Interplanetary ballistics/ meteor impacts

[Kato]

I've been thinking about the good old concept of meteor impacts and their destructive capabilities. But not on the planetary extinction level scale and more on the local disaster level.

Is anyone aware what's the best way to get some rough but realistic estimates on such events? e.g. how to know what kind of meteor would be enough to destroy a small town? Should I just run any mass through E=m/2 v^2 and compare to the effect of conventional weapons? How relevant is the process of atmospheric entry? (my guess is not big, but then 'it will break apart in the atmosphere' is a common argument against the dangers of small asteroids. And I know, mass to surface ratio and all that, but where is the line?)
Would the vicinity be affected worse than by a similar bomb? Would it be hard to tell them apart afterwards?


But please, no explanations that reference what happens in 'fictional story X', even the better sci-fi authors tend to be inaccurate about this, I feel.

(I'm not sure if I have to point out that I don't intend to redirect any asteroids for nefarious reasons :smalltongue: I'm just curious and I know we have many people smarter than me around)

[halfeye]

I've been thinking about the good old concept of meteor impacts and their destructive capabilities. But not on the planetary extinction level scale and more on the local disaster level.

Is anyone aware what's the best way to get some rough but realistic estimates on such events? e.g. how to know what kind of meteor would be enough to destroy a small town? Should I just run any mass through E=m/2 v^2 and compare to the effect of conventional weapons? How relevant is the process of atmospheric entry? (my guess is not big, but then 'it will break apart in the atmosphere' is a common argument against the dangers of small asteroids. And I know, mass to surface ratio and all that, but where is the line?)
Would the vicinity be affected worse than by a similar bomb? Would it be hard to tell them apart afterwards?


But please, no explanations that reference what happens in 'fictional story X', even the better sci-fi authors tend to be inaccurate about this, I feel.

(I'm not sure if I have to point out that I don't intend to redirect any asteroids for nefarious reasons :smalltongue: I'm just curious and I know we have many people smarter than me around)

It depends on a lot of variables:

1/ Anything that comes in via orbit has an upper limit on its velocity, but is quite likely to hit, whereas something that comes in at random on an actual collision course can be any velocity at all up to c, but a miss is a miss and no second chances and the odds of a hit are miniscule.

2/ The type of asteroid matters, a rubble pile is quite possibly going to disintegrate high up no matter how big it is, a comet may be a bit tougher, some rocks explode, and a solid piece of iron will hit the ground if it doesn't burn first.

3/ The mass, but until you know the first two, that's not so important.

4/ Is it spinning, and if so how fast, probably not significant most of the time.

I am by no means an expert, but I have thought about this a time or two.

[gomipile]

You might find the two tables in the Airbursts section of this article useful:

https://en.m.wikipedia.org/wiki/Impact_event#Airbursts

The units are in kilotons and megatons. You can put that and the airburst altitude into NUKEMAP:

https://nuclearsecrecy.com/nukemap/

Combining those two resources should help you get a helpful estimate.

[Sermil]

I've been thinking about the good old concept of meteor impacts and their destructive capabilities. But not on the planetary extinction level scale and more on the local disaster level.

Is anyone aware what's the best way to get some rough but realistic estimates on such events? e.g. how to know what kind of meteor would be enough to destroy a small town? Should I just run any mass through E=m/2 v^2 and compare to the effect of conventional weapons? How relevant is the process of atmospheric entry? (my guess is not big, but then 'it will break apart in the atmosphere' is a common argument against the dangers of small asteroids. And I know, mass to surface ratio and all that, but where is the line?)
Would the vicinity be affected worse than by a similar bomb? Would it be hard to tell them apart afterwards?


But please, no explanations that reference what happens in 'fictional story X', even the better sci-fi authors tend to be inaccurate about this, I feel.

(I'm not sure if I have to point out that I don't intend to redirect any asteroids for nefarious reasons :smalltongue: I'm just curious and I know we have many people smarter than me around)

I would start with known real events, and look at what scientists think caused them. In particular, look at some of the smaller impact craters from https://en.wikipedia.org/wiki/List_of_impact_craters_on_Earth.

An example close to what you want might be the Wabar craters (https://en.wikipedia.org/wiki/Wabar_craters). It hit with a Hiroshima-level impact, after most likely entering at a shallow angle and breaking up during entry. Fortunately, it was in the middle of the desert and pretty no one was around to see it.

A more powerful impact would be the Barringer Crater (https://en.wikipedia.org/wiki/Meteor_Crater). It's about half a mile in diameter. They estimate the actual impact energy at 10 megatons, which is a large fusion nuclear bomb, so enough to destroy a large city. Wikipedia says by the time it hit, it was about 160 feet across, and was probably much larger when it hit the atmosphere.

If you really want to go down the rabbit hole, there's an entire Earth Impact Database (http://www.passc.net/EarthImpactDatabase/index.html) with 190 confirmed craters.

[halfeye]

It's probably worth mentioning that cities, towns or villages being hit are very, very unlikely events, there is much more of the surface of the Earth that is sea, desert, jungle, farmland, countryside or marsh.

[factotum]

It's probably worth mentioning that cities, towns or villages being hit are very, very unlikely events, there is much more of the surface of the Earth that is sea, desert, jungle, farmland, countryside or marsh.

Given a big enough explosion that doesn't matter so much. If a meteor like the 1908 Tunguska one hit pretty much anywhere in Europe then a small town of some sort would be caught in the blast radius.

[halfeye]

Given a big enough explosion that doesn't matter so much. If a meteor like the 1908 Tunguska one hit pretty much anywhere in Europe then a small town of some sort would be caught in the blast radius.

Sure (I think, what was the blast radius again? there are parts of Britain that desolate that this might not be true), but what were the actual odds of that? I don't know, but I would be surprised if they weren't quite small. Most of the world isn't as densely populated as Europe even now, and less so then.

I'm basically saying, if the OP wants, for story reasons to have a town wiped out, that's okay but it's an editorial decision, it's not a thing that would happen often by random chance on the real Earth.

[factotum]

Sure (I think, what was the blast radius again? there are parts of Britain that desolate that this might not be true)

Tunguska destroyed trees over 770 square miles, which, assuming a circular area, would be a blast radius of 16 miles or thereabouts. That's actually rather lower than I thought it was, so ignore my above statement.

[gomipile]

Tunguska destroyed trees over 770 square miles, which, assuming a circular area, would be a blast radius of 16 miles or thereabouts. That's actually rather lower than I thought it was, so ignore my above statement.

Still, a 32 mile diameter circle isn't inconsiderable. You'd be hard pressed to find anywhere in Europe south of Scandinavia where there'd be zero casualties from that.

[Telok]

https://impact.ese.ic.ac.uk/ImpactEarth/

http://down2earth.eu/impact_calculator/

https://www.purdue.edu/impactearth/

http://www.killerasteroids.org/interactives/impact/index.php

My local uni has exactly three astronomy classes, one of which is the 400 level cross chem/bio/astro astrobiology class. In one of the classes the 'practical lab' includes renting time on a computer controlled telescope 2000+ miles away, running calculations for two hours, and submitting a write up of data points on near a Earth asteroid. So for finishing the class you get to be a "published scientist" when the data comes out about two months later.

As a side effect you end up knowing a bit about finding impact calculators.

[halfeye]

Still, a 32 mile diameter circle isn't inconsiderable. You'd be hard pressed to find anywhere in Europe south of Scandinavia where there'd be zero casualties from that.

Yes, it's probably big enough that in Europe you would find more town sized hits than misses.

Britain isn't all south of Scandinavia. Then there's the Alps.

Europe is still atypical.