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What happens when asteroids (or other small bodies) "collide" with a gas planet? In my head it would either go through it or just stay "inside" it due to the strong gravity. I tend to believe in the latter one, but if this is the case, does this mean that the gas giants are full of small bodies inside them?

Thank you.

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    $\begingroup$ It would be crushed into pieces by tidal forces before it hits the surface of the atmosphere. And inside the atmosphere it would be destroyed by friction and the fragments would lose speed and spiral in towards the center of gas planet. Somewhere down there the materials of the asteroid gets into equilibrium with the surrounding gases. But huge red giant stars like Betelgeuse on the other hand, might have even companion (neutron)stars orbiting "inside" them. Jupiter no. I'll leave the answer to a real physicist. $\endgroup$ – LocalFluff Apr 6 '15 at 14:10
  • $\begingroup$ @LocalFluff would you care to quantify? I think the Roche limit for rocky material would be inside Jupiter. Certainly, something of low density, like a comet, might break up before it gets to the surface, but not something with say 5000 kg/m$^3$. $\endgroup$ – Rob Jeffries Apr 6 '15 at 15:30
  • $\begingroup$ @RobJeffries It would be a huge project for me to calculate that, and since others do it much better I lack motivation to do so. Maybe and likely a solid asteroid, like 16 Psyche, could enter the atmosphere before it breaks up. But I see no other destiny for it than to quickly liquify and join the general differentiation of its new environment. I think it's pretty obvious that the drag from the dense gas would prevent any solid object from orbiting inside a gaseous planet. Obvious enough to not having to go into the numbers (cause I'm lazy). $\endgroup$ – LocalFluff Apr 6 '15 at 16:53
  • $\begingroup$ @LocalFluff: I think a reasonably dense object like a rocky or iron asteroid would have more than enough internal strength to hold itself together for a while even inside the Roche limit. Very large objects like planets behave like fluids (for example, Earth is round because it's close to hydrostatic equilibrium), but smaller objects behave like solids. A body small enough to be non-spherical isn't big enough for tides to be a huge influence. $\endgroup$ – Keith Thompson Apr 6 '15 at 19:13
  • $\begingroup$ @KeithThompson "A body small enough to be non-spherical isn't big enough for tides to be a huge influence." But then what about comet Shoemaker–Levy 9? The expectation is that most objects orbiting out there at Jupiter are pretty icy and soft. But anyway, once inside the atmosphere any object would quickly decelerate and dive into the depths where it will be crushed by pressure. $\endgroup$ – LocalFluff Apr 6 '15 at 21:22
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A very well documented example of a small celestial object with a gas giant is the collision between Comet Shoemaker-Levy 9 with Jupiter. As it was a comet that was tracked, the whole process of the collision was well documented and observed.

Though the collision occurred in July, 1994 - the first effects occurred 2 years earlier, according to the NASA Technical Report "Shoemaker-Levy 9 and the tidal disruption of comets", when the

break-up of Periodic Comet Shoemaker-Levy 9 into multiple pieces following its grazing encounter with Jupiter

This represents one of the first stages of the collision that could occur as the celestial object approached the large planet. In this case, this breakup resulted in Comet Shoemaker-Levy 9 to form a train of fragments to eventually collide into Jupiter - the full process at this stage is shown in the diagram below:

enter image description here

Image Source: Tufts University

The train of fragments was captured by the Hubble Space Telescope (shown below):

enter image description here

Image Source: NASA

Each fragment was designated a letter (e.g. Fragment A etc). According to the article "The collision of Jupiter and Comet Shoemaker-Levy 9." (Zahnle and Mac Low, 1994), these fragments were in the order of about 100 m to 5 km in size, heading straight towards Jupiter.

Then to the main event - the collision of the comet fragments with Jupiter.

An example was from Fragment G, one of the larger fragments, when it collided with Jupiter (image below):

enter image description here

Image source: NASA

As you can see, the explosion was spectacular - about 21 fragments impacted into Jupiter over a number of days, given Jupiter's relatively fast rotation, the impacts became quite distributed on the top of the clouds of Jupiter, as shown in the ultraviolet image below (the circle shadow below is the moon Io):

enter image description here

Image Source Air and Space Museum

There have been several impacts into Jupiter (and presumably into the other gas/ice giants as well), another one that underwent scientific scrutiny was an asteroid 'the size of the Titanic' colliding with Jupiter in 2009, reported in the article "New evidence that asteroid, not comet, struck Jupiter in 2009", describes a comparison between the 2009 asteroid and Comet Shoemaker-Levy 9 are described as:

The dark debris, the heated atmosphere and upwelling of ammonia were similar for this impact and Shoemaker-Levy, but the debris plume in this case didn't reach such high altitudes, didn't heat the high stratosphere, and contained signatures for hydrocarbons, silicates and silicas that weren't seen before. The presence of hydrocarbons, and the absence of carbon monoxide, provide strong evidence for a water-depleted impactor in 2009.

Overall, the article mentioned the likely process during any collisions of a celestial body with a gaseous planet, using the example from Jupiter:

Plunging through Jupiter's atmosphere, the object created a channel of super-heated atmospheric gases and debris. An explosion deep below the clouds – probably releasing at least around 200 trillion trillion ergs of energy, or more than 5 gigatons of TNT

and the celestial object itself - vaporised, totally destroyed in the resulting explosion.

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We have observed impacts with Jupiter

In each case the object was destroyed in Jupiter's atmosphere.

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