Impact events and you.

I’m working on a setting for my next RPG and I have a couple of questions.

How big an impact event would you need to have a very likely extermination of (very nearly) all humans? I’m talking almost immediately from heat, earthquakes, tsunamis, 900 kph winds and activated volcanoes. Not attrition from a later die off due to an impact winter.

10km ball of nickle/iron? 20km? Anybody know of any references or specifically expert enough to help me out?

Hard to say, since I don’t think there has been a known impact in the last 300 million years that would be have been large enough to have that kind of effect.

The asteroid that caused the mass extinction at the end of the Cretaceous is thought to have been about 10 km in diameter. However, most extinctions were probably not instantaneous (except for species confined to the immediate area of the impact). Extinctions probably would have taken place over a period of months, years, or even millenia.

Humans would be much more resilient against mass mortality immediately after an impact than most animals of similar size. Even ordinary buildings would provide some protection from impact effects, and many humans would be in well-protected sites in underground structures, mines, etc. Given any warning of the impact survival in the immediate aftermath would be quite high. And because of stored food, humans would be likely to survive an impact winter.

To kill almost all humans immediately even in areas remote from the impact site you would need one hell of an impact.

I suspect that it will be more an issue of proximity than size. Someone on the opposite side of the planet in the middle of a land mass won’t get hit by any blast unless the impact is so massive that the whole world has just kersploded.

Do you really need the whole population of humanity to die, or just the ones within reach of the characters?

Well my hope was to have widespread geological upheavals that destroy most of the shelters so only a very few pockets remain. My initial thought was to have the impact site in central Asia so and that half of the world would be largely cooked and the remainder would have the geological problems.

Would it be possible that a large enough impact (below the amount that just renders the Earth’s surface molten) could cause a series of 10+ Richter scale quakes on the opposite side of the planet?

I know practically nothing about geology, so I may be so silly here it is stunning.

Oh yeah, in answer to your question, it would need to be the entire planet for the final arc of the campaign to make sense.

This little calculator is handy:
Solar System Collisions: Earth Impact Calculator

It lets you specify land only strikes, or land and sea, and specify composition, velocity and size of the impactor.

for example, a 25km diameter iron body striking Brazil at 25 km/s yields:

That right there is pretty awesome, thanks. I assume the earthquake would weaken as it wrapped to the other side of the planet, but it seems like that would work well enough. A big enough quake could collapse the majority of shelters and the exposed could die from the impact winter.

This is a serious question. What is an RPG? I’d really like to know.


Role Playing Game. Like D&D. Played with pencil and paper, typically by mega-nerds.


I have a followup question:

How likely is it that something like cracked tectonic plates causing new volcanoes or every volcano on the ring of fire goes apeshit crazy? Is it reasonable to pepper the US with active volcanoes because of the shock?

Or Rocket Propelled Grenade, a sure sign of mega-nerdiness in Afghanistan.

On The subject of vulcanism, you might try reactivating the Rio Grande Rift:

Exactly what happened 65 million years ago is debated since with a lot of the evidence it isn’t completely clear exactly when things happened. One thing you may want to factor into your doomsday scenario though is fire. There’s a layer of soot all around the earth which tends to indicate that the entire world burned after the Chicxulub impact. Exactly how and why is also debated, but it’s easy to imagine that the impact throws up all kinds of hot stuff and it literally rains fire as this stuff floats around the earth and comes back down.

Cities are going to burn. Food supplies are going to burn. People are going to burn. It’s basically going to be hell on earth. When 99 percent of the world’s food supply disappears overnight, there’s going to be all kinds of violence as the survivors scramble for the remaining 1 percent. So, it’s hell on earth with the survivors shooting at each other while the world burns around them.

This isn’t quite as instant as maybe you are looking for, but it’s not a slow dwindling death. We’re talking about a major population loss in a very short amount of time. There will be small pockets of survivors all over the place, but maybe that’s what you want anyway.

Another thing to consider is that the shock wave spreads out as it travels around the earth and comes back to a point on the opposite side of the earth from the impact. You aren’t just going to get major earthquakes at the site opposite the impact, you’re going to get a crust breaking major event that’s probably going to spark off all sorts of volcanic activity.

No, not on current geological knowledge (at least on the short term). Volcanoes occur on plate margins because plates are subducting and melting there. You couldn’t get more magma without vastly accelerating the amount of melting, and I don’t see how an impact could do that over the time scale you want. The only way to get more volcanoes in the middle of a plate would be to initiate some new hotspots like Yellowstone there. Any mechanism would be speculative and would probably not happen over the short term.

I think it’s somewhat controversial the extent to which a large impact could caused increased volcanism. But if it did occur, it wouldn’t happen over a short period, but more likely hundreds or thousands of years.

There’s one additional point to be taken into account – the Chicxulub impact and the hypothecated Permian-Triassic impact were during periods when the Deccan and Siberian Traps respectively were active, meaning rather extensive vulcanism before, during, and after the impact event.

Regarding the planetide ashfall that engineer_comp_geek mentioned (actually much less than worldwide, but still extensive), one explanation is a large-area conflagration, but far from planetwide (say about the area of ten US states, for example), with ash suspended in the atmosphere and falling out over a much wider area.

In terms of the opposite side of the planet, quakes will generally weaken. However, one thing to remember is that the convergence point (the point exactly on the opposite side) is one of the worst places to be, since the seismic waves spread out around the globe and then converge on that point at the opposite side.

I’m not sure how your setting is going to work, but one of the problems you face is that anything that wipes out a large percentage of people instantly is guaranteed to kill everyone over the long run. If it’s OK that everyone in the setting dies within a few years or decades, then you’re good. If you want long-term survival, then you have to reduce the number of short-term casualties.

Thanks for all the replies, I really appreciate it.

I think I have a better understanding thanks to your help.

I found this site and it seems like it has some good info:

So according to the site my 50km iron asteroid hitting central Asia straight on at 70km/s would cause a nearly thousand mile diameter crater, would subject the US to a moderate but noticeable earthquake after half an hour, then after 8 and a half hours 830 mph hot winds will hit destroying or seriously damaging anything weaker than a hardened bunker. Then firestorms followed by a ten year impact winter.

No increased vulcanism, but I think that’s enough to drop humanity to a low enough level to make the campaign make sense.

One of the conceits is that small government and private shelters survived here and there across the world. After the winter started dying down the few shelters that were able to hold out began trying to rebuild a society.

I’m still working on it and the specifics of the event were necessary to get down before I got too far.

You may want to take into account the division between the northern and southern hemispheres. Unless your impactor hits near the equator, one hemisphere is going to be affected much more than the other.

Just to nitpick, an iron object going at 70 km/sec isn’t terribly plausible. Iron objects generally come from the asteroid belt, and would have velocities more in the range of 20 km/sec. We can solve this problem by making it an ice object instead- comets come from farther out in the solar system, and so have higher velocities. Comet Hale-Bopp has a diameter of about 60 km, and might do nicely:

If it happened in Central Asia, there would be a noticeable earthquake in about half an hour in the US (well, in Pittsburgh, this will vary on where in the US you are), and 337 mph winds about nine hours later.

Oh I know the speeds and size are unlikely, part of the campaign necessitates that it is an extra solar object that appeared without (much) warning from behind the Sun.

OK. A long-period comet could do that, too. We didn’t discover Hale-Bopp until July of 1995. It made its closest approach to Earth in March 1997 and reached perihelion in April 1997. We wouldn’t know it was headed right for us right away, since you need several observations to figure that out.