for a little background see
http://news.nationalgeographic.com/news/2004/04/0414_040414_earthkillers.html

>> In 1908 an asteroid believed to be about 60 meters (197 feet) in diameter exploded in the atmosphere over Siberia. The resulting shock wave knocked down trees for hundreds of square miles.

>> On March 18 of this year, an asteroid measuring perhaps 50 meters across passed Earth at a distance of 30,000 miles (48,000 kilometers). It was announced with 23 hours notice.


Why do asteroid impacts cause explosions when they impact with Earth ?
:eek:

It is due to Kinetic energy; not really much to do with relativity at all actually.

The velocity of an asteroid is very high, several kilometers per second; you multiply this velocity by the mass, and you get a very large momentum- this momentum is instantly converted into energy when the asteroid hits.
It is as simple as that.

SF worldbuilding at
www.orionsarm.com/main.html

The Tunguska asteroid exploded at an estimated 6 km above the ground, while in the
atmosphere and still moving. From what little I know, friction and a mechanical shock
increased in the stoney astroid as it traveled through the atmosphere, until the heat
and shock increased to the point of releasing the kinetic energy in an explosion equivalent to an atomic bomb. Zarkov, I suppose you have other ideas?

On a related topic, some people are suggesting tunguska was actually a natural gas/ air explosion. On the evidence of the destruction around the area and lack of actual bits of asteroid.
but apart from that, yes, you have kinetic energy going to heat, due to friction and impact force.

>> until the heat and shock increased to the point of releasing the kinetic energy in an explosion equivalent to an atomic bomb.


So it is just mechanical energy transferred to the surrounding ground !

I would not expect a largish hunk of rock to get very hot throughout, by passing momentarilary through our atmosphere ( how long ? a couple of minutes???)
... it would be just surface heat, even if the outside was somewhat vaporised.

Unless ?

>> around the area and lack of actual bits of asteroid.

This is the most interesting part... in a lot (?most) examples there is NO BITS of the asteroid to be found..... this to me is very curious....

Maybe ?

As the meteor burns up in the atmosphere (remember it is moving very fast...faster than a returning spacecraft), it breaks apart because the materal vaporizes from the heat and shock of the entry. At a certain point (if the meteor is not too big to survive till the ground), the thermal and mechanical shock is too much for the meteor and it breaks apart very quickly. That increases the surface area exposed to the atmospheric conditions. All this energy going into vaporizing and expanding the material into plasma and gas is terrific. So the explosion of all that material changing state at once can be quite large and will release a good deal of energy similar to a nuclear bomb air burst.

As the meteor burns up in the atmosphere (remember it is moving very fast...faster than a returning spacecraft), it breaks apart because the materal vaporizes from the heat and shock of the entry. At a certain point (if the meteor is not too big to survive till the ground), the thermal and mechanical shock is too much for the meteor and it breaks apart very quickly. That increases the surface area exposed to the atmospheric conditions. All this energy going into vaporizing and expanding the material into plasma and gas is terrific. So the explosion of all that material changing state at once can be quite large and will release a good deal of energy similar to a nuclear bomb air burst.

Maybe it wasn't an asteroid.......Maybe!

Atta Boy :)

>> The velocity of an asteroid is very high, several kilometers per second
>> remember it is moving very fast

to travel 1000 km would take say 10 minutes.....

You claim in 10 minutes or less, a hunk of rock several kms accross or more gets heated enough to break apart by heat conduction....

Ho Ho Ho and a merry christmas :)

Comets, are hunks of rock, when passing near the Sun, they must get direct heat radiation far greated than anything Earth can claim to give via friction in 10 minutes.

Comets exploding in space ???? don't think we have seen any near the Sun.

I think though, from the data of that comet that hit Jupiter, the observed effect was far greater than expected ( by a magnitude of at least 10)... indicating there could be far more to this than anyone expects...

????? Cosmic chemistry

Asteroids passing near the sun experiences a different form of heat then an asteroid passing through the atmosphere. When passing near the sun, the asteroid experiences solar radiation which has wave-like properties. When passing through the atmosphere of Earth, it experiences heat due to friction which is a property of particles. They're two completely different forms of heat. Granted the sun also emits particles but those particles doesn't cause the asteroid to heat up because their is no outside medium to conduct heat just above the surface of the asteroid.

And also, small asteroids explode in the atmosphere because of the enormous amount of stress that those rocks experiences upon its descent through the atmosphere. That stress is caused by the enormous friction due to the atmosphere and at some point, the asteroid reaches its "breaking" limit and fractures ... releasing all the stored energy is had from its momentum coming in. At least that's my opinion of it.

You claim in 10 minutes or less, a hunk of rock several kms accross or more gets heated enough to break apart by heat conduction....

Ho Ho Ho and a merry christmas :)

Comets, are hunks of rock, when passing near the Sun, they must get direct heat radiation far greated than anything Earth can claim to give via friction in 10 minutes.

Perhaps you could do some rough calculations, and let us know what you figure out?

If I were going to have a wild ass guess, I'd also expect a sun-grazing comet to be heated more (but less mechanical stress, obviously). But, a wild-ass-guess really isn't worth much, is it?

edit -
Where did you get "several km across" from?
It seems that a stony asteroid of that size would certainly make a nasty crater (ie a world killer), but that's not relevant for the Tunguska event.

Comets exploding in space ????

Comets break up far and near (http://www.jpl.nasa.gov/releases/2002/release_2002_170.html)

I think though, from the data of that comet that hit Jupiter, the observed effect was far greater than expected

I think you'll find that the most publicized "expectations" (hundreds of megatons of TNT) were based on an acknowledged wild-ass-guess of the size of the fragments (1km diameter).

Much higher calculations (several million megatons) were also published based on guesses of 3-4km diameter fragments, but were less publicized.

The size of the fragments was guessed at by a couple of methods - two different models of tidal disruption were used to attempt to calculate the original comet size. Results ranged from about 2km to 10km, giving equally wide guesses for fragment size. Hubble imaging was used to give an approximate upper bound of 3-4km diameter for the largest fragments.

Thanks Pete, yep I suppose the size of the fragments that hit Jupiter would be all important.


An interesting first hand experience was the Challenger disaster....

certainly bodies were recovered, gloves and various pieces of wreckage... not all charred.....???

I believe it has something to do with e=mc^2. I'm sure some of the physics guys here can explain exactly how.

As Erburacum45 said, the formula you need is for kinetic energy:
E = (mv^2)/2

m (mass) is in kilograms, v (velocity) in metres per second.

At the moment of impact (or atmospheric breakup), most of a projectile's kinetic energy is converted into light and heat. I can't pull exact figures off the top of my head, but at the kind of speed impactors travel - up to 70 times that of sound - the energy released per kilogram of mass is over ten times the energy released by a kilogram of high-grade conventional explosives. Needless to say, a ball of rock & ice 100 metres across would weigh tens of thousands of tons; your explosive force would be greater than most nuclear weapons.

Thanks Pete, yep I suppose the size of the fragments that hit Jupiter would be all important.


An interesting first hand experience was the Challenger disaster....

certainly bodies were recovered, gloves and various pieces of wreckage... not all charred.....???

I believe the velocity and structure of Challenger was significantly different to a stony or icy asteroid.

More importantly, the Challenger fragments (and corpses) weren't travelling as asteroidal or cometary speeds when they made their unscheduled return to Earth.

More importantly, the Challenger fragments (corpses) didn't care how fast they were travelling when they made their unscheduled return to Earth............The Challenger is so far removed from being classified as an asteroid or comet, that it isn't even funny............

Atta Boy

Anyone seen the Discovery Channel's 1997 documentary "Three Minutes to Impact"? That covered much of the theory relevant to this topic, and caught the wave of impact hysteria which dominated popular science in the late 1990s.

Today, it's GM foods people worry about. In the 1980s, it was AIDS. What's next?

More importantly, the Challenger fragments (and corpses) weren't travelling as asteroidal or cometary speeds when they made their unscheduled return to Earth.

I really don't think u could call them corpses now, could u? :rolleyes:

Certainly othe parts of the re-entry vehicle were "heated" by re-entry.

BY the way, someone mentioned Tunguska. Has there been any exact explanation on how the explosion occured?

The best guess is that it was a tiny cometary fragment; being water ice, such a fragment would leave little residue for the investigating scientists to find.

---------------------
SF worldbuilding at
http://www.orionsarm.com

>> being water ice

More like solid hydrogen peroxide.

LoL!
You just can't let that one go, can you!