Meteor propter hoc

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Dec. 14 2008 6:09 PM

Meteor propter hoc

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In Auckland, New Zealand recently a warehouse was set ablaze. It was quite the inferno, needing a huge effort to quell it.

Phil Plait Phil Plait

Phil Plait writes Slate’s Bad Astronomy blog and is an astronomer, public speaker, science evangelizer, and author of Death From the Skies!  

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No one knows what started it... but rumors are spreading that it was a meteorite that did the damage. Several people saw a fireball in the sky, and it happened around 10:00, around the time the fire started.

Case closed, right?

Photoshop of a meteor over Auckland fire

Bzzzzt. Nope. I will almost guarantee a meteorite did not start this fire! Why not?

Meteors are chunks of rock or debris that enter the Earth's atmosphere. They violently compress the air, heating it up -- it's not friction that does the heating, contrary to common belief. But common wisdom also says that meteorites would hit the ground still burning hot, and cause fires wherever they land. And after all, we've seen it in countless movies!

However, there's a piece of info you should remember here: that's in the movies. In real life, meteorites don't work that way. A small meteoroid (the solid part of the glowing meteor) will burn up rapidly, leaving nothing to hit the ground. If it's somewhat bigger, like the size of a car, it'll explode high in the atmosphere, and then pieces of it will rain down. However, those smaller pieces fall relatively slowly, and have plenty of time to cool down before they hit. The recent fireball over Canada shows that, as did a rain of meteorites that hit Chicago a few years ago did too. No fires were caused by those rains of rocks from space, because they were cold when they hit.

A piece of rock or metal large enough to retain its heat when it impacts the ground would be pretty big, like over 100 meters across. Those tend to be a bit more obvious when they impact, since they explode with a yield equal to that of a 15 megaton blast.

That might do a bit more damage than start a warehouse fire. Had something like that been the cause of the Auckland warehouse fire, there wouldn't be anyone left in the city to report it. There would be a smoking hole a mile across.

I've heard reports like this one many times. They always -- always -- turn out to be non-extraterrestrial in origin. Just because a bright meteor was seen does not mean it caused the fire! That's a logical fallacy: post hoc ergo propter hoc, "after this therefore because of this." There's a reason that's called a fallacy.

So that's why I think a meteorite didn't cause this fire, and I'm pretty sure there's a more down-to-Earth explanation here... at least one that started off down-to-Earth and didn't just end up that way.

Tip o' the Whipple Shield to BABloggee Bret Hall.

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