow to detonate in -- would do the job.

I saw a JPL report a while back indicating you don't have to drill into it. A standoff nuclear detonation (setting the bomb off next to it) will cause the surface to ablate (evaporate) and provide propulsion in the direction opposite the detonation.

Against a 40 m asteroid, the preferred mitigation option is probably just to track it closely and send out an evacuation order a few days before impact. Which would probably just mean a notice to airmen/mariners to avoid a particular thousand or so square miles of ocean on a particular day, but even evacuating a similar area of e.g. Siberian desert would be cheaper than a crash program to develop an anti-asteroid missile. And the odds of hitting an urban area are on the order of a thousand to one against, compared to the certainty of spending a billion or so dollars on the missile if you go that route.

If it does seem likely to hit a city, or if letting it hit one of our deserts would Make Us Look Weak In Front of the Commies or some such, a missile with a simple inert metal slug would suffice to fragment a 40-meter rock; Deep Impact made a 150-meter crater in a comet. And just shattering the rock will suffice, because unlike the old arcade game any asteroid fragment much less than 40 meters just makes pretty upper-atmosphere fireworks.
The odds of success for an interplanetary ballistic missile developed and fired in anger on two years' notice would not be high, so you'd probably want to e.g. contract three teams (NASA, DARPA, USAF?) and have each of them make a test shot at a defunct spacecraft in high orbit before the real thing. So this isn't going to be cheap.

A 400-meter asteroid, we'd need something nuclear, but the bit where Bruce Willis lands and digs a hole to bury the bomb is pure fantasy - doing a U-turn at twenty thousand miles an hour is absurdly difficult. Instead of the fuel and engines and untested guidance system you'd need for that maneuver, just send a bigger bomb and detonate it at close approach. The X-rays and neutrons will boil off a layer of the asteroid's surface, and the asymmetric expansion of the vaporized-asteroid cloud will push the rock vigorously aside. Against a 400-meter asteroid, a one-megaton hydrogen bomb would do, but it would probably be safer to send four or five of our a href="">300-kiloton models at intervals of a few days and refine the targeting after each shot. Again, this isn't going to be cheap.

But more important than any of this, the odds of a 40-meter rock hitting a city any time in the next twenty years, never mind two years, are about 0.005%. The odds of a 400-meter rock hitting anything but ocean, desert, or wilderness, 0.003%. The odds of a rock large enough to extinctify the human race, less than one in a billion and there's nothing we could do about it anyway. So, while I'd love to be the guy in charge of designing the anti-asteroid missiles, the rational economic analysis is to keep watching the skies and keep our evacuation plans up to date.

I suspect in a mass panic we could launch a thrown-together Orion in as little as a month, and exponentially more every following month.

This assumes essentially all possible planetary resources are devoted to the effort, of course.

if there's a limited pot of money and political will to spend on averting high-death scenarios, wouldn't nuclear disarmament be a significantly better investment?

spoken like a true asteroid.

How much political will is necessary to get Russia and China to agree to stop Iran from developing nuclear weapons?

less than would be required for a multi-trillion-dollar 'defense system' that would make any of reagan's Star Wars fantasies look like the laser pointers you tease cats with?

Ibaien: You don't seem to understand the nature of the project. It would not involve a "death star" capable of vaporizing asteroids that approach the Earth. Rather, the idea is to identify potential hazards and divert them well in advance of their close encounter with the planet.

This requires very little political will -- who is really opposed to the idea of not getting hit by asteroids? There is no pro-asteroid impact lobby. No country's national interest is served by asteroid impacts.

And your concerns about "shouldn't we be working on nuclear disarmament instead" are completely irrelevant to the topic.

we can barely resupply our LEO space station these days; I'm not at all sanguine about our prospects for asteroid diversion. as for political will, we as a species can't even agree to take climate change or energy depletion (serious, immediate, and evident risks) seriously. why would we spend a single dollar or a single vote on mystery asteroids? and as for disarmament, that's even simpler a problem than the two I just named. why bloviate about potential risks with hypothetical solutions when we're unwilling to tackle real risks with clear solutions?

I can speak only for myself, but I enjoy a medium amount of chaos, perhaps to include an asteroid landing somewhere on the other side of the earth. Or Florida.

Just before you run around saying "nobody."

"why bloviate about potential risks with hypothetical solutions when we’re unwilling to tackle real risks with clear solutions?"

Yeah, we've got asteroids, volcanic eruptions, tidal waves, hurricanes, and that real problem, climate change. Although it's maybe a little slower in progress than climate change, what about continental drift? These big land masses cruising around and bumping into one another can raise all kinds of hell. Isn't it time we added tectonic forces to our list of worries? Maybe start funding a program to develop ways to pin down South America so it can't slide into Japan? We should be very concerned for the next generations whose beach front property could be become a mountain range.

Asteroid defense is a global public good. Countries will want to free ride, even if every country's national interest is served by it. Obviously it takes political will.

It's the same with climate change mitigation: no country's national interest is served by climate change, but each country wants the other ones to take on the burden of handling it.

I don't think it's that cut and dried. Reason being: if every country unilaterally destroyed their nuclear arsenals, that may make a conventional war more likely, which can still be a pretty high death scenario. Plus, even if everybody disarmed, there's no guarantee they stay disarmed.

America's nuclear weapons act as a deterrent against chemical/biological attacks.

There's also a credible commitments problem. How do we know that the Russians and Chinese have also fully disarmed? Full inspections of every facility?

Disarmament only works when you're talking about battleships that are hard to hide. It's easy to hide some ICBMs for use "just in case".

"America’s nuclear weapons act as a deterrent against chemical/biological attacks"


It's probably better to invest in something that could actually happen.

But does this mean that you won't be supporting #SMOD2016?!

Does SMOD2016 sell bunker stickers?

Perhaps the asteroids are sexy because there's still a veil of mystery around them. So many things unknown.

There's also big risk related to volcano eruptions but the knowledge development by geologists has killed the fear. There's evidence of big volcano eruptions that affected life on Earth. The measurement of the shockwave on the atmosphere of Mount Pinatubo largest eruption yielded the estimate of a 70 megaton equivalent explosion, 4300+ times Hiroshima bomb but less than 1000 people dead. There was an economic impact but the Philippines are still there

However, none of these numbers and facts are going to make any human heart pump faster........after all nothing happened and it's just geology. If there is a god with good sense of humor, boring tectonic events will keep killing more people and disappearing species from Earth than sexy asteroids :)

There's also the not unimportant fact that we know where the volcanoes are. Mount Pinatubo is not going to erupt in Chicago.

The local damage is one part. But doesn't the ash cloud / SO2 etc. of a really large volcano have the potential to disrupt global climate severely?

Exactly, you feel safe because some nerds in the past developed knowledge on how volcanoes work. The link Ray shared is also about some nerds that need money to discover asteroids, make models and predictions of their trajectories.

There are two approaches to the issue: a) Knowledge that shows near misses, most events are relatively small, people can be evacuated or "it's going to fall in Antarctica", or b) Building the weapon you shoot every time you're scared by a rock that gets near to Earth.

Both approaches are important, but which one makes you feel safer?

Well, a) makes me feel safer, of course.

But I wanna shoot the big gun!

(Incidentally, this applies to a lot of social issues.)

Super-volcanoes can definitely cause global issues, but those are pretty rare phenomena and there probably isn't much that can be done about them, anyway.

We have reason to believe we could avert an asteroid strike if we knew about it in time. We cannot avert a volcanic eruption.


Sounds like a great movie, The Core is as close as I can get to a movie about dropping nukes into volcanoes. Although they dropped them in the core not a particular volcano ... Hence the name.

Why not? Why couldn't we just drill down and let stuff ooze out?

"We cannot avert a volcanic eruption." Well not with that attitude.

Agreed. In my neck of the woods (Denver), the big question is when (not if) Yosemite is going to blow again, and the probability that this will be in the next couple hundred years is real. This would take out a huge part of the Western U.S., and lead to dramatic global cooling that would wreck crops in much of the world for a year or two.

Alas, there isn't a heck of a lot we can do other than carefully monitor it, not build urban centers nearby, and flee if it looks likely to blow (much as the people of Santorini did in ancient Greece before a volcano erupted on that Aegean island ca. 1500 BCE).

You've confused Yosemite with Yellowstone.

It's Yellowstone, not Yosemite, and it doesn't sound like anything's going to happen there anytime soon:

According to the analysis of earthquake data in 2013, the magma chamber is 80 km (50 mi) long and 20 km (12 mi) wide. It also has 4,000 km3 (960 cu mi) underground mass, of which 6–8% is filled with molten rock. This is about 2.5 times bigger than scientists had previously imagined it to be; however, scientists believe that the proportion of melt in the chamber is much too low to allow another supereruption.

What, no mention of B612 charity? See more here:
Seems AlexT and TC are stealing the limelight from B612...

I'm not paying for your boondoggle in the sky.

I believe in a faith based approach to the second coming.

Agreed. I prefer my boondoggles to be right here on Earth where I can see them, dammit.

These are such rare events that we aren't greatly increasing our risk by waiting 100 years to do it, but the cost would presumably be much lower with whatever new technology we have by then. Moving prematurely costs much more, like building the Panama Canal by hand instead of with earth-moving equipment.

While this reasoning can be used to put off this investment indefinitely, it is likely that within 100 years the cost will become so small that some country will simply cough up the few billion that will be required.

BTW, another great video.

In the case of asteroid diversion physics works against that principle. To divert an asteroid, you want to displace it by some distance at a specific time. If you start exerting a force on an asteroid 10 years early, you'll need to exert 100 times more force over those ten years than if you had started exerting force 100 years early. There's also the consideration that propulsion and energy production technologies haven't been improving at the exponential rate that silicon chips have. You can't apply Moore's Law to everything.

But that doesn't chage the prior thinking -- if the costs of applying 100F is less than the cost now of appling F-- then we're much better off waiting the 100 years.

...waiting *90* years. If you were to wait 99 years you would need 10000 times as much force! It's a subjective assessment, but I don't think we can count on the cost of diverting dropping so much as 100 fold in 90 years. It hasn't been going down that fast. I'd much prefer we start the diversion program right away. At the very least, the fact that the scale of the problem grows larger as time goes on should be weighed against whatever pace of improvements in propulsion are expected. It should move the schedule forward somewhat, even if not all the way to the present.

This is one area where the "Great Stagnation" is real.

While there are lots are areas where huge gains from new technology are likely, solar system astronomy, solar system space flight, and the physics and engineering involved in blowing up or changing the course of asteroids, is not one of them. All of these technologies are quite mature, although often underfunded. The problems are hard, but they aren't complex. All of the relevant laws of physics relevant to this task have been established to essentially perfect accuracy for at least half a century. We know how to make extremely powerful solar space based telescopes to detect them. We have pretty much mastered all of the technologies that could be brought to bear to blow up or move an asteroid. There may be some room to improve the computational power of computers tracking asteroid locations and predicting trajectories in the next several decades, but with parallel processing using ordinary networked PCs we already have gillions of times the computational power that we did a few decades ago.

Pretty much, all of the relevant technologies for this task are ready for prime time and aren't going to get a lot better in the next century.

Let's not forget those events that while they don't destroy civilization, do ruin your day:

Reminds me of my favorite philosophy project--the oxford center for the study of existential risks:

Considerations of the risk of future global annihilation may counteract the implications of a 0% discount rate between current and future lives!

"There are a million asteroids in our solar system that have the potential to strike Earth and destroy a city " -Emphasis on the CITY part. NASA has found the planet-killers.

Question/Note: The same technology used to protect earth from asteroids could be used to turn asteroids into weapons. Asteroids could be "steered" into hitting certain targets. Possible?

Of course - but why should anyone discuss deacdes of SF speculation at a serious web site like this one?

(Heinlein, Drake - nothing compared to a GMU econ professor whose apparently proudest achievement is being named the Bartley J. Madden Chair in Economics at the Mercatus Center, which has no actual connection to GMU apart from sharing an Arlington address -

Yipes. Yes, possible, although economics may be our saving grace on that score. If you are going to destroy a city, it is probably a lot easier and cheaper and stealthier and faster to get your hands on an H-bomb and drop it, than it is to send a rocket into space and point an asteroid our way - a project to large to hide that would take many years or decades once the rocket is launched.

So, no one believes it was a comet only slightly larger than those that strike the Earth every day?

great post!

Might do better supporting some of the commerical astroid mining efforts. The you get the public good aspect for free in a sense they have incentives to find the larger asteroids and possible to design and build tools that allow the asteroid obits to be manipulated.

Should also add that probably the best defense system is diversify the locations human occupy. Space collonization is probably a more viable tactic than the planetary defneces given existing technology.

Space collonization is probably a more viable tactic than the planetary defneces given existing technology. -

There are very few things harder than space colonization to manage. Lobbing nukes at asteroids isn't one of them

lobbing nukes at asteroids isn't a solution and the challegnes to getting the resources to the asteroids in time to do much about them is the same problem as getting people somewhere else. Certainly building a bigger space station is probably easier. Putting people on the moon a proven task -- colonizing a challenge but I don't buy the technical issues are so huge -- time consiming yes. Once the solar panles are the energy is cheap and digging holes would be easier on the moon than on earth due to gravety. We already have the material technology to then seal the interior of a cavern. We have the technology to perform all the farming and plant gowing activities that would be need to start generating an atmosphere. Sublunar living solve the problem of radiation exposure without a surface atmosphere.and magnetishpere.

"Planetary Defense" sounds something like "Environmental Protection".

Calling it defense produces better funding than calling it environmental protection.

Nuclear waste cleanup was billed as an anti-proliferation effort, so the law makes all nuclear waste the property of the U.S. government and has 100% public funding of clean up efforts. The spending in inadequate, but the results are much better than CERCLA (the Superfund law) dedicated to cleaning up non-nuclear toxic waste, primarily with funding from responsible parties determined in litigation. This has dramatically underinvested in toxic waste cleanup, delayed cleanups much more than nuclear waste, and led to immense dead weight transaction costs for litigation that have eaten up lots of the potential cleanup funding.

I'm confused as to where to look for the easter egg. There's nothing in the video about the 80s.

I can't even parse the sentence about identifying the easter egg.

Now there's an extinction risk we can do something about.

Jeans are also a public good, because there are many people whom I do NOT want to see in underwear!

Pledging to Kickstarter/IndieGoGo/etc campaigns that are doomed to fail is one of the greatest, cheapest moral superiority gambits available.

It should be relatively obvious that the best planetary defense is Orion, the 1950s-era nuclear propulsion concept. High impulse, high mass, high delta-v. And the engineering challenges appear minimal.

We could really easily build a battleship-sized Orion and mothball it against the need. Seems sort of ridiculous not to, honestly.

We're undergoing a mass extinction and you folks think asteroids are where the focus should be regarding defense of the planet?

Nice to see a sane person in this comment thread. The rest seem to be just smart enough to be real dangerous.

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