Re: Earth-grazing asteroids as a military resource

On 12/03/2025 12:47 am, Martin Brown wrote:

On 11/03/2025 13:18, Bill Sloman wrote:

On 11/03/2025 11:43 pm, Carlos E.R. wrote:

On 2025-03-10 15:28, Bill Sloman wrote:

 

The historical record - in terms of meteor craters big enough to have survived for a few million years - demonstrates that big earth grazing asteroids are pretty rare. I imagine that somebody has worked out what the distribution is, at least roughly.

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There is evidence of dangerous "objects" hitting the earth and causing destruction in the "historic" age.
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Example: https://en.wikipedia.org/wiki/Tunguska_event
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We were just fortunate that it hit a non populated area, otherwise it could have destroyed a city. The explosion was between 3 and 50 megatons.

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You really are a twit. If you had bothered to read all the way through my post, you would have found exactly the same url (so it shows up twice in your post, which is a touch comical).
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And the object didn't explode - it just came apart. Lots of very fast moving, very hot rocks rocks (it does seem to have a stony asteroid, which is presumably why it didn't make all the way down to the ground) would have produced a huge shock wave, so it might as well have exploded, but calling it an explosion implies that the energy emerged suddenly, rather than just coupling into the atmosphere when the air got dense enough to have a significant interaction with the fast moving rock.

 The exact dynamics for Tunguska are still a bit unclear but assuming it was a typical rock ice composite material then it probably did to a very good approximation explode once the hypersonic shockwave from impacting the denser atmosphere exceeded the binding forces holding it together. Most sources describe it as an explosion at about 6 miles altitude.
 https://www.nasa.gov/history/115-years-ago-the-tunguska-asteroid-impact-event/
 No pieces of it have ever been identified as reaching the ground.
It is assumed that most of it vapourised.
 Finding meteorites is a lot easier in Antarctica than on Arctic tundra.

The surface of rocky asteroid falling through the atmosphere will get very hot, but it doesn't spend much time in the atmosphere so the core of the asteroid won't.
The surface shell will expand and peel back - and with a rock ice composite some of the superficial water will turn into high pressure steam and expand any crevices it can get into.
You've got to think about a progressive ex-foliation. At some point the high pressure steam may get to the core of the asteroid, and if that happens before it hits the ground the core - which will still be moving very fast - has a chance to get hot as well.
Reducing this down to an explosion at a single point is an over-simplification, if handy one.
--
Bill Sloman, Sydney