Are meteorites hot or cold when they hit Earth?
Movies always depict meteors as flaming balls of fire, streaking across the sky, and igniting anything they touch after they reach the earth...Is this true? I know that they get hot as they enter our atmosphere, but I also know that it is pretty cold in space, so they start out with quite a chill. Do meteors really get hot enough to keep them flaming all the way to the ground? If a meteor fell at my feet, could I touch it? Would it be cold or hot?
This is a good question, and one that we really don't have a great answer for. It's true that the chunks of rock and/or ice that form meteorites have been travelling through space for at least millions of years, and are therefore very cold when they begin their descent through our atmosphere. As they hit the atmosphere, the outside of the rock begins to heat up (forming the "fusion crust" on the meteorite). The hot outside begins to ablate (or be stripped off), which removes heat. The meteorite falls through the atmosphere in seconds, so for larger rocks, only the outside part has time to be heated. So the question is: When they hit, are they still cold because the hot parts of the rock were removed via ablation, or does the outside manage to get hot enough to burn things?
Unfortunately, there really aren't very many meteors that are picked up directly after they've fallen, so it's hard to do good statistics on which ones are hot or cold. So far it seems that some of each have been found. For example, this FAQ lists reports of meteorites (compiled by Don Blakeslee of Wichita State University) that have been touched soon after they fell, and some people reported that the rock was hot, some that it was warm, and some that there was frost on the outside! These reports are all of a qualitative nature, usually based on the testimony of a small number of people.
We certainly don't have big fires starting when meteorites hit Earth, so although they may singe grass or burn someone, they definitly don't hit the ground as a flaming fireball, the way you sometimes see it depicted in movies.
Many astronomers believe that small rocks hitting the ground should not be hot. In a Science@NASA article about the recent fireball over Pennsylvania, written by Tony Phillips, the planetary scientist Don Yeomans is quoted as saying,"Rocky asteroids are poor conductors of heat. Their central regions remain cool even as the hot outer layers are ablated away... Small rocky meteorites found immediately after landing will not be hot to the touch."
In their meteorite FAQ, the American Meteor Society says "The ablation process, which occurs over the majority of the meteorite's path, is a very efficient heat removal method, and was effectively copied for use during the early manned space flights for re-entry into the atmosphere. During the final free-fall portion of their flight, meteorites undergo very little frictional heating, and probably reach the ground at only slightly above ambient temperature." However, they point out that there really aren't many reports, and those we have are often "prone to hearsay".
So, in summary, we don't really know what temperature meteorites are when they fall. The problem is that there really isn't much quantitative data to base an answer on! However, many astronomers believe that small meteorites should be barely warm, or even cool when they hit the ground. The temperature probably varies depending on the size and composition of the original rock. For example, some materials might ablate more efficiently than others, or conduct the heat better. It's an interesting question, though, and one I wish we had a better answer to!
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