David Hudson asked:
Why do things burn up on entering Earth's atmosphere?
Dave - The main reason why things heat up when they hit the Earth’s atmosphere is they've got huge amounts of kinetic energy - they're going incredibly fast.
When they bash into the Earth’s atmosphere, most of the heating is actually because the air they bash into hasn’t got time to get out of the way, so the air gets compressed; and when you compress air, it gets hotter. You may have noticed this if you've ever pumped up a bicycle tyre very, very quickly: the end of the pump gets hot.
So, the air in front of the inbound object - such as a meteor or even an asteroid - heats up, and that starts to erode the surface of the object and you get this tail of hot, burning material, which you see as a shooting star.
With very small things, because the friction is so much larger compared to their mass, they tend to lose their speed much more gently very high up in the atmosphere, so they slow down more gently and don’t get as hot. And once they slow down enough, they just drift down like dust through the atmosphere.
So, it is conceivable that something like bacteria clinging to a small dust grain could survive re-entry from space, whereas a big lump of rock would melt very quickly.
It's also to consider that the faster a gas is going the more kinetic energy it has and increase in kinetic energy means an increase in temperature Pmb, Sun, 20th Oct 2013
Regarding the two explanations given above, they are completely equivalent. One is based on a microscopic level and the other on a macroscopic level.
It doesn't matter what I understood.
"Of course it does. You confused ram pressure with fluid pressure in this particular case."
By the way. For those of you who have a problem with using the term "temperature" other than as the scalar (i.e. Cartesian scalar/invariant) that one uses in thermodynamics, i.e. as in reentry temperatures I strongly suggest that you not get bogged down with concerns about who used a word correctly or not and train your thoughts back to the real question, i.e. Why do objects burn up on entering the Earth's atmosphere?