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View Full Version : Meteor impact - how big to mirror Hiroshima or Nagasaki?

DyerWolf
2007-Sep-07, 06:35 PM
I've seen a lot of "world killer" videos lately, but none that talk about the smaller effect.

Any idea on how big a meteor or comet would have to be to have an effect similar to a nuclear blast?

01101001
2007-Sep-07, 07:01 PM
Any idea on how big a meteor or comet would have to be to have an effect similar to a nuclear blast?

Are you after the smallest physical size that might resemble, say, Hiroshima? Largest? A lot depends on speed, angle and composition, etc. Diameter is not the only variable. And, does it have to reach the ground? That might be difficult.

Just a for-instance, if we aim for 15 kilotons of TNT, about Hiroshima bomb sized, playing with the Earth Impact Effects Calculator (http://www.lpl.arizona.edu/impacteffects/):

This puny, slow meteor yields about 11 kilotons in an airburst:

Distance from Impact: 10.00 km = 6.21 miles
Projectile Diameter: 10.00 m = 32.80 ft = 0.01 miles
Projectile Density: 3000 kg/m3
Impact Velocity: 11.00 km/s = 6.83 miles/s
Impact Angle: 45 degrees
Target Density: 2500 kg/m3
Target Type: Sedimentary Rock
Energy:
Energy before atmospheric entry: 9.50 x 1013 Joules = 0.23 x 10-1 MegaTons TNT
The average interval between impacts of this size somewhere on Earth is 6.0 years
Atmospheric Entry:
The projectile begins to breakup at an altitude of 47100 meters = 154000 ft
The projectile bursts into a cloud of fragments at an altitude of 28200 meters = 92600 ft
The residual velocity of the projectile fragments after the burst is 7.98 km/s = 4.96 miles/s
The energy of the airburst is 4.50 x 1013 Joules = 0.11 x 10-1 MegaTons.
No crater is formed, although large fragments may strike the surface.

cjl
2007-Sep-07, 10:27 PM
Here's another one that would give about a 26 kiloton airburst, but a little lower altitude so it might have a much more significant effect on the ground:

Distance from Impact: 6.00 km = 3.73 miles
Projectile Diameter: 6.10 m = 19.99 ft = 0.00 miles
Projectile Density: 8000 kg/m3
Impact Velocity: 17.00 km/s = 10.56 miles/s
Impact Angle: 60 degrees
Target Density: 2750 kg/m3
Target Type: Crystalline Rock

Energy:
Energy before atmospheric entry: 1.37 x 1014 Joules = 0.33 x 10-1 MegaTons TNT
The average interval between impacts of this size somewhere on Earth is 7.9 years

Atmospheric Entry:
The projectile begins to breakup at an altitude of 13400 meters = 43900 ft
The projectile bursts into a cloud of fragments at an altitude of 8780 meters = 28800 ft
The residual velocity of the projectile fragments after the burst is 7.59 km/s = 4.71 miles/s
The energy of the airburst is 1.10 x 1014 Joules = 0.26 x 10-1 MegaTons.
Large fragments strike the surface and may create a crater strewn field. A more careful treatment of atmospheric entry is required to accurately estimate the size-frequency distribution of meteoroid fragments and predict the number and size of craters formed.

cjl
2007-Sep-07, 10:33 PM
And here's what you would need to equal Castle Bravo (http://upload.wikimedia.org/wikipedia/commons/5/5d/Castle_Bravo_Blast.jpg), the US's largest ever thermonuclear test:

Distance from Impact: 15.00 km = 9.31 miles
Projectile Diameter: 50.00 m = 164.00 ft = 0.03 miles
Projectile Density: 8000 kg/m3
Impact Velocity: 17.00 km/s = 10.56 miles/s
Impact Angle: 90 degrees
Target Density: 2750 kg/m3
Target Type: Crystalline Rock

Energy:
Energy before atmospheric entry: 7.57 x 1016 Joules = 1.81 x 101 MegaTons TNT
The average interval between impacts of this size somewhere on Earth during the last 4 billion years is 1.0 x 103years

Atmospheric Entry:
The projectile begins to breakup at an altitude of 14100 meters = 46300 ft
The projectile reaches the ground in a broken condition. The mass of projectile strikes the surface at velocity 15.1 km/s = 9.38 miles/s
The impact energy is 5.98 x 1016 Joules = 1.43 x 101MegaTons.
The broken projectile fragments strike the ground in an ellipse of dimension 0.156 km by 0.156 km

DyerWolf
2007-Sep-08, 02:09 AM
Thanks. I'll have to play around with that. Gives a lot more info than I would have guessed. Any idea what "distance from impact" means as far as the program is concerned?

01101001
2007-Sep-08, 02:26 AM
Any idea what "distance from impact" means as far as the program is concerned?

When you get an impact, as I recall, it details affects on you, at that distance, survivability and such. Yeah, with an impact, you get:

Crater Dimensions:
Seismic Effects:
Ejecta:
Air Blast:

And (at least some of) those results depend on distance.

neilzero
2007-Sep-08, 02:24 PM
My guess is you will, all but surely be injured 9.31 miles from ground zero, but you have a 50% chance to live with good medical care. Neil

mugaliens
2007-Sep-09, 07:03 AM
Wow.

I'm glad that meteor impacts, traditionally, have less of an impact than castle bravo!

astromark
2007-Sep-09, 08:17 AM
Quote;...
"Thanks. I'll have to play around with that. Gives a lot more info than I would have guessed. Any idea what "distance from impact" means as far as the program is concerned?... end quote...

Given that you now have this. ' Earth impacts program.' You will quickly see that it is easy enough to ask the questions regarding size composition and velocity of an impacting object. The distance from that point of impact is critical to you surviving such provided that surviving is an option available. Fiddling with the numbers can produce a 'New asteroid belt' between Mars and Venus. Yes your distance from impact is important. What did you want to know?

DyerWolf
2007-Sep-09, 12:22 PM
Given that you now have this. ' Earth impacts program.' You will quickly see that it is easy enough to ask the questions regarding size composition and velocity of an impacting object. The distance from that point of impact is critical to you surviving such provided that surviving is an option available. Fiddling with the numbers can produce a 'New asteroid belt' between Mars and Venus. Yes your distance from impact is important. What did you want to know?

I hadn't gotten to the point of asking how close could I stand to the thing and still live to tell the tale. That's a pretty cool feature. Too bad they don't have it tied to a 3d program that can animate the impact... That would be really cool.

I was thinking about a story I'd read (Niven, Pournelle???) about aliens that used asteroids as their artillery against us humans.

From another thread, I think I remember reading that it would almost be easier to design your own nuke from scratch than to grab a big enough asteroid and throw it at the earth with any hope of hitting a specific target.

Grashtel
2007-Sep-09, 01:20 PM
I hadn't gotten to the point of asking how close could I stand to the thing and still live to tell the tale. That's a pretty cool feature. Too bad they don't have it tied to a 3d program that can animate the impact... That would be really cool.
Cool but very impractical, the load on the server would be much higher and it would take up a lot of space and use tons of bandwidth compared to the current text only version.

I was thinking about a story I'd read (Niven, Pournelle???) about aliens that used asteroids as their artillery against us humans.
I believe that the book you are thinking of is "Footfall" by Niven and Pournelle that features elephant like aliens dropping a big asteroid into the Indian ocean to clear themselves a beachhead. While they used kinetic weaponry as artillery it was Thor type projectiles (pretty much guided crowbars) rather than simple asteroids.

From another thread, I think I remember reading that it would almost be easier to design your own nuke from scratch than to grab a big enough asteroid and throw it at the earth with any hope of hitting a specific target.
Depends on how big a target you are shooting at, if you just want to hit a specific ocean or continent and don't mind where on it then its not so difficult, for smaller targets something with more predictable aerodynamic behavior and active guidance (eg the "smart crowbars" of the Thor weapon system) is much easier to aim.

DyerWolf
2007-Sep-10, 02:28 PM
I'd forgotten the "guided crowbar" part of the story.

So, if you wanted to use a kinetic energy weapon against a city, the Thor weapon is more practical than just throwing a big rock from space.

Any idea how big the crowbars were?

Is it simply a matter of mass such that I could still use the calculator above to figure out size and mass? I.e. would a truckload size (3800 cubic feet, 1,865,800 lbs) chunk of iron mined from an asteroid be sufficient?

(I tried to convert a 107.54 cubic meter chunk of steel into the calculator - but don't remember how to derive the diameter given the spherical volume (I made my truck into a sphere)) - Any help?*

*Sorry, feeling math-lame today.

JohnBStone
2007-Sep-10, 10:34 PM
Any idea how big the crowbars were?
Crowbar size for surface targets (e.g. tanks) or telegraph pole size for underground bunkers.

http://en.wikipedia.org/wiki/Kinetic_bombardment