# Kind of a dumb question

1. Dec 8, 2009

### Kronos5253

Okay, this is kind of a dumb question, but I did some searching online and still couldn't come up with an answer, although I think I many know it anyway.. BUT! Here it goes:

Why do objects only burn up entering the atmosphere from space, and not vise versa?

2. Dec 8, 2009

### player1_1_1

I think its density gradient but not sure;]

3. Dec 8, 2009

### tmyer2107

When objects are in space orbiting Earth they are moving very fast, a lot faster than when they take off. When they re-enter the atmosphere they are moving so fast the friction with the air makes them "burn up". Eventually, the object will slow down due to air resistance and reach a terminal velocity.

This is just what I always assumed happened based on my common sense so I could be wrong.

4. Dec 8, 2009

### twofish-quant

Because when you are going down you are moving faster than when you are moving up. By the time a spacecraft is moving very fast, it's already outside of the earths atmosphere.

5. Dec 8, 2009

### BigSteve

That is correct. Orbital velocities are on the order of km per second.
Entering the atmosphere causes severe heating due primarily to friction with gasses in the atmosphere.
This can be exploited in what is known as aerocapture. Where a spacecraft traveling to a planet such as Mars enters its atmosphere (sometimes multiple times) at an angle that allows it to slow down and attain orbit. If it hits too shallow it will simply skip off and keep going, and if it hits too steep it will burn up and/or crash into the planet surface.

6. Dec 8, 2009

### Phrak

Of course, if the intent is to establish an orbit, you want it to skip.

7. Dec 9, 2009

### Lsos

By the way, it's not the fact that you're entering the atmosphere that causes things to burn up, it's the fact that the things entering the atmosphere are usually going VERY fast.

For example, this Spaceship2 that is supposed to take tourists to space soon, does not need heat shields. This is because despite going to space, it's going much, much slower that a satellite, and therefore does not experience the extreme heat when reentering the atmosphere.

8. Dec 9, 2009

### blitz.km

Objects, when entering the Earth's atmosphere are a lot more faster relatively due to constantly having a downward acceleration.
So, they burn up due to air resistance.

9. Dec 9, 2009

### gmax137

So, we're saying that when a satellite or the shuttle is launched into orbit, the engines continue to burn (increasing the velocity) long after the craft has risen above the atmosphere? I'm just asking.

10. Dec 9, 2009

### rcgldr

The reason is speed. When a rocket is launched, it continues acceleration after it leaves the atmosphere to a very high speed while in space. When it re-enters the atmosphere, it's speed is much higher and the heat mostly due to extreme compression of the air (as opposed to friction) combined with no way to remove the heat results in burn up of the object isn't properly designed to handle the heat.

11. Dec 9, 2009

### alexgmcm

Yeah, one of my professors was talking about this. The idea that it is mostly due to friction seems to be a popular misconception.

12. Dec 12, 2009

### Kronos5253

Guess so, cause that's actually what I was thinking it was haha.

13. Dec 12, 2009

### Kronos5253

Which brings me to another question...

Does that mean if there's no compression of the air, there's no heat, therefore no danger? For example, if you had a solid, pointy piece of metal that was in orbit, moving the same speed that the Earth is rotating, and shot it through the atmosphere toward the Earth, there'd be no compression of the air correct? Depending on size of course.

14. Dec 13, 2009

### Lsos

"Moving at the same speed that the Earth is rotating" is actually quite slow. The Concorde easily went faster.

The point is that orbital speed is WAY beyond that. About 17,000mph. At that speed many interesting phenomena occur which no layman would even consider. It's so fast that convention logic of what shape would work for reentry and what shape wouldn't, simply breaks down. And, at that speed no matter what you do, there will be heat, and lots of it. It's just a question of how you deal with it.

For example, a solid pointy piece of metal was actually tried, and it quickly burned up. Turned out that a blunt, highly "non-aerodynamic" shape is actually better at reentry, because the accumulation of compressed air underneath it actually serves to push the heat and horror out of the way of the vehicle.

15. Dec 13, 2009

### Kronos5253

Ah okay, well thank you for the answer :)

16. Dec 13, 2009

### sermatt

What If an astronaut who's flying perpendicular to Earth (not in orbit) starts entering the atmosphere? Will he still fry despite having a considerably lesser than orbital velocity?

17. Dec 14, 2009

### Lsos

I mentioned above that Spaceship2, which is supposed to take tourists to space soon, does not need heat shields, since it is going considerably slower than orbital. So, simply entering the atmosphere is not enough to fry you. You also need to be going pretty fast. Keep in mind though, that even if you started from a stop, the earth's gravity will accelerate you towards it, possibly enough that you might end up going dangerously fast (I don't know though, I haven't done the calculations).

18. Dec 14, 2009

### sophiecentaur

If you had enough energy (fuel)left when returning to Earth, you could hit the atmosphere at a more reasonable speed by using retro rockets (or face backwards).

I still think that, with suitable control (which is a major problem, I know) it is conceivable to 'fly' in and reduce speed slowly enough to reduce the rate of heating. It would have to involve a long spiral path.

19. Dec 14, 2009

### Lsos

Yeah but if you really think about it, the amount of fuel that would require, and the amount of fuel it would require to bring that fuel up in the first place...it would just be a waste. It's easier, cheaper, and makes more sense to put a bunch of heat tiles instead of making rockets 2x bigger. It's not a problem that needs to be solved...

20. Dec 14, 2009

### sophiecentaur

I totally agree that retro rockets are far too expensive a proposition but the 'flying in' solution would remove a significant risk, I think.
Most solutions to most problems in Space revolve around the Energy situation. (Just like down here, only worse).
ps The rocket would have to me quite a bit more than 2X bigger - I think there's an exponential law involved.