Rain of Iron and Ice #3

How meteorite impacts work:

This section begins by differentiating meteors from meteorites.  If a meteor doesn't complete its fall to Earth, if it burns up on the way, it remains a meteor.  If a meteor falls from the sky and hits the Earth it becomes a meteorite. 

As a meteor enters the Earth's atmosphere it begins to be ablated (the name for the process in which a projectile melts and vaporizes) and the rate at which this occurs is based on the density of the air in which the meteor is traveling (which doubles every 5K) and how fast the object is going.  This means that a faster moving object will penetrate to a lesser depth into the Earth's atmosphere than a slow moving object. 

The book brings up that since asteroids and meteorites regularly impact the Earth or other planets, if they weren't replenished there would be no more and meteor showers would stop.  However this isn't the case so scientists came up with some hypotheses: 1, that Jupiter's gravity acts on harmonically orbiting asteroids slowly pulling their orbit away from normal sending them on tracks through the inner solar system or 2, that comets with long orbits are disturbed by Jupiter's gravity and sent on shorter orbits through the central solar system.  Astronomers then calculated the approximate number of asteroids and meteors that are created and how many each hypothesis would produce and the numbers suggest that both hypotheses are correct, about 50% of newly created Near Earth Asteroids come from bodies dislodged from the Asteroid Belt by Jupiter and the other 50% come from comets that Jupiter's gravity effects in such a way to have them orbit by the Earth too.  This is relevant because comets and asteroids have different chemical makeups and so they'll do different things when they come into contact with the Earth's atmosphere. 

Not all impacts leave lasting evidence.  Since they are random the vast majority will land in the ocean and disappear without a trace.  Additionally events like the Tunguska meteorite occur with unknown frequency.  The meteorite exploded with great force, but because it didn't create a crater the evidence of felled trees would only last 100 years or so.  Despite not impacting the Earth the Tunguska meteorite could still have caused plenty of damage if it had come only minutes or hours later and exploded over a populated city.  The only evidence that will remain of the Tunguska event will be small spheres of magnetite which no one will be looking for because there isn't a crater.

Here is what happens when there is an impact explosion: the kinetic energy of the object is almost immediately converted into heat which produces a small, exceedingly hot fireball of vast energy and pressure.  The fireball briefly reaches temperatures of several hundreds of thousands of degrees and the energy released is significantly greater than that of the Sun before expanding faster than the speed of sound and producing high amounts of x-rays.  The fireball will continue to expand until its pressure is equal to that of the surrounding atmosphere.  At this point the fireball, which is still ridiculously hot, will begin to rise from the burning remnants of whatever was on the ground and allowing the relatively cold surrounding air to rush in.  These winds can reach hurricane speeds and serve to fan the blaze around ground zero.  The fireball may release so much energy that surfaces illuminated by it can spontaneously burst into flame while anything within the range of the fireball is vaporized (this can occur even with relatively small explosions like the one in Halifax, Nova Scotia on December 6th 1917 during which many people were never found having been consumed by the explosion).  Due to the forces involved the volume of matter excavated by the explosion will usually be about one hundred times greater than that of the impacting body (Lewis 59).  The rapid excavation creates a shock wave both in the air and in the ground causing buildings several kilometers away from the explosion to have the ground beneath them pushed outwards.  The glass in these buildings will shatter and then accelerate to near the speed of sound becoming deadly projectiles directed at the people who may be unfortunate enough to be around (or in the case of the recent Russian meteor, the people who went to their windows to see what had happened).  Aerial explosions typically effect a larger area less severely and impacts or explosions in water will have a similar effect except that the water will rush in to fill the gap left by the explosion, rising up in the center before splashing down again (this process can repeat several times).  Such an initial explosion and the splashing that follows will often create tidal waves or tsunamis that will devastate shorelines nearby.

This is the kind of thing that some sort of prevention strategy would try to... well... prevent, and why shooting a rocket at an asteroid is usually a bad idea.  If a rocket were to hit an asteroid and blow it up, usually the asteroid would simply be in many smaller pieces and these pieces could each go through the process described above.  Fun stuff.