Question about leaving and entering our atmosphere?

Question

I know that when something enters our atmosphere there is all the heat and burning coming in. Like comets etc that usually burn up coming in or space ships coming back in have to have heat shields etc. I have never heard of this happening when there are rockets or space ships leaving the atmosphere. Is there that same burn when leaving earth as there is when coming back in? Or is it only when an object is coming into our atmosphere that it is burned up? If there is no burn going out why not?

Answer 1

Dear MJGC...

Good question... The answer lies in "the way" objects come into our atmosphere and leave it.

Objects entering our atmosphere are traveling very fast - something like 20,000 miles per hour or faster. Hitting the fringes of our atmosphere they begin to generate heat from friction with the few gas molecules in the thin atmosphere up at high altitudes. Falling lower, they encounter more and more molecules of gases, making more, and more heat.
And, as you stated, the objects most often burn up from all that heat.

Objects leaving our atmosphere from the surface of the Earth start out at velocity = Zero Mph. They accelerate up through the dense atmosphere and begin to reach high velocity only at high altitudes. So, while heating does occur, it does not occur in the same way. Also, second stage propulsion units kick in after about 2 minutes and further lift the space vehicle (rocket, or whatever) even higher up into the very thin atmosphere as the rocket ship accelerates. So the highest velocities are encountered when the atmosphere is the thinnest.

The angle of departure also is larger than the angle of penetration on descent. This means that objects leaving the atmosphere are subjected to this heating effect for a shorter period than those entering and being slowed down by friction with the atmosphere alone.

A lot of the re-entry heat generation could be eliminated if the re-entering rockets had sufficient fuel on board to operate "Retro-Rockets" for a short time and lower their velocity. Sadly they do not have this extra fuel. So, they must re-enter at a shallow angle and let the air of Earth's atmosphere slow them down. And there is your heating problem...

Answer 2

It's all about Kinetic Energy.

An object in orbit has A LOT of kinetic energy.

Kinetic energy is expressed by the equation KE = 1/2 * mv^2.

KE is the kinetic energy in Joules.
m is the mass of the object in kilograms.
v is the velocity in meters/second.

As I said above, an object in Earth orbit has A LOT of kinetic energy. I'll let you plug in the numbers for a 5,000-kilogram spacecraft travelling at a speed of 23,000,000 meters per second (orbital speed). You will see that the KE is some ridiculously huge number.

To slow that spacecraft from orbit down to zero speed on the ground (0 kinetic energy), you need to bleed off all that kinetic energy. The intense amount of heat given off due to air friction during re-entry does that-- You are using the atmosphere as a brake, putting as much un-aerodynamic surface area as possible against it (the heat shield of the spacecraft).

Going into space on the other hand means you have to ADD kinetic energy. Going up, you are trying to minimize friction with the air, which means you use a pointy rocket with efficient aerodynamics, so most of the rocket thrust is being used to generate kinetic energy and not dissipated as air friction.

Hope this helps.

Answer 3

The heat on reentry is caused by friction with the atmosphere. Meteors are traveling very fast--often over 100,000 mph. The shuttle isn't going that fast--but it is traveling 18,000 mph.

That much speed means that the friction between the objectt and the air is extreme-and so a LOT of heat is generated.

But, when a rocket leaves the atmosphere, it starts off at zero speed--and by the time they accelerate to those really high speeds, they're already above the atmosphere. They do heat up some--but not like they do re-entering theatmosphere at those much higher speeds.

Answer 4

You have a decent question; air friction does work no matter which way you're going. Very fast jets have to contend with high heat from tearing through the air much like a reentering spacecraft.

The thing is that (for now) the chemical rockets used to get the space shuttle, tv satellites and so on up into space travel much, much slower going up than down. That's because they have a lot of weight to carry, with rather inefficent engines---all against the force of gravity most of the way.

If we ever invent some high-tech antigravity drive that lets us leave the atmosphere at incredible speeds, then we will have to contend with the problem of friction going both ways.

Answer 5

As a retired Submariner I have experienced the hull compressing as depth increases. So knowing this compression is caused by increased pressure on the hull it leads me to a question. How much does the Space Shuttle expand when leaving earths atmosphere?