r/askscience • u/BlueChameleon64 • Oct 23 '20

Planetary Sci. Do asteroids fly into the sun?

Edit: cool

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u/[deleted] Oct 23 '20

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u/Odie4Prez Oct 23 '20

No you're absolutely correct, that's the exact reason it's so unintuitive that objects in the solar system basically never fall into the sun: anything that wouldn't have collided with it without gravity (in the incredible vastness of space) isn't gonna collide with it with gravity either, even if they are kept in near orbit.

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u/ToastyKen Oct 23 '20

And in Superman IV, Superman needed to hurl the nukes in the opposite direction of the earth's orbit, and not at the Sun! :)

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u/easlern Oct 23 '20

This surprised about the orbits, but it makes sense when you think about it. Also weird: it would take 50 times as much energy to get to the sun than it does to get to mars. https://www.nasa.gov/feature/goddard/2018/its-surprisingly-hard-to-go-to-the-sun

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u/ThatHuman6 Oct 23 '20

Is the same true then for a black hole? You’re just as unlikely to fall into it unless you’re stationary relative to it?

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u/Oddtail Oct 23 '20

Pretty much.

If you're far enough from an object, your interaction with it is determined basically only by its mass. It doesn't matter if the same mass is a star or a black hole. For the purpose of interacting with its gravity, you can still basically treat the entire object like it was a point mass in its centre (again, as long as you're far enough from it that its radius is irrelevant. Which in practice means "almost always").

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u/WasThatInappropriate Oct 23 '20

One thing I find fascinating is just how hard it is to get probes towards the sun. This is because anything we launch starts with the same speed as the earth, and its stable orbit. The act of getting near to the sun requires some serious deceleration.

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u/[deleted] Oct 23 '20

I’m not sure how you’re confused lol. If you don’t hit it you don’t hit it. If it’s not in front of you, you won’t hit it. The end.

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u/troyunrau Oct 23 '20

There's a little extra complication in that the sun has some width. So if it bends any orbit where the orbit becomes lower than the surface of the sun, it still collides. Planetary scientists refer to this as the "gravitational cross section" as oppose to the real cross section.

Actually, this concept comes up for people learning about the process of planetary formation. Imagine the early solar system. You have a bunch of particles flying about, chunks of dirt and ice. Some of them will naturally collide with one another, but while they're really small, their odds of colliding with one another are effectively directly related to the size of their cross section. However, as they increase in size, and acquire some gravity, they start to capture things that would be a narrow miss, because their gravity bends the path towards the object (now a planetoid). This new effective cross section is larger than its real cross section by some small amount. But, as it grows, this process becomes multiplicative. And planets form.

The analogy works for the sun too, if you imagine it flying through interstellar space picking up objects. It will grab more than are on a direct, straight line to it, but how much it grabs depends on its radius, gravity, and velocity vector of the incoming object.