There seems to be a perception in the community that the Earth would somehow be better off being struck by an icy comet rather than by a similar sized rocky or metal object. This is, generally, not the case. Let’s take a look at why not.
For the purpose of our discussion, we are going to assume that a one cubic mile object is about to strike the Earth traveling at the relatively slow speed of the Persid Meteor Shower. That’s about 33,000 miles per hour. An object coming directly from deep space or a comet approaching the sun could go much faster. I once got stuck for four hours in a traffic jam outside the baseball stadium in St. Louis and missed a very good concert so we’ll assume the stadium is ground zero.
The common argument is that an icy object would be less dense and so would cause less damage. On a global scale, there would be less total energy in the impact and slightly less particulate matter in the atmosphere and ejected back into orbit after the event but only slightly less and I’m afraid it would make no difference what-so-ever to our doomed baseball fans.
Here’s the problem. At a velocity of 33,000 miles per hour. When the object begins to be warmed and slowed by the atmosphere, say 100 miles up give or take, it is only 10.8 seconds from impact. This means that the entire column of air underneath it only has about 11 seconds to leave the area. Or, put another way, the air between the Earth and the object would have to move at 33,000 miles an hour to get out of the way.
Super computer modeling shows that this is not what happens. What happens is that the leading edge of the object, regardless of material, superheats into a gas or plasma, the air superheats into a gas or plasma, the stadium superheats into a gas or plasma, the upper level of the earth superheat. As for the fans, well, go ahead and put the onions on your hot dog, you’ll never live to regret it.
Once things have gotten hot enough to to turn concrete dugouts and Sy Young award winners into plasma, the issue of rocky vs. icy is only going to be interesting only to scientist in bunkers on other continents who are wondering when it’s going to stop snowing mud.