Why do rockets have to hit the atmosphere at an angle on reentry to not burn up?

Question

I remember this from Apollo 13, they had to hit the atmosphere at an angle, if they came in too directly they’d burn up. My stupid layman thought is that I’d want to come in directly because if the atmosphere is making me burn up I’d want to take the directest and shortest route to landing so that there’s less atmosphere to burn me up. Obviously that’s not how it works, why not

In: Planetary Science

Answer 1

Air surrounds the Earth. When you press into it, it compresses. As it compresses, it gets hot. If you press into it fast enough you compress it enough that it heats up to a temperature hot enough to melt rock. That’s what’s happening in a meteor (shooting star).

Astronauts don’t want their spacecraft to melt, that’s bad (see Space Shuttle Columbia disaster). At a shallow angle you get hot for longer, but not hot enough to melt. That’s what the spacecraft is going for. Too shallow and you skip off, like a rock skipping on a pond.