The edge of the atmosphere

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The boundary isn't a sharp distinction...the atmosphere continues to get thinner and thinner...

"Space" is generally recognized as altitude above 100KM...or roughly 300,000 feet...

The air is extremely thin up there. Humans generally can't survive long term above about 20,000 feet (they can with acclimatization, Look at mountain climbers, but lose the cabin pressure in an airplane above that, and you won't last long).

And it's really, really cold...then even though it gets really hot, it's still really, really cold...

http://en.wikipedia.org/wiki/K%C3%A1rm%C3%A1n_line
 
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I thought that outer space was considered to be 50 miles altitude.

edit: OK, 50 miles is the Air Force's definition.

I like the Karmann criteria because it is based on an actual physical law, but it is very aircraft-centric.
 
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I think that's the USAF definition...where you get "Astronaut" wings...

But any definition is simply an arbitrary determination of where to draw the line...the air just keeps getting thinner..
 
Originally Posted By: mjoekingz28
Can anyone explain this?

What conditions are present when leaving Earth's atmosphere and going into space AND are the conditions equal upon re-entry?


Further, the conditions of the atmosphere are the same (temperature, pressure, etc.) but the conditions for the vehicle are far, far different because of the physics.

To orbit the Earth, you're going about 18,000 miles per hour. A rocket going up gets well out of the atmosphere before exceeding a few thousand miles per hour.

But upon re-entry, it's going close to 18,000 miles per hour and using only friction with the air to slow down, so, the speed in the atmosphere is far higher, meaning that the vehicle heat load is far, far greater than it is when leaving...just look at meteors! Solid lumps of metal, primarily iron, simply burn up when entering the atmosphere.

That's why a thin aluminum skin can survive the launch, but a really heavy, often ablative, heat shield is required for re-entry...
 
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Originally Posted By: Kira
Thank you for putting the word ablative into my vocabulary. Kira


In Latin, it was a specific case of a noun, but it has another English meaning..involving ablation...

11.gif
 
Originally Posted By: Kira
Thank you for putting the word ablative into my vocabulary. Kira


It was a cromulent opportunity.
 
Originally Posted By: eljefino

It was a cromulent opportunity.


I had to Google that one!

And now, I know why I've never heard of it before...but like Lisa, I am a bit of an iconoclast...
 
Originally Posted By: Astro14
The boundary isn't a sharp distinction...the atmosphere continues to get thinner and thinner...

"Space" is generally recognized as altitude above 100KM...or roughly 300,000 feet...

The air is extremely thin up there. Humans generally can't survive long term above about 20,000 feet (they can with acclimatization, Look at mountain climbers, but lose the cabin pressure in an airplane above that, and you won't last long).

And it's really, really cold...then even though it gets really hot, it's still really, really cold... As Helios airline found out the fatal way about ten years ago. Every one on board but two died from hypoxia. The crew was not familiar enough with the 737 they were flying to correct a problem with the ECS. Two flight attendants tried to get the plane down but it ran out of fuel and crashed into a mountain.

http://en.wikipedia.org/wiki/K%C3%A1rm%C3%A1n_line
 
Originally Posted By: Astro14
Originally Posted By: Kira
Thank you for putting the word ablative into my vocabulary. Kira


In Latin, it was a specific case of a noun, but it has another English meaning..involving ablation...

11.gif

As I recall, Air & Space had an article which described the original 7 astronauts less than enthusiastic
reaction to being told by an egghead how the "heat shield" worked.
 
Originally Posted By: Astro14
Originally Posted By: Kira
Thank you for putting the word ablative into my vocabulary. Kira


In Latin, it was a specific case of a noun, but it has another English meaning..involving ablation...

11.gif



small Difference in pronuciation (at least in the HS Latin I took..) Ablative(as in Case) is a short A sound between the l&t, while this meaning is pronounced with a long A sound in that spot.

I first ran across this sense of Ablative, while Watching StarTrek Deep Space Nine, when the first Got the USS Defiant. it had a Cloaking Device, but no Shields. instead it had Ablative armor over the Hull.
 
+1 to Astro14.

Somewhere there is a movie of a shuttle launch that continues into orbit and shows velocity, altitude, lat., long., time elapsed, etc., through MECO (main engine cut off). Quite fascinating to watch. Velocity continues to increase as the drag drops off.

Most of the meteors you see at night are about the size of a grain of sand. Hardly any mass at all. But they're going in excess of Mach 20, depending on many factors. Some much faster. The trail you see isn't due to the grain of sand burning, it's due to the air ahead of it being heated extremely hot because the air molecules can't get out of the way fast enough. Many survive reentry, all depends on what they're made of, their reentry trajectory and others. Every year, the Earth gains lots of tonnage of mass from these objects. Of course, the vast majority of it falls into the oceans due to their ~ 70% coverage of the Earth's surface.
 
How many tons of mass does Earth lose each year from the loss of high altitude volatiles into space?
 
Originally Posted By: fdcg27
How many tons of mass does Earth lose each year from the loss of high altitude volatiles into space?

?????? Excuse me?
 
Look into high altitude manned balloons in the 1930s . The first to visit the edge of space did it in a balloon gondola .
 
If it wasn't for Thorium decay in the core, we wouldn't be here....

The heat of radioactive decay has kept the core liquid, which provides the magnetic field that shields us from the worst of space weather...things like solar wind, with the occasional CME...
 
I am always amazed at our tiny little planet hurtling through space with its micro thin layer of atmosphere barely held in place.

We are quite lucky...
 
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