How does the Earth's atmosphere slow down rockets?

03 October 2017

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Question

When a rocket is returning from space, how much does Earth’s atmosphere slow it down?

Answer

We put this question to planetary scientist David Rothery...

David - We rely on Earth’s atmosphere to slow the rocket down. If you’re thinking of a re-entry capsule such as the ones Apollo used or such as we used to get back from the International Space Station, you have a blunt end. You have tiles to absorb the heat because the compression when you hit the atmosphere, heats the atmosphere so much that you’d burn the spacecraft up if it wasn’t properly protected, so you slow down using the atmosphere. It’s much easier to land on any planet with an atmosphere than onto an airless body where you do need to use rockets. But if people have been seeing the Falcon rocket landing back on the barge from which it has taken off, that is using rocket power to slow it down. So both techniques are used but you use less fuel if you use the atmosphere to slow you down and a parachute to make the final bit of descent, for example.

Chris - Thank you, David. And, of course, the temperature you reach is humongous, isn't it, because you’re doing adiabatic compression of the atmosphere in front of the capsule so you get a thousand degrees or so?

David - Absolutely. More than a thousand I think. It’s usually described as friction with the atmosphere, but it’s not friction, it’s not rubbing against it. It’s slamming into it and compressing it. That’s why shooting stars are glowing in the sky. It’s the compression of the air in front of the grain of dust coming in. It’s not friction, it’s compressing the air to make it hot.

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