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submitted 3 weeks ago by [email protected] to c/[email protected]

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[–] [email protected] 1 points 3 weeks ago (1 children)

For one thing, the Apollo shield started in the very thin upper atmosphere, and they came in at an angle that meant they bled off as much speed/energy as possible in that thin upper atmosphere before going into the thicker atmosphere.

I don’t know that that makes a huge difference to the physics involved, though it certainly may have.

Of course it will make a difference. The whole challenge is about managing the heat build-up, which is the energy per second (i.e. power). If you hit the thin upper atmosphere you're encountering less material, so less friction / pressure, so less heating. It means you can keep the heat on the heat shield in a manageable range, rather than putting it at a temperature where it would melt or explode.

the air cushion begins heating itself up instead of the object, reducing the amount of heat the object receives.

No, both heat up. The air cushion transfers its heat to the object next to it. At the kinds of pressures we're talking about, you might even be getting nitrogen plasma rather than just nitrogen gas.

But it would also tail off as the bore cap heated, reducing stresses on it as it went higher.

If it went high enough for that to matter. If it disintegrated in the lower atmosphere it wouldn't matter that the air got thinner in the upper atmosphere.

chunks of meteorites bigger than a meter have made it through the atmosphere, for instance

Is a metre the original size, or the final size? Also, reverse meteors (something starting with its maximum speed in the lower atmosphere) are doing things the hard way. Rather than getting slowed down initially by the thin upper atmosphere and then only hitting the thick atmosphere once they're slower, they start out in the thickest atmosphere. OTOH, a meteor is a random collection of rock and metal formed by gravity in space. A pure metal plug cast on Earth is probably going to be a lot less prone to breaking apart.

the bore cap starting at the bottom of the atmosphere means that it’s likely it experienced less fracture stress, since the air would’ve accelerated with it rather than being static.

That doesn't make sense to me. Something in a thicker medium is going to experience more stress. Try pushing a cracker through the air vs. through water vs. through gelatin. Which medium will cause the cracker to crack first? Obviously it's the thicker medium.

[–] [email protected] 0 points 3 weeks ago* (last edited 3 weeks ago)

Most of this is going to be "eh, agree to disagree" because we just don't have enough data. But I do want to call out a couple of things:

No, both heat up. The air cushion transfers its heat to the object next to it.

Over time, yes. But the bore cap doesn't have very much of it. Heat transfer is not instantaneous; would it be long enough for the air to transfer its heat to the object, before the object reaches the Karman Line? Radiation is pretty quick (like, speed-of-light quick), but conduction is much slower; particularly when one of the bodies (the air) is an insulator. And with iron being an excellent conductor, any heat transferred will be spread throughout the body more quickly than it can be absorbed.

If it disintegrated in the lower atmosphere it wouldn't matter that the air got thinner in the upper atmosphere.

True, but it's not like there's a line (er, well, I mean, not a physical demarcation...there is the Karman Line, but...ah, you know what I mean). Atmospheric density is a decreasing gradient from the ground to the Karman Line. So as it approaches its mechanical and physical limits, the amount of energy acting upon it decreases millisecond by millisecond. Is that enough to save it? Shrug. Not enough data. But it's possible.

Is a metre the original size, or the final size? [of the meteorite chunk]

Actually it's almost three meters, and as far as we can guess that was about its original size. Though in fairness, it was entering the atmosphere at a steeper angle and may even have come down entirely in "dark flight." Still, there are other large meteorites which have impacted at a size greater than 1 meter across, though obviously we have no way to confirm exactly how big they were before they landed.

Rather than getting slowed down initially by the thin upper atmosphere and then only hitting the thick atmosphere once they're slower, they start out in the thickest atmosphere. [...] Something in a thicker medium is going to experience more stress. Try pushing a cracker through the air vs. through water vs. through gelatin. Which medium will cause the cracker to crack first? Obviously it's the thicker medium.

True! But remember, the "reverse meteor" (great phrase, btw) is not hitting the stationary atmosphere at full speed like a regular meteor (or space capsule) does. The iron plug accelerated (incredibly quickly, but it did accelerate) while already in contact with the air above it. This means that the air accelerated at the same rate the iron did, reducing the fracture forces that would seek to crack it. Imagine the difference between swishing your hand in a swimming pool vs. slapping the surface of a swimming pool; it may require more force, but it won't hurt as badly.

OTOH, a meteor is a random collection of rock and metal formed by gravity in space. A pure metal plug cast on Earth is probably going to be a lot less prone to breaking apart.

Oh, great point, and one I hadn't thought about. Something that's an aggregate of 80% iron and 20% "other stuff" isn't going to have nearly as much tensile strength as a homogeneous plate of iron.