Science Questions

Why burn up on entering Earth's atmosphere?

Sun, 10th Apr 2011

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Question

David Hudson asked:

Why do things burn up on entering Earth's atmosphere?

Answer

Dave -   The main reason why things heat up when they hit the Earth’s atmosphere is they've The Leonid meteor shower 1833got 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.

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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


Not really.
If I put a gas cylinder in a car and drive it around it doesn't get hotter.
Bored chemist, Sun, 20th Oct 2013


You misunderstood what I was referring to. When you hit the atmoshere with an initial speed of the orbital speeds then the kinetic energy of the gas relative to the object is huge. Since the temperature of a gas is related to the kinetic energy of the particles the faster the gas hitting you the hotter it will be.

The rms speed v of a molecule of mass m in gas of temperature T are related by the expression

v = sqrt(3kbT/m)

For nitrogen at room temperature v = 509 m/s. However consider the speed at which nitrogen molecules hit the space shuttle whose orbital speed was 7730 m/s. Imagine what the equivalent temperature is! Pmb, Mon, 21st 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.
Pmb, Tue, 22nd Oct 2013

It doesn't matter what I understood.
What you said was "increase in kinetic energy means an increase in temperature" which is plainly wrong, as I showed with the example of a gas cylinder in a car.
Bored chemist, Tue, 22nd Oct 2013


Of course it does. You confused ram pressure with fluid pressure in this particular case.

I.e. you mistook what speed and what temperature I'm referring to. You're referring to the temperature of a fluid type of gas (a gas being a type of a fluid). I'm referring to the increase in temperature caused by the bulk motion of the gas. I.e. I'm speaking of ram pressure and you're thinking of fluid pressure. If you calculate a rough temperature from the space shuttle reentry speed you'll find an impressive temperature from such ram pressure.

I'm not guessing at any of this by the way. One of my good friends is an astrophysicist at MIT's center for astrophysics (that’s it’s old name). When I make a statement of certain things outside my area of expertise I make sure to double-check myself with an expert in the field (I know a lot of experts in various fields of physics). In this case I double-checked with my MIT-astrophysicist friend.

The faster gas particles hit an object the hotter the object gets. That’s it, plain and simple from a microscopic point of view. You're a very sharp person so I'm surprised you didn't follow what I was saying. I think it has to do with the definition of temperature. Pmb, Tue, 22nd Oct 2013

"Of course it does. You confused ram pressure with fluid pressure in this particular case."
No, I didn't mention pressure at all. However it seems you are muddling the PV energy into this somehow.

What you said was "increase in kinetic energy means an increase in temperature" which is plainly wrong, as I showed with the example of a gas cylinder in a car.
The gas has more kinetic energy but it is not hotter.

And it can't possibly matter who your friends are- that's a variation on the theme of this logical fallacy.
http://en.wikipedia.org/wiki/Argument_from_authority

It's not that I didn't follow what you were saying, it's that you didn't say what you thought you did.
Seriously.
Do you accept that a kilogram of gas in a cylinder at 10 metres per second (WRT some observer) has a kinetic energy of 50 Joules (from that observers POV)?

Do you accept that it is at the same temperature as it was before it was accelerated to that speed?

Do you accept that this contradicts your earlier assertion that "increase in kinetic energy means an increase in temperature"?
Bored chemist, Tue, 22nd Oct 2013


Nobody suggested that you did mention it. I was referring to the concept of ram vs fluid pressure. This is what we’re dealing with here.


Nope.


Nope. If you insist on taking this in a vacuum and ignore what I’ve explained later then I don’t see the point in going over this again.


Please go to  http://en.wikipedia.org/wiki/Atmospheric_entry and learn what is meant by reentry temperatures please.


That is quite incorrect. No such fallacy that you suggest/imply actually exists. I see that you failed to note that I’m not basing my article on authority. That in itself is a logical error on your part. I quite clearly and explicitly stated that I merely confirmed what I know to be the case with my friend.


I’m very surprised you referred to this as such. You lost one “sharp point” when you pointed to this as if referring to an authority was a logical fallacy. :)    Please go back and read that article more closely for the article you referred to reads


Authority is an important source of knowledge. The next time you’re in a library you should see if they have a text in logic. If they have Practical Logic: An Antidote for Uncritical Thinking – Fifth Edition by Douglas J. Soccio and Vincent E. Barry. In particular please turn to page 54 and read the section labeled Authority. In particular



Uh oh. This is a red flag and I avoid them like the plague. I find comments like “Seriously” to be a tad condescending so I avoid them at all costs. With that note this ends my part in this thread. You’re not much in a listening frame of mind anyway.

One last thing before I go - Please learn more about what scientists mean when they speak of the temperature of reentry. You’re confusing it with they typical thermodynamic definition of temperature and as I keep trying to explain there is another sense that temperature finds use in, i.e. temperature of reentry.  It’s not the same thing as the temperature of an object which is a scalar and thus independent of ones frame of reference. I explained above but you keep failing to get this all too important point, i.e. there are different meanings of the usage of the term ‘temperature” here.
Pmb, Tue, 22nd Oct 2013

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?

As I've explained above the kinetic energy of the atmosphere's gas molecules hitting the reentry vehicle is what causes it to heat up so much. Energy is being transfered from the atmosphere's gas molecules to the molecules that the reentry vehicle are constructed of. Pmb, Wed, 23rd Oct 2013


Not really.
If I put a gas cylinder in a car and drive it around it doesn't get hotter.


Because the gas in your cylinder is not traveling to the carburetor/F.I./intake fast enough to cause heat...it's an entirely different subject than a solid object going into the earths atmosphere...geeze

Why even ask this question...my 16 year old knows the answer...friction...any friction will cause heat...the more friction/speed, the more heat. Gas going through the lines, fuel pump and into the intake is only around 3, 4, 5 lbs of pressure..unless it's a dragster lol. scienceofscience, Sat, 26th Oct 2013


Not really.
If I put a gas cylinder in a car and drive it around it doesn't get hotter.


Because the gas in your cylinder is not traveling to the carburetor/F.I./intake fast enough to cause heat...it's an entirely different subject than a solid object going into the earths atmosphere...geeze

Why even ask this question...my 16 year old knows the answer...friction...any friction will cause heat...the more friction/speed, the more heat. Gas going through the lines, fuel pump and into the intake is only around 3, 4, 5 lbs of pressure..unless it's a dragster lol.


You may be missing the rather subtle repartee between Pete and Bored Chemist. Anyhoo, I believe the heat associated with the meteor is more accurately described by adiabatic compression, not friction. We use the fire piston as a demonstration at our museum:


A small piece of combustible material is placed at the bottom of the cylinder, rapid compression of the air increases the temperature and the material ignites. The ideal gas equation PV=nRT gives a pretty good discription of the process.

http://www.kansasmeteorite.com/Meteor/cosmic.mpg (right click and open in new tab to see it in action)

disclaimer: as with anything on the internet or other media, verify the information for yourself if it is important to you. distimpson, Sat, 26th Oct 2013


Nobody suggested that you did mention it. I was referring to the concept of ram vs fluid pressure. This is what we’re dealing with here.


Nope.


Nope. If you insist on taking this in a vacuum and ignore what I’ve explained later then I don’t see the point in going over this again.


Please go to  http://en.wikipedia.org/wiki/Atmospheric_entry and learn what is meant by reentry temperatures please.


That is quite incorrect. No such fallacy that you suggest/imply actually exists. I see that you failed to note that I’m not basing my article on authority. That in itself is a logical error on your part. I quite clearly and explicitly stated that I merely confirmed what I know to be the case with my friend.


I’m very surprised you referred to this as such. You lost one “sharp point” when you pointed to this as if referring to an authority was a logical fallacy. :)    Please go back and read that article more closely for the article you referred to reads


Authority is an important source of knowledge. The next time you’re in a library you should see if they have a text in logic. If they have Practical Logic: An Antidote for Uncritical Thinking – Fifth Edition by Douglas J. Soccio and Vincent E. Barry. In particular please turn to page 54 and read the section labeled Authority. In particular



Uh oh. This is a red flag and I avoid them like the plague. I find comments like “Seriously” to be a tad condescending so I avoid them at all costs. With that note this ends my part in this thread. You’re not much in a listening frame of mind anyway.

One last thing before I go - Please learn more about what scientists mean when they speak of the temperature of reentry. You’re confusing it with they typical thermodynamic definition of temperature and as I keep trying to explain there is another sense that temperature finds use in, i.e. temperature of reentry.  It’s not the same thing as the temperature of an object which is a scalar and thus independent of ones frame of reference. I explained above but you keep failing to get this all too important point, i.e. there are different meanings of the usage of the term ‘temperature” here.


You said it  "in a vacuum"
There's nothing in the original post to clarify it

"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"
Explaining later that you didn't actually mean what you wrote doesn't make what you wrote correct.

And, if it makes a difference who you friends are then I must be right because my mum taught one of the Spice girls.

And if you are prepared to base your opinion on the use of one word like "seriously" then you seriously need to rethink that.
It may well be condescending- but that's not a problem.
Condescending language is designed for talking down to people who have got stuff wrong.
Now I'm going to drive really fast past some gas bottles and watch them burst due to the raised temperature. (relative to me they will have a high kinetic energy and, according to you, that makes them hot).


BTW, you have falsely attributed this to me "Although certain classes of argument from authority can constitute strong inductive arguments, the appeal to authority is often applied fallaciously"
Whether it's true or not isn't important: I didn't say it.

Science of science,
You missed the point, the gas isn't being used as fuel, it's just being transported. It gets kinetic energy- but it doesn't get hotter- which (as I'm sure you will agree)  contradicts PMB's rather absurd assertion that "increase in kinetic energy means an increase in temperature"

And the answer isn't friction. Some of it is, but most of it is due to adiabatic compression as DTStimpson has pointed out.
(It's the same process that heats the air to ignite the fuel in a diesel engine) Bored chemist, Sat, 26th Oct 2013

Back in the depths of ancient history, what you said was this
"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"
And it's still wong.

Still, at least I agree with you about this bit
"I just realize what the problem I had communicating what I was saying. When I said temperature it was incorrect."

And, just to clarify the point, if you put a thermometer in a gas cylinder and drive it around, it still doesn't get hotter just because it's moving.


What I think you still need to work on is this bit
"When that's done then when the increase in kinetic energy due to the increase in the bulk motion of the thermometer  "
Relative to the gas and the cylinder the thermometer shouldn't be in motion.
There should be no "bulk motion" of the thermometer. Bored chemist, Sun, 3rd Aug 2014



Plain nonsense.

There was even such experiment - shoot objects (hundred or thousands times) to container with liquid, and measure increase of temperature of liquid.
Kinetic energy of moving object was converting to higher temperature of liquid.

Gas has >1000 times smaller molecules density than liquid/solid matter. So increase of temperature will be much smaller, but it doesn't mean there will be none.
UltimateTheory, Sun, 3rd Aug 2014

Having read this thread with interest, I suspect that if anyone asks me this question I shall say "friction".  Naive? Perhaps, but it may be what the person is looking for.

This reminds me of an incident, many years ago, when our daughter was at school, she asked her mother a question about geology. 

Mother: "Why don't you ask Dad that; he knows more about it than I do."

Daughter: "Yes, but Dad does go no a bit." Bill S, Mon, 4th Aug 2014

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