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Comment by astro123 | Hacker News Reader

The argument is actually really really simple. If it started from within the solar system, it would not (barring some three body interaction) have enough energy to escape. But, we know from observations that it does have enough kinetic energy to escape. There are two options,

1. It may have stolen some energy from another object (an accidental gravity assist), but we know that it didn't come close enough to anything big on its way though.

2. It came from outside the solar system and so entered with some velocity, and therefore will leave with roughly the same velocity it came in with.

We have well known mechanisms to eject objects from star systems so it isn't crazy to have things passing through. No-one questions it because high school level physics is enough to show why it is the only reasonable explanation.

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> We have well known mechanisms to eject objects from star systems so it isn't crazy to have things passing through.

We know it's 'possible' for a tennis ball to go through a wall. But it's not actually within the realms of the possible.

So I guess I still question the 'crazy' part. What are the mathematics here.

If a planet breaks into 5000000000000000 (Earth / Oumuamua) pieces how many hit another solar system? I'd expect zero, but have no idea.

I think it's more likely 1. We missed the fact it went close to another body or it's in a complicated gravity assist eons in the making.

See https://www.space.com/43015-interstellar-visitor-oumuamua-no... for the math.

But you're still focusing on the wrong thing. The idea of "a complicated gravity assist" getting something going this fast in the Solar System is laughable. The only thing in the Solar System heavy enough to do a gravity assist on the necessary scale is the Sun itself, and it started nowhere near the Sun.

That link is great thanks.

For it to be real I'd expect billions of things passing through yearly, maybe daily. For something as large as Oumuamua I'd intuitively expect millions or billions of small rocks coming through.

"We find that such objects collide with the Sun once every 30 years, while about 2 pass within the orbit of Mercury each year."

But I worry about anything like this talking in human years, like "30 years". I'd expect every million years or millions a year when talking about the universe.

I understand as our technology grows, first we find yearly events and then improve. But the theoretical must make sense to me in non human timelines.

There could well be millions or billions of sand particle sized objects coming through constantly.

The Solar System is big. Almost all of them will pass through and miss everything. We literally have no way to notice them.

Incidentally we have spotted a second interstellar object. https://en.wikipedia.org/wiki/2I/Borisov is more what we were expecting them to look like.

It is more likely that we underestimated the likelihood of seeing such objects rather than us experiencing an extremely unlikely event

Arguments from probability still have to take into account physical evidence, like velocity.

Also, most (interior) walls are far weaker than you might expect.

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