# Why things easily leave earths atmosphere but burn up while re-entering the same atmosphere

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Why things easily leave earths atmosphere but burn up while re-entering the same atmosphere

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Rockets are going slower on launch. And even then hearing is an issue. Part of the “ Max Q” callout during launch is slowing down during the point of maximum dynamic pressure, aka friction.

At least that’s what Kerbal Space Program taught me

Atmosphere is denser near the ground and thinner near space. As a rocket goes up, it is accelerating. It’s going slow when it is moving through thick air, and faster only in the thin air. When it reenters, it is going extremely fast, and will hit thicker air while still going faster, unless it wastes a lot of fuel actively decelerating.

When you’re leaving Earth’s atmosphere, you are slowest where the atmosphere is the thickest (near the ground). You speed up as you go up, while the atmosphere is thinning out, which means air resistance and compression of the atmosphere is not as significant.

When returning to Earth, the spacecraft is traveling at very high speeds (assuming it was in orbit). The spacecraft will slow itself down a bit in order to stop orbiting and start the re-entry process, but it’s not by a lot. Additionally, once you’re not in orbit, gravity will start to increase your speed instead of just make you orbit. That means that the spacecraft is “accelerating” as the atmosphere gets thicker and thicker (it’s not really getting faster, because the atmosphere is getting in the way and slowing it down, but gravity is still trying to make it go faster, which means that much more air resistance).

In that process of slowing down from the atmosphere, the spacecraft heats up a lot; there is both air resistance/friction and compression of air affecting the spacecraft, as it is traveling so fast that the air in front can’t get out of the way fast enough, and so compresses. Resistance/friction and compression both increase temperatures, which directly heats up the spacecraft to a ridiculous degree. Another way of looking at it is that the spacecraft is losing both velocity (kinetic energy) and height (potential energy) the entire way down; that energy has to go somewhere, so it gets turned into heat.

The thing about space is, it’s easy to get there. Staying there is the hard part. To stay in orbit, you have to go very fast “sideways”. Spacecraft gain much of that speed while above the dense atmosphere. On reentry, objects descend back into the atmosphere without losing much speed (actually speeding up at first as they descend). Hitting air at such high speed compresses the air in front of the spacecraft, heating it to many thousands of degrees.

Being in orbit is a lot less about being *up* than it is about being *fast*. The reason rockets go up first and then roll to start going fast is so that they can get above the thickest parts of the atmosphere where drag would slow the rocket down. If there were no atmosphere, it would be more efficient to go closer to a tangent to the surface – more sideways. But with the atmosphere, it’s more efficient to burn fuel and get over the air.

Conversely, coming out of orbit is less about going down than it is going slow. How you slow down doesn’t matter – once you slow down you will fall. You could use fuel and thrust in reverse to slow down, but you would have to carry that fuel up in the first place. Instead, just use a tiny bit of fuel and slow down *just* enough to touch some air and the drag from the air will slow you down.