From my understanding of the word gas giant, it is a planet composed of entirely a gaseous atmosphere, and so planets Jupiter and onwards fall in this category.

That being said, what would stop a spacecraft from just going right through any of these planets?

For earth, a spacecraft would firstly rapidly disintegrate approaching the surface of earth, and if it happens to still have part of itself, it will collide with the surface.

For gas giants like Uranus, if a spacecraft is able to counter the gravitational forces and slow down, could it actually "go through" Uranus?

  • $\begingroup$ One issue with gas giants is the definition of the "edge" where the planet begins. Rocky planets have atmospheres who's extreme edges an orbiting or interplanetary spacecraft might pass undamaged, depending on design and alitutde. and solid/liquid sphere where it wouldn't. However, for a gas giant, the point where the planet "begins" is often defined by its optical properties, I think something like opacity = 1. The density falls rapidly above this point and increases rapidly below this point. This will be a hard question to answer without interpreting what "right through" means. $\endgroup$
    – uhoh
    Feb 18 '18 at 3:41
  • 1
    $\begingroup$ No. The center of Uranus is hard and hot like the center of the Earth. Even just 1/10th (or less) of the way inside would crush any spaceship we know. See: en.wikipedia.org/wiki/Galileo_Probe and what-if.xkcd.com/139 but you could aerobreak around the outer edge. That's kind of going through, just not through the middle. $\endgroup$
    – userLTK
    Feb 18 '18 at 3:53
  • $\begingroup$ Basically you ask until which radius the planet can be just a manageable atmosphere? There should be data about their inner pressure and density. Through a gas giant surely not but inside yes, providing the vehicle stands dynamic P , or just P if it comes to halt. $\endgroup$
    – Alchimista
    Feb 18 '18 at 10:26
  • $\begingroup$ BTW, if there's no requirement to come out the other side in one piece, then you could pass through anything, except a black hole, if you're moving fast enough. :) $\endgroup$ Feb 20 '18 at 22:16

In short, No.

Side detail: Uranus and Neptune consist likely of 20% gas and 80% rock, coming from simple density considerations. They have large inner cores with masses around $\rm 12-14 \; m_{earth} $, and something like $\rm 2-3 \; m_{earth} $ of gas on top of them.
Jupiter and Saturn are true gas giants. They both have $\rm 5-20 \; m_{earth} $ of solids in them and the rest of the $\rm 320 \; m_{earth} $ (=Jupiter) and $\rm 95 \; m_{earth} $ (=Saturn) are gas.

However even if Jupiter and Saturn were 100% gas, you couldn't fly through them.

The main reasons for this is

  1. Enormous density, pressure and temperature. A gas mass of several tens of Earth masses will collapse under its own gravity to enormous central densities, temperatures and thus static pressures.
    The densities that this gas will be compressed to, can have densities comparable and exceeding that of water, which is a good rule-of-thumb for the center of Jupiter (typical calculations show gas densities around $\rm 20 g/cm^3$, source).
    The static pressure will crush your spacecraft, the high density will near instantly slow you down to zero due to strong friction, and the high temperatures will just melt or evaporate your spacecraft.

Just because something is gaseous, doesn't mean you can soar through it, as you would on a calm day with a glider on a light breeze.

There are other factors that complicate even getting there:

  1. Re-entry heat. The cold upper atmospheres of the gas giants increase the mach numbers $v / c_{sound}$ at re-entry by up to a factor of two. As recently discussed on space exploration the re-entry heating that a spacecraft experiences scales as the 8th power of the re-entry speed or more. So a spacecraft would experience around $2^8 = 256$ times more heat load. The engineering challenge to survive this already would be considerable.
    I bring this up, because as @uhoh pointed out, it is unclear whether you want to just slice the atmosphere or fly through the $r=0$ coordinate of the planet.

  2. Core erosion. Funny enough, the solid core is probably not even there anymore. Recent work has shown, that under the enormous pressures given at the center of a gas giant a phenomenon called 'core erosion' will take place. In this, typical rock-composing elements prefer to exist in a gaseous phase rather than in a solid phase at those high pressure, thus the solid core will dissolve into the surrounding gas.
    This will further increase friction when trying to fly through the gas.

  • $\begingroup$ Are you sure Uranus & Neptune are 80% rock? The Wikipedia article on "ice giant" says they're mostly ices, not gas, but also not rock. $\endgroup$
    – Allure
    Feb 19 '18 at 8:53
  • $\begingroup$ @Allure: Well, wikipedia is a good starting point for doing research, but does not necessarily represent absolute knowledge, or even the state of research in the respective field. Anyway, you're correct in pointing out that it's probably not 100% rock. More like a rock/ice mixture. This comes mainly from two lines of argumentation: 1.) The ice giants must have accreted most of their mass far beyond the water-ice line in our solar system, and water is very abundant in the universe $\endgroup$ Feb 19 '18 at 9:52
  • $\begingroup$ 2.) if you want to match the given mass and radius of a planet in a structural model, you need to make assumptions about the composition. Given laboratory data about the high-pressure behaviour of rocks and ices we can have a rough idea about the mixing ratio between ice and rock, but our models are far from being developed enough to point to a single, unique composition. Thus I found it justified to speak a bit sloppily here about just rock. Anyway, for the point of the question this is also irrelevant. $\endgroup$ Feb 19 '18 at 9:54
  • $\begingroup$ +1 but answers shouldn't really have wrong stuff in them, even if it's irrelevant wrong stuff; it set's a bad example for others who are new or less conscientious. If there's not a good way to show that it is likely to be rock and not anything else with high density, it would be better to not assert that it is rock. $\endgroup$
    – uhoh
    Feb 21 '18 at 6:07

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