This is the fourth week of my two week series on natural disasters. And it's the end. Really. I mean it this time. For those who have slogged it out, a grateful tip of the hat. The corrections, suggestions, general feedback and new information in the comments has been much appreciated.
Fill in the blank. The Earth suffers an impact from an asteroid or comet generating more energy than the explosion of the atomic bomb at Hiroshima every __ year(s).
Do your own work, please, no peeking. Have your answer ready? OK. The real value is one. As in the Earth is walloped by a twenty-kiloton impact event, on average, once a year. To get that value, I'm not skewing the average by pulling in lots of rocks that rained onto our planet early in its history. This estimate is for the Earth today, and it comes from Eugene Shoemaker, co-discoverer of the comet that so spectacularly splashed into Jupiter in 1994. How is it possible that we could experience a twenty-kiloton event year in and year out and most people are completely unaware? Well, to start with a lot of the Earth is covered in water, so most of those impacts take place at sea. Dense as the population may seem, even when it comes to land there are a lot of places on the planet not exactly favorable to people (i.e. covered in ice, desert, or very high mountains) and some of the impacts also happen in those regions. Still, it seems like a regular explosion of this magnitude ought to be noticed (and would leave the Earth looking like a teenager in bad need of acne cream).
The biggest reason these impacts don't make the 6PM news is that the bulk of these objects are vaporized high in the atmosphere. The size of the asteroids involved is around 5-10 meters across – around the size of a school bus. Relative to the Earth, most of them are coming in between 15 and 25 km/sec. Which is fast. When space bus whaps into the atmosphere at that speed, it's not like taking a stroll through a gentle breeze, it's more like taking a run into a brick wall. The result of these collisions is a high-altitude flash of light and perhaps a distant thunderclap. What eventually reaches the ground in most cases are the pulverized remains of the asteroid – bus dust. Even though the asteroid doesn't reach the ground in one piece, this is still referred to as an impact event.
On Earth, the effects of water and wind mask a hard truth that's visible in the craters that mark the surface of most bodies in the Solar System from the Moon to Mercury – there's a lot of crap up there, and a lot of it will eventually impact with something else. In addition to that one big 10 meter and above impact, each year brings around 500 meteoroids down to about 1 meter. Another 80,000 objects down to the size of a quarter can be expected, along with a fantastic array of dust, grit, and gravel. In all, the total weight added to the Earth each year averages around 75 million kilograms -- just be glad it doesn't all arrive in a lump. The Earth itself is the product of several impacts. Early in the history of the Solar System, there were dozens of dwarf planets orbiting in the space now occupied by Venus, Mars and Earth. It took a series of enormous collisions (including the one that spawned the Moon and trimmed off enough of Earth's atmosphere to make life possible) before we got to a planet approaching the size and consistency of what we see today.
Though there aren't any bodies of that size threatening impact today (at least, none in the inner system) the Earth is still the regular recipient of visitors.
On the morning of 30 June 1908, the sky above Krasnoyarsk Krai, Russia was lit by a flash far brighter than the spring sun. A trader 65 kilometers to the south reported that "the sky split in two" and a blinding blue-white glare brought with it a heat that made him worry that this clothing would catch on fire. Tribesmen to the west noted a series of overwhelming thunder claps followed by an enormous crunching and roaring – the sound of massive trees being sheered away. When the pressure wave reached them, it tossed them several meters across the shaking ground. From a hundred kilometers away, the sound was like the firing of dozens of artillery shells, and observers noted a flash followed by a tower of smoke rising into the clear sky. Sounds of rumbling, groaning in the Earth, and distant explosions were heard for several minutes.
Four years later, the first expedition coming into the area of Tunguska discovered an blast zone in which trees had been stripped of leaves and branches, scorched, and toppled. The total area leveled was over 2100 square kilometers – about three times the size of New York City. It was recognized almost immediately that this was an impact event, but despite decades of diligent search, no large crater was ever located. This was another air burst – occurring 5-10 kilometers above the ground. For years some scientists thought that the Tunguska blast had been caused by a comet, or even by something as exotic as a nugget of antimatter, because it was expected that an ordinary asteroid would impact the ground. There is still some mathematical support for the idea of a comet, but the best contender for what exploded in the sky that day is a common form of stony asteroid. It wasn't until recently that we figured out that those fragments of space rocks that reach the Earth as meteorites are the exception, not the rule.
The lack of a crater doesn't mean there was no destruction. The object involved in the 1908 impact over Russia was probably 50-80 meters across (more of a space school building instead of a bus), and the resulting energy was around 1,000 times that of the Hiroshima explosion. Fortunately, the region of Russia was (and still is) very lightly populated, and the only known victims of the disaster are around 80 million felled trees. But of course, it could happen anywhere.
The good news is, an event the size of Tunguska is expected only around every 1,000 years. As with all such statistics, that doesn't mean that any given year is guaranteed space-rock free, it only means that the odds against such an event happening in any particular year are pretty good. Even better, the same things that make the regular 20 kiloton impacts pass without notice also apply to this 1,000x greater impact – a lot of the Earth is not densely populated. If a 50-100 meter asteroid were to plunge down directly above a dense urban center, it would be a disaster to equal the worst a hurricane or earthquake could do. Blast damage could spread across hundreds of miles and the area at the center of the destruction would be utterly wiped out. On our Teller Scale of Intolerably Large Disasters, let's score it a 0.8 – big enough to level a city, but unlikely to bring down a nation without complicating factors.
But of course, asteroids come bigger than the Tunguska size. The danger can be seen in the effects noted on the Torino Impact Hazard Scale (yes, there's a scale for that). Unlike the scales for tornadoes, hurricanes, earthquakes, and volcanoes, the Torino Scale does double duty. The numbers on the scale don't just define how bad an event would be, but also how likely. This is because many small objects in space may be seen so briefly that an exact path is difficult to predict. The idea is not to classify the damage of an event after it has happened, like the Richter Scale of quakes, but to designate a potential object in space as a possible impactor. A bus-size rock, even a building–sized rock, is hard to spot from millions of miles off when these things are often nearly as dark as soot and don't exactly have their high beams on (Anyone want to present the incoming congress with a bill to require that all large asteroids be painted yellow and mounted with flashing strobes? I give it 50-50 odds of passing.) It's also difficult to determine the exact influence on a passing object after a close encounter with Earth or another planetary body. Sometimes the odds of an object coming around to tangle with Earth on an upcoming orbit are just that, odds.
So a 0 on the Torino Scale doesn't mean that the object in question wouldn't make one ungodly whomp if it did impact Earth, it means that the object's path through the Solar System is well understood and safely well shy of Earth over the next several hundred years. Just about every object we know of is a "0." In the middle of the scale – from 5 through 7 – you have objects which are expected to pass close to Earth, and whose impact would cause devastation ranging from regional to global. These are objects that need close scrutiny to see if they should be moved into the top zone of the scale.
Objects receiving a designation of 8, 9, or 10 are all but certain to impact the Earth. The difference between the three is a matter of size. A Torino 8 object is something similar to what happened in Tunguska – 50m or so across, ready to impact with the force of a very large hydrogen bomb. Such an impact could be locally devastating, could kick up moderate Tsunamis if it happens at sea, and could have temporary effects on global climate.
An object warranting a Torino 9 extends up to things nearly a kilometer across. The impact of something this big would cause a level of destruction that no hurricane or earthquake could come close to matching. It would cause absolute destruction over an area of hundreds of miles, trigger Tsunamis that could circle the globe, and make an enormous impact on global climate lasting years or decades. Such an event is expected every 10,000 years on average. There is a growing cadre of geologists and anthropologists who suspect that such an event may have occurred over the North American continent around 13,000 years ago, helping to usher out many of the continent's large mammals and terminating the early but technically advanced Clovis Culture (mark me down as a skeptic on this, as I don't find the evidence so far compelling). A series of elliptical lakes in Argentina may also mark the impact of fragments left over from the explosion of an object on this size, around 10,000 years ago. On the Teller Scale, an impact like this would rate around 1.5 to 2.0 – probably enough to take out any nation near the impact, possibly enough to mark the fall of global civilization depending on the size of the object and where it strikes.
At the top of the Torino Scale are objects which are certain to impact and which exceed 1km in size. Even at the bottom of this scale we're talking a Teller 2.0+ event. An object of this size is very unlikely to disintegrate in the atmosphere. It would punch through, reaching the ground in a matter of seconds, pass through ocean, soil, and rock to create a crater several kilometers deep and tens (or hundreds) of kilometers across. The material that splashed from the center of that crater would be hotter than the surface of the sun. A supersonic blast wave would ring around the world and a expanding wall of superheated air would rush out to set forest aflame across half a continent. Areas untouched by that disaster would be visited by rains of flaming magma and scalding downpours of ashy hot rain. The atmosphere would be roiled by steam, vaporized rock and salt, smoke, and ash. Frantic hurricanes would race from the boiling sea around the impact site. Everything near the blast would be dead in minutes (if not seconds), and around the world plants and animals would begin to die as asteroid winter settled over the smoke-shrouded planet.
In other words: it would be bad. However, maybe not as bad as you think. Near the town of Manson, Iowa is a buried crater some 40 kilometers across. It marks the point where a stony asteroid about two kilometers in diameter vaporized part of America's inland sea in the Cretaceous Age. The Mason impact must have been an enormous disaster, wiping out creatures across the continent and around the world. When the idea began to circulate that the Cretaceous and its menagerie of dinosaurs had been brought to a close by an impact event, many people turned to Manson as a good candidate for the disaster. However, a closer look showed that Manson actually dated from about 9 million years before the end of the Cretaceous. Despite the size of this impact, there doesn't seem to have been any spike in the rate of extinctions associated with this event. The world went through the storms, through the winter, and kept on going (dinosaurs and all).
But 9 million years after Manson, a 10km object struck at the edge of what's now the Yucatan Peninsula, leaving behind a crater 110km across... and a world where the only dinosaurs wear feathers. An impact event like that which capped the Cretaceous would undoubtedly be a Teller 3.0 event. It would be the end for mankind as a species.
By looking at the moon, scientists can get a pretty good idea of just how often something this size comes screaming to Earth. The answer looks to be around every 10 million years. Not every one of this objects has as much impact as the "dinosaur killer," but they're all bad news.
Okay, time for a review.
Of the natural disasters we've looked at, hurricanes and earthquakes can, in the worst of situations, topple a nation especially when (as we've recently seen in Haiti) these disasters come in conjunction with other problems. Other catastrophes, like the aging of the sun, are a sure threat to end the Earth's existence but are both extremely distant in time and utterly inevitable. Finally there are other possible events that I haven't bothered to discuss, such as the nearby explosion of a supernova which could quick-fry Earth with a directed blast of gamma. The thing is, nothing like this seems to have occurred in the Earth's history so far and which we have no reason to suspect such events of being more likely in the near future ("near" as in "next billion years or so"). There are also events like long term natural climate change (from a return of the ice age to tropics at the poles), but I'll deal with that when the subject of our own intervention in Earth's atmosphere comes up. In terms of "end of the world" scenarios, these events are too small, too distant, too unpredictable, or too slow to be interesting.
That leaves us with only two contenders when it comes to natural events likely to topple human civilization (or humanity itself): volcanic eruptions and asteroid impact. Both of these categories of events can scale from the inconsequential to a threat to mankind's existence. Both of them is not just likely, but nearly certain to occur well before we need to think about anything like the Sun's impending retirement.
Thankfully, neither of these events is all that likely at any given time. Super-volcano explosions of a scale that could threaten global civilization come along about once per 10,000 years. Asteroid impacts of that size are probably less common. Say... once in 100,000 years. Decent odds, but neither of these threats is so negligible that they can be or should be ignored.
For super-volcanoes there's probably nothing we can do to halt the disaster. Instead, funding should (are you listening, Bobby Jindal?) be devoted to study and monitoring of volcanoes. Putting some bucks into planning how we might react to such an event is also a good idea. In a world where it's hard to get people to plan beyond the next quarter much less the next year, planning for somethingthat may be further away than the reign of Ramses II doesn't come naturally, but when the payoff could be measured in billions of lives saved, it's worth a small upfront investment. For asteroids, there's another factor entirely. With some advances in space technology, we might not only see these objects coming, we might find ways to divert them (ways that don't involve Bruce Willis and the worst movie ever filmed by a major studio). Right now, the highest level of any object rated on the Torino Scale is a 2. Such an object is very unlikely to impact the Earth, and it's expected that additional study will punt this candidate back to a 0 "not going to hit us" rating. Let's hope it stays that way. But let's invest a bit, just in case.
When it comes to natural disasters, the good news is that there is no sword hanging over our heads. Despite the frothing disaster porn on History International, there's no planetary alignment, galactic plane alignment, surge in solar storms, death star, death planet, or death tiddly wink in our immediate future. The Earth has supported a wide variety of complex life for hundreds of millions of years, and there's every reason to believe that's not about to change.
But that doesn't mean we're safe. In 1838, a young Abraham Lincoln delivered a speech in Springfield, IL.
At what point then is the approach of danger to be expected? I answer, if it ever reach us, it must spring up amongst us. It cannot come from abroad. If destruction be our lot, we must ourselves be its author and finisher. As a nation of freemen, we must live through all time, or die by suicide.
Lincoln was talking about America, but he might as well have been speaking for the world. If there is a threat to our civilization... we're it.