When Meteors StrikeThere seems to be this perception that the Earth would have less damage if struck by an icy comet rather than a rocky meteor of the same size.  With the number of movies and documentaries that show objects entering the atmosphere and super-heating with friction, I can see where this idea can come from.  Unfortunately this is typically not the case.

I was recently in a lovely debate over IRC on this subject with some college students that believed an object of less than one cubic kilometer made of mostly ice would disintegrate to such a degree that only a small crater would be created on the Earth, whereas a solid object of the same size would cause enough damage to wipe out half of Canada.  While I typically avoid heated discussions over IRC and similar forums, I was compelled to give these students a little physics lesson.

For the purposes of this discussion, let’s assume that a one cubic kilometer-sized comet was on a direct collision course with the Earth.  Let’s also say this comet was traveling at the same speed as the Perseid objects, which is a relatively slow 52,800 km/h.  Objects coming from the far reaches of our solar system or beyond could go much faster.  Since I just moved from Vancouver and everyone there is impatiently waiting for the “next big quake”, let’s use that city as ground-zero.

The common perception is that an comet or meteor made of mostly ice would be less dense, and therefore create less damage.  On a global scale, this is true.  There would be less total energy in the impact and slightly less dust and other particulate matter in the atmosphere, as well as less matter ejected back into orbit after the event.  However, the difference would be so slight that it wouldn’t make one bit of difference for the unfortunate residents of Vancouver and everyone within a radius of several thousand kilometers.

The problem is quite simple … we simply do not have enough atmosphere.  When the object begins to slow and heat up due to friction with the Earth’s atmosphere (let’s say, in the lower regions of the Thermosphere – about 100 kilometers up), it’s only 10.8 seconds away from impact.  Remember, the object’s initial speed relative the Earth is 52,800 km/h.  This means that the entire column of air underneath the object has less than eleven seconds to leave the area.  To put this another way, the air between the Earth and the meteor/comet would have to move at 52,800 km/h to get out of the way.

Supercomputer models of this scenario shows us that air does not get out of the way for objects at this speed.  Instead, the leading edge of the meteor, regardless of its composition, super heats into a gas or plasma.  The air underneath also super heats into gas or plasma.  The city of Vancouver would also super heat into gas or plasma.  The upper atmosphere also super heats, as heat rises and air is pretty good at thermal distribution.  What about the people in those cities?  Well … hopefully they wouldn’t feel anything past the initial boom of impact.

Once things have become hot enough to turn concrete, steel and water into plasma, the issue of rocky objects vs. icy is only going to be interesting to scientists hiding in bunkers in Europe, Asia and Africa while they wait for the skies to stop snowing mud.