Kinetic impact energy question

[Usquanigo]

This is a little bit of a flight of fancy, so indulge me if you would be so kind.

Let's say you had an object weighing about 4lbs moving at roughly orbital velocities (rough estimates, 1.75kg @ 7600m/s). Now, lets say you could have that slam into something without the effects of atmospheric drag getting in the way.

How is up to you, one could imagine a magic portal, or simply assume it is re-entering, held together, and strikes at 7600m/s (but in that case, also ignore all effects of heating, shockwave, etc).

According to what I've found online, [math]KE=\frac{1}{2}mv^2[/math] with the result being in Joules... and, 1 ton of TNT containes 456.056 Joules.

So... based on that, the 4lb example would yeild a little less than 110 kilotons of TNT (in terms of Joules of energy).

Now, Little Boy was between 13 and 18 KT, and Fatman was about 21 KT. We know what they did. So scale Little Boy (Hiroshima) up by almost 100 times..... :blink:

Ok, so, that brings me 'round to the question - how would that force be applied? Meaning, with a nuke, the blast either pushes off of the Earths surface and radiates out and up, or, it's an air-burst and radiates down and out with nothing but air to push off of.

But they measure in KT or MT (of TNT / equiv). So would the force, wouldn't the force be entirely concentrated into the area of the impactor? Or would it explode out like a TNT blast of the given size at the impact location?

 

[RisingFury]

By my calculations Q = 4.6 *10^9 J/ton for energy released by TNT and 5*10^13 J stored in the impactor, that puts it at roughly 10 000 tons of TNT or 10 kilotons of TNT.



To answer the question, all this energy would be dissipated as heat and blast - look at animations of impacting meteorites.

 

[Usquanigo]

Yes, I just now saw that someone posted bad information and I didn't dig deeply enough. :facepalm: They said 4.189x109 J when it should have been 4.189x10^9. Doh!

Still... an impressive Little Boy amount of energy release though. (just an order of magnitude LESS impressive. hehe :embarrassed: )

Well, with a meteorite, it's carrying a lot of thermal energy and a large shockwave. What I'm thinking of would be more like.... well, like throwing a projectile in space and having it collide with an object, at a relative velocity of 7600m/s. Or like a projectile with a hypothetical zero drag (amount), slamming into a building, or armor plate, or the ground. The atmosphere might come in to play once the impact occurs, but not before (if you can imagine such a thing - I know I'm way outside physics on that end)

 

[Jarvitä]

To answer the question, all this energy would be dissipated as heat and blast - look at animations of impacting meteorites.

That would depend entirely on the shape and density of the impactor. A phone pole of wolfram could probably survive re-entry and impact without losing that much velocity.

 

[Quick_Nick]

Here's a simulation of an asteroid with about half the mass and half the energy.
http://impact.ese.ic.ac.uk/cgi-bin/...s=1&theta=45&wdepth=&wdepthUnits=1&tdens=2500
I think you converted Joules to MegaTons TNT wrong.

EDIT: Bah, 3 replies show up

And impacts release quite some heat. IIRC, the impact of SMART-1 into the moon released enough heat and light that it was visible from Earth. There's a lot of friction when you've got a relative velocity of several km/s.

 

[Usquanigo]

Here's a simulation of an asteroid with about half the mass and half the energy.
http://impact.ese.ic.ac.uk/cgi-bin/...s=1&theta=45&wdepth=&wdepthUnits=1&tdens=2500

Cool link. :thumbup:

The only trouble is that it involves a rock actually falling through the atmosphere, and dumping all that energy into the air.

And impacts release quite some heat. IIRC, the impact of SMART-1 into the moon released enough heat and light that it was visible from Earth. There's a lot of friction when you've got a relative velocity of several km/s.

Yeah, that is true. But I guess that's the real question I'm asking. Would it be an explosion - like you set off the equivalent amount of explosive at the point of impact (line taking a stick of dynamite, and double sided taping it to the wall, for example), or would he be focused and concentrated at the point of impact (more like a shaped charge blowing in the direction of the blast). I guess it's a question of area effect vs point of impact. (then again, maybe it's the same thing and I don't know it, which is why I'm asking lol)

Seems that impact (much the same as what I'm thinking about here), would have created a lot of back blast, so maybe that's what I'm trying to say (sorry, the words escape me sometimes) - is that back blast/area effect part of the calculated figure, or is all that calculated energy delivered directly into the point of imact (with the back-blast, heat, light, etc, being secondary effects of that energy transfer - which would, in the final analysis, increase the amount of total damage done to the target and immediate area [over the calculated value alone])

 

[tori]

Most of the energy would be directed in the direction of travel, i.e. most of it would be used to produce shockwaves deep in the Earth. A smaller portion would be used to push the ground apart, creating a horizontal shockwave (a "ripple") and a crater (just like a regular asteroid). The rest would be released as radiation (white light and infrared mostly - bolides* are a pretty good illustration of the color that friction at these speeds produces.). Deep Impact also made a nice flash.

So a 10 kt impact would deal less surface damage than a 10 kt nuclear airburst, because it invests a lot more energy into the earth moving department.

[ame=http://www.youtube.com/watch?v=Jhf4ezudqQU]*[/ame]

 

[N_Molson]

The [ame="http://en.wikipedia.org/wiki/Tunguska_event"]Tunguska event[/ame] is an impressive example of the meteroid's destructive power. We were very lucky that time, Moscow, London, Paris or New York could have been removed from the map as well :shifty:

 

[tori]

Indeed, if atmosphere is added into the game, it brings out the destructive power and efficiency of the rock. When atmosphere is present, a 10 kt nuke produces a similar blast to a 10 kt asteroid, because then only a small remainder of the kinetic energy makes it all the way to the ground (a bunch of "pebbles" travelling at their terminal velocities, about 100 m/s).

 

[Quick_Nick]

Btw, I believe most meteors come in at greater than escape velocity. 7.6km/s is fairly low. That "velocity squared" part of the energy equation could make that significant.
Idk if a satellite impact is much different from a meteor...hm... I bet it's all the same when you hit that hard.

 

[tori]

If the incoming object is not a satellite of Earth, then it has to come in at escape velocity or greater (>= 11.2 km/s). The average is around 17 km/s I believe.

 

[Aviator]

just remember, that your object will only exert its full 10 kilotons of force if the other object brings all of its peices to a complete stop.

now i seem to think that your rock would punch right through whatever it hit and continue - leaving a large amount of debris and a messy hole, but still retaining most of its velocity.

so unless you can totaly transfer all of your kinetic energy into your target they wont receive a nuclear bomb equivilent force.