I assume things burn up when they enter our atmosphere because they are traveling at very high velocities. I also assume this applies to things we send up, and back down, from outer space (i.e. the space shuttle). Why don’t they just slow down to a crawl before they re-enter the atmosphere?
Basic physics. The shuttle can’t slow down to a crawl before hitting the atmosphere without using as much fuel as it took to get to orbit. The shuttle can’t carry that much fuel so it converts kinetic energy (motion) into heat from hitting the atmosphere. Do a search, there are numerous threads on this topic.
The simplest and most efficient method of slowing down a spacecraft is to hit the atmosphere and use friction* of the air. The heat shields are basiclly the “brake pads” for the spacecraft. Like the pads on your car, it’s bad news if they overheat and fail, but the risk can be minimized with proper design and usage. Rocket engines are more complex and therefore less reliable than heat shields, and they require lots of fuel.
*Technically “compression” might be a better term, but both are good enough for now.
In orbit, speed is directly related to altitude: going faster = moving higher, and vice versa. Therefore, there is only one possible orbital speed at the altitude where they start hitting atmosphere.
This is only true for circular orbits. Eccentric orbits form an ellipse, and at the closest approach the vehicle is going faster than it would be in a circular orbit of the same altitude.
An extreme example is the Apollo re-entry trajectory. These orbits were very eccentric, and the capsule re-entered the Earth’s atmosphere approximately 50% faster than the space shuttle does.
Now, it would be very difficult to enter Earth’s atmosphere at the farthest approach of an eccentric orbit, since you must have been inside the atmosphere before that. So, the only way to enter at a slower speed than a circular orbit would be to have retro rockets and a boatload of fuel, as Padeye mentioned.
You have this backwards: the greater the altitude, the lower the speed, and vice versa. It’s somewhat counter-intuitive, but to get into a higher orbit, you fire your forward thrusters to slow your speed.
If you doubt this, consider the planets: closer to the sun, the shorter the planet’s year.
Also see this site:
There’s also the fact that if you’re going too slow – that is, you counteract the acceleration from gravity very effectively – you can bounce off the atmosphere like a stone on a pond.
–Cliffy
That can’t be right, can it? If you’re in a car driving round a banked oval circuit, and you step on the gas, the car rises up the bank.
Surely what happens is the spacecraft increases its orbital speed, moving to a higher orbit, and when it switches its engines off it decelerates to a new, lower orbital speed?
You’re both right, sort of.
In an elliptical orbit, the spacecraft will be moving fastest when it is closest to the planet.
When comparing two circular orbits, the one closer to the planet will be faster, both in terms of orbital speed and time it takes to complete one orbit.
However, to get to a higher orbit, you must increase your speed. You will then lose speed as you climb to the higher, slower orbit.
The reason we use aerodynamic braking to slow from orbital speed is that it’s simply the most mass-efficient way to do so. With current engine technology a heatshield capable of surviving entry at orbital speed weighs much less than the fuel that would be needed to slow down before reentry. If we someday invent a rocket engine with a fantastically high fuel efficiency - fusion powered rockets or something like that - then it might become more efficient and safer to actively slow down before hitting the atmosphere.
Yeah – the reason the outer planets have longer years isn’t because they move slowly but because one degree of their orbit is a much, much longer distance than one degree of an inner planet orbit. Isn’t it?
–Cliffy
Smaller orbits are faster because orbiting is falling - the closer you are, the stronger the pull of gravity and the faster you fall. Very roughly, anyway.
No, the slower orbital speed makes a difference too. Jupiter’s orbit, for example, is about 5 times longer than Earth’s but its year is 12 times longer.
Um…no. Methinks you are confusing orbital speed with orbital period (or angular rate). At comparable speeds, the lower you are the less your orbital period (and the higher your angular rate). So, in a sense, going into a lower orbit makes you go “faster”, i.e. your orbit period is shorter than it would be at the equivilent (linear) speed in a higher orbit, and indeed, orbital ballisticians often say, “to speed up, you slow down; to slow down, you speed up.”
However, for a circular orbit, forward-directed thrust is going to put you in a lower, not higher orbit. Most orbits, though, are not circular (except for some satellites, especially geostationary sats) and the ballistics are more complex. You might thrust forward at some point to make your orbit more elliptical and the aphelion further from the planet (though you’re generally better off adding your thrust to do this) but your mean altitude is going to be less. In an elliptical orbit, as you indicate, a greater distance from the prime focus has a correspondingly slower orbital speed per Kepler’s Second Law.
As for the question posed by the OP, as others have quite adequately explained, heating is due to energy “waste” from slowing down the craft, analogous to the brakes in your car. (The heat is primarily due to compression, as scr4 alludes to, rather than anything like friction.) By coming in at a more shallow angle they could reduce skin temperatures and skip along the atmosphere, more slowly losing speed; the problem is that this makes the reentry path more difficult to predict and control, especially for a ballistic object (which is what the Mercury and Apollo spacecraft were) and even for a lifting body (like Gemini or the Shuttle) such fine control isn’t really possible. Besides this, it is a dangerous regime with rapidly changing aerodynamic conditions, and they’d rather get through the region as quickly as possible rather than linger and have a few tiles flake off or a heat shield crack.
Someday we may have the power to “stop and drop” a spacecraft from orbit without using the atmosphere as a skidpad, but it won’t be anytime soon. The amount of extra fuel you’d have to carry is prohibitive.
Stranger
Here’s my WAAAYYY oversimplified understanding for a stable orbit:
potential energy (described in part by the distance above the earth)
+
kinetic energy (described in part by the speed of the satellite)
= “magic number”
An increase in potential energy (i.e. further from the earth) allows a smaller kinetic energy (slower speed) while still keeping the combination the two where we need it to be for a stable orbit (what I’m foolishly calling the “magic number”).
HOWEVER – decreasing you speed WILL NOT move you into a higher orbit, it will simply lower your total energy beyond that required for a stable orbit and you will soon be burning up in the atmosphere.
To reach a higher orbit you must first speed up, so that you energy is greater than that required for stable orbit. This will result in a larger mean distance to the earth. Once you reach the desired height, to maintain it you must THEN slow down to a speed LESS than what you started with. Since you have increased your potential energy, your kinetic energy must decrease to still maintain the sum of the two equal to the “magic number”
As I said this is my oversimplified understanding of the mechanisms involved. If I’ve misstated anything I’m sure someone smarter will be along shortly to chastise me 
To clear up the Jupiter/Earth issue:
In general, speeding up in a circular orbit will put you in a higher orbit. You actually have to do a second burn to regain a new circular orbit. But the fact remains: Accelerate in a circular orbit and you’ll go higher.
But, once you get to that new orbit, you’ll actually be going slower than in your old orbit. The energy taken to work against gravity and gain orbital altitude will drop your speed.
The simple formula for the orbital velocity of a satellite in circular orbit is V = (gr)[sup]1/2[/sup]. The g is the graviational acceleration at that point.
Comparing the Earth’s orbit with Jupiter’s, it’s important to recognize that there’s a huge difference in g for the two planets. That is, the strength of the acceleration due to the Sun’s gravity is far stronger for Earth than Jupiter. Since the strength of g decreases as the square of distance, the g in the above equation falls off much faster than r as you get farther from the sun.
So it’s true that Jupiter, being much farther away from the Sun, also has a slower orbital velocity than Earth. But, still, if you wanted to move the Earth, which is currently moving a lot faster than Jupiter, out to Jupiter’s orbit, you’d have to add an impulse. You’d have to move our planet even faster to throw it out of it’s current orbit and let it “fly” out to the orbit of Jupiter. By the time it got there, it would be going slow. Even slower than Jupiter is now. Then you’d add even more velocity to keep it in a circular orbit, instead of falling back towards perigee at the old orbit in a big ellipse.
This whole process is a called a Hohmann transfer. It wouldn’t really work too well for big, hard-to-move planets. But for small objects like artificial satellites which can be given practically instantaneous boosts, it’s an efficient way of moving orbits. It’s an energy minimizing transfer, but isn’t practical for a lot of cases.
Not neccessarily. It will put you into a lower eliptical orbit which may or may not intersect the earth depending on the height of the original orbit and the amount you slowed.
Well see this is where my understanding may be lacking. I was under the impression that for a given stable orbit, if you slowed down, the combination of kinetic and potential energy would be less than that required for stability. In other words your orbit would deteriorate until you got the big burn.
Am I missing something?
Entering into the worlds atmosphere at 17 and a half thousand miles per hour it would burn up.
The “orbit” you are describing is a descending spiral, which requires some kind of constant velocity-reducing force (atmospheric drag, perhaps) to decrease altitude. This requires taking energy out of the system; either slowing the satellite down or reducing its altitude without the corresponding increases in altitude or velocity respectively.
Ballistic orbits come in the various flavors of conic sections; circles and ellipses for stable capture orbits, parabolas and hyperbolas for escape (fractional) orbits. The energy for any stable orbit remains the constant sum of U and V (potential and kinetic energy). Orbits can actually be described in terms of energy without regard to their specific kinematics, and it is usually easier to do so in the general case, though if you actually want them to be in a particular place at a certain time with a given velocity you have to deal with all those numbers and coordinate systems and that crap.
Terminal phase ICBM RVs (reentry vehicles) travel much faster than this at terminal speed, and survive until the fusing system turns them into a flash of neutrons and gamma rays. Spacecraft like Apollo enter much faster but are designed to shed velocity (unlike RVs, which are designed to slice through the air as cleanly as possible) so that they’re only moving a few hundred miles an hour when they deploy drogues/parachutes/parawings.
Stranger
The orbital velocity decreases inversely with the square root of the distance from the primary for a circular orbit. For an elliptical orbit the speed in orbit is never correct for the distance from the primary. At apogee you are going to slow so the vehicle falls toward the primary. At perigee you are going too fast so the vehicle moves away from the primary.
I think what happens in going to a higher circular orbit is that you point the rocket thrust vector up and fire the rockets to increase the velocity and thus the kinetic energy. As the vehicle moves upward against the earth’s gravity the velocity is reduced as kinetic energy is converted to potential energy. When the desired orbit distance is reached small adjustments are made by firing rockets to get the correct velocity vector aligned tangential to the desired circular orbital path.