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Comment by GlenTheMachine | Hacker News Reader

I'm curious about whether an object with a density this low could survive the gravity gradient it experienced during the solar flyby.

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As am I. The paper dedicates one sentence to that question:

"... another aspect that needs to be studied is whether its extremely low density could be maintained while in the parent system, during its long interstellar journey, and when entering the solar system."

Without analyzing the structural integrity of such a fluff ball it's tough to give any more or less merit to this concept than the others.

Just how close did it get to the Sun? My memory is that it got about as close as Mercury to the sun. It should have warmed up a fair bit, but the gravity gradients shouldn't be ripping it apart.

Unless it's very large why would there by much of a gravity gradient?

It never came all that close to the sun, so the gradient on different parts of it wouldn't be that extreme.

There wouldn't be a large gradient. But a big ball o' fluff presumably has essentially no internal cohesion; it's sort of like a liquid in that way. So even very small gradient forces could be enough to disperse it.

So lets run the numbers:

At Perihelion it was 38198320 km from the sun = an acceleration of 0.090962666582226 m/s^2

It's 1km long so on the other end the acceleration is 0.090962661819574 m/s^2

Subtract: .000000004762652 m/s^2 which is such a small amount I'm not sure how to put it into context.

Multiply by 1 metric ton = 1/5 weight-force of a single snowflake on earth.

Which means each metric ton of material needs to handle just that much force trying to separate it.

i.e. it won't be ripped apart in the slightest. Over centuries it might elongate a bit (and it's already elongated).

Interesting. Thanks for the analysis.

So, continuing this general line of thought: what's the largest tidal force it would have experienced? Differential solar pressure? Would it have encountered anything on its journey that should have dispersed it?

> what's the largest tidal force it would have experienced?

I have no idea of its past obviously, but near us this would be it, at closest approach to the sun the tidal force is greatest.

> Differential solar pressure?

Solar pressure is very low, it just applies that force for a very long time. It would act to compress it (flatten it), slightly.

But I don't know what orientation it had when approaching the sun. It could have been tumbling and randomized any force.

> Would it have encountered anything on its journey that should have dispersed it?

Vaporization from heating is the only thing energetic enough that I can think of.

But maybe other people can think of other things.

I'm curious about the force/event before it got to our solar system. If the original object was protoplanetary, what kind of forces would be required to knock an object of this current size out of a stars gravitational hold, and into interstellar space? I'm guessing we can assume that due to the current size of this object, it would have had to have been part of a much larger event.

Possibly it didn't? Since we saw it after solar perigee, perhaps it is in it's spread out state and it was more compact while approaching the sun.

and the stress and strain of rotating about its own axis, especially if the optical pressure is not perfectly central, it could speed up to high speeds.

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