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When we talk about orbits we often imagine a ship or station orbiting a planet outside its atmosphere.

Is it possible a ship could go into orbit inside its atmosphere (perhaps to hide from other craft) or would it be torn apart by the gravity and atmosphere of the planet?

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    $\begingroup$ I have lost many a Kerbal to this endeavour. $\endgroup$ – Aaron Lavers Jun 15 '16 at 3:20
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    $\begingroup$ @AaronLavers, best if they come compressed inside a science-station that took years to assemble... but after all, if you can float inside that atmosphere... that would not be an orbit, so forget about this. $\endgroup$ – Confused Merlin Jun 15 '16 at 10:52
  • $\begingroup$ Most answers here are telling you "no" because they conflate orbital velocity with atmospheric velocity. In principle, especially for a gas giant, there's nothing preventing them being similar. $\endgroup$ – imallett Jun 16 '16 at 7:20
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    $\begingroup$ @imallett If you have an alternative answer, then by all means please post it as an answer (instead of as a comment). Remember that comments are intended to request clarification of or suggest improvements to the post that the comment is attached to. $\endgroup$ – a CVn Jun 16 '16 at 7:35
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    $\begingroup$ what-if.xkcd.com/138 $\endgroup$ – Qwerky Jun 16 '16 at 12:11
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It would lose speed due to drag and fall in.

If you're thrusting to maintain speed, just fly like a plane and don't try to orbit. The hypersonic speed of orbital velocity would be conspicuous anyway, not a good way to hide.

Do you have any idea how fast orbital velocity is? Low Earth orbit is about 17500 miles per hour. Imagine doing that while still having air around you. The heat of "reentry" is caused by ram pressure: imagine that all the time! Now realize that around Jupiter the needed speed would be one or two orders of magnitude faster several times greater: about 94200 miles per hour as estimated by Michael Kjörling.

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    $\begingroup$ I rather doubt myself that the estimate is accurate to more than two significant figures at most, especially because the starting figure (the value for escape velocity, $v_e$) is accurate only to three significant figures and the $v_o$ calculation is only an approximation to start with. The resultant velocity does however nicely illustrate the magnitude involved. $\endgroup$ – a CVn Jun 16 '16 at 7:50
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Like JDługosz wrote, what will cause problems in the scenario you describe isn't so much your orbit as the fact that you are within the gas giant's atmosphere.

I'm going to use Jupiter here to have some specific gas giant to use for examples. Feel free to look up the relevant data for any other gas giant, or come up with your own.

For the case we are interested in, a small mass orbiting a much larger mass where the radius of the orbit is equal to the larger body's radius (just dipping your toes into the Jovian atmosphere), orbital speed can be approximated as $$ v_o \approx \frac{v_e}{\sqrt{2}} $$

The escape velocity of Jupiter is approximately 59.5 km/s, so to dip our toes into the atmosphere we get an orbital velocity of approximately $$ v_o \approx \frac{59~500~\text{m/s}}{\sqrt{2}} \approx 42~100~\text{m/s} $$

To give an idea of how freakishly fast this is, it's equivalent to approximately 152,000 km/h or 94,200 miles per hour. It gets you between the Earth and the Moon in 2.5 hours. In mid-1976, an airplane managed to get to 3,530 km/h, which is about 1/43 of the orbital speed at the edge of Jupiter's atmosphere. The best we have managed on anything resembling a repeat basis is around 2,500 km/h, or 1/60 of what you would need.

For comparison, Jupiter's wind speeds peak in excess of 150 m/s. While quite a stiff gale, that's nowhere near orbital velocity; by the above estimate, about 1/280 (and that's assuming that top wind speeds occur in the uppermost layers of the atmosphere, which might not be the case). With such a large difference between orbital speeds and wind speeds, we can largely ignore wind speeds for the purposes of this question; even in a perfect situation, wind speed will contribute less than 0.36% of the required velocity. (Interestingly enough, according to the same source, Jupiter wind speeds have a peak very near the equator, which works well for us.)

Given that Jupiter has an equatorial diameter of 142,984 km and that the circumference of a circle is $\pi d$, 42.1 km/s gives an orbital period (if you can call it orbital) of $\frac{142984 \pi}{42.1} \approx 10~700~\text{seconds}$ or just under three hours. For comparison, Wikipedia gives Jupiter's sidereal rotation period ("day") of 9.925 hours (a shade over 9 hours 55 minutes).

For comparison, to get into a reasonably stable low Earth orbit you need a velocity of approximately 7.8 km/s (corresponding to an orbital period of about 90 minutes). To go to the Moon (which is pretty close to escape velocity), you need about 10.5 km/s relative to the Earth. Actual Earth escape velocity is 11,186 m/s. Compare Apollo by the numbers: Translunar Injection and look at particularly the Earth Fixed velocity figures for the various lunar missions.

Let's say you can somehow handwave the issue of absolute speed away. (After all, you got there somehow, and that already takes quite a bit of speed.) Let's also say that your craft is a very, very long, perfect cylinder with a forward cross section of 1 square meter, built to handle constant hurricane-level wind speeds. Every second, you are moving through 42,100 meters of atmosphere. That means that every second, your craft will need to push aside 42,100 cubic meters of atmospheric gases while maintaining its speed (at least if you plan on staying at that altitude). Wikipedia gives the composition of Jupiter's atmosphere as approximately $89.8 \pm 2.0 \% ~\text{H}_2$ and $10.2 \pm 2.0 \% ~\text{He}$. Despite the fact that these two gases are among the lightest known, and that the density is going to still be low at the altitude we are talking about, pushing aside over 40,000 cubic meters of gas per second is going to cause some massive drag.

And that, my friend, is what will cause your craft to heat up, lose speed very quickly and eventually descend into the atmosphere, ruining your day.

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  • $\begingroup$ My back of the envelope was that the rate at which I fall is about the same (1 g) but I need to travel an order of magnitude farther to fall around the limb of the planet. $\endgroup$ – JDługosz Jun 15 '16 at 2:13
  • $\begingroup$ What if a gas giant was rotating extremely rapidly, and had a diffuse atmosphere extending out to near the point where orbital velocity is not too much higher than rotational velocity? Obviously there won't be atmosphere right out to that point, since that's the point where material is flung outwards. Rotating that fast would make it closer to a disc than a sphere. This is probably not plausible, since having atmospheric pressure holding an atmosphere up close to the limit is probably an unstable equilibrium, if there's any equilibrium at all. Ejection or gravitational collapse. $\endgroup$ – Peter Cordes Jun 15 '16 at 15:14
  • $\begingroup$ @PeterCordes The atmospheric gases would be extremely diffuse at that point, to the point that you could barely argue that you are in the atmosphere at all. (Remember, LEO Earth satellites experience drag due to Earth's atmosphere, to the point that the ISS needs periodic reboosts every few months to maintain its altitude.) I also have doubts that a gas giant would hold together under such conditions. And of course, unless the rotational vector aligns perfectly with the orbital vector you are still going to have problems. $\endgroup$ – a CVn Jun 15 '16 at 15:18
  • $\begingroup$ Yes, of course this only works at the equator. I'm talking about a situation extremely different from Earth or Jupiter, where the rotation is so fast that the outer-most layers of the atmosphere are nearly orbiting. The question is whether it's possible under any circumstances to have some not-too-diffuse atmosphere moving at only a couple thousand meters per second below orbital velocity. It's certainly plausible that there's no way for this to be stable, but it's not obvious. $\endgroup$ – Peter Cordes Jun 15 '16 at 15:26
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    $\begingroup$ @PeterCordes At that point, if the gasses were nearly in orbit, they would bleed off into space because of stellar winds $\endgroup$ – wedstrom Jun 15 '16 at 16:00
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While JDługosz and Michael are scientifically correct with their answers, I think we are overlooking something here.

If your fictional spaceship can sustain flight indefinetely, it can go inside the atmosphere and fly around the planet on its own. This is not the same as orbiting, as it requires powered maneuvers, but more like flying a plane on Earth. The conditions of the atmosphere of gas giants we know of is a lot different from Earth's own, though, so this is going to take some handwaving or more serious research. But with sufficiently advanced technology, this would be doable on Jupiter.

XKCD has something to say about that:

Our Cessna can’t fly on Jupiter; the gravity is just too strong. The power needed to maintain level flight is three times greater than that on Earth.

If your spaceship is approximately the size and weight of a Cessna 172 Skyhawk, and can provide three times as much thrust as that airplane, then it can (probably) fly safely on Jupiter (just be careful you don't enter any storms). For larger, heavier ships, it's just a matter of scaling the power up as needed.

Now you might say this brings another problem into question, you'll need continuous application of power. And if you won't take renewable power sources that you can use while in there, this strategy is a poor one. Indeed, that's why you wanted to be in orbit in first place - you don't want to spend fuel to keep your craft flying inside that atmosphere.

Alright. Just pop one or more balloons open. Granted, you might need a lot of really large balloons, but again... You would be able to hide inside Jupiter's a gas giant atmosphere, while largely avoiding falling into the planet core. Just be careful to avoid any storms.

Edit: as JDługosz and Ross pointed out, a balloon filled with Hydrogen or Helium in the atmosphere of Jupiter would not be buoyant, unless you could heat these gases to be hotter than the surrounding atmosphere, and even then it might not be buoyant enough. This balloon idea would very probably not work on the gas giants of our solar system... It would work on a fictional gas giant that has an atmosphere composed of heavier elements, though. I got this inspiration from a videogame (Windforge) for which the world is a gas giant with an nitrogen/carbon dioxide/oxygen atmosphere, and the player and NPC's navigate through it in airplanes and zeppelins. So... Some parts of handwaving and suspension of disbelief may be necessary for this idea to be considerable.

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    $\begingroup$ JDługosz already discussed flying through the atmosphere in a manner similar to an airplane. That's hardly rocket science. (Haha.) $\endgroup$ – a CVn Jun 14 '16 at 22:01
  • $\begingroup$ Yes, but he suggests flying at orbital speed (which would be more kerbalistic than realistic :D). I am suggesting slow, regular everyday on Earth flight instead of orbiting inside an atmosphere. $\endgroup$ – Renan Jun 14 '16 at 22:05
  • $\begingroup$ I said "and don't try to orbit" and don't be conspicuous with such high speed. $\endgroup$ – JDługosz Jun 15 '16 at 2:24
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    $\begingroup$ Balloons filled with what in a hydrogen atmosphere? The only thing bouyant would be hot hydrogen. So you need power for your hot air balloon anyway. $\endgroup$ – JDługosz Jun 15 '16 at 2:26
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    $\begingroup$ If you harvested the hydrogen for fuel, then fuel would be prevalent. However, eventually you would use enough of it to create an area of lower density, thus giving yourself away to a race with advanced enough technology. You would be invisible at a distance. $\endgroup$ – cybernard Jun 16 '16 at 4:00
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Hide in orbit

Hiding spacecraft is really easy because space is so big. Jupiter already has a lot of rubble orbiting it, if you orbit with the rubble, and turn off your engines and radio, no one will notice another piece of rubble, especially if your ship is dark grey and dull.

Turn the heating low and insulate well to hide better in infra red, but as the rocks are heated tidally and by collisions, even a warm spacecraft will be non obvious.

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    $\begingroup$ Europa has a surface temperature of around 50-110 K. That's pretty cold, even for the exterior of a spacecraft. $\endgroup$ – a CVn Jun 15 '16 at 12:19
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    $\begingroup$ @MichaelKjörling Io on the other side is almost melting... $\endgroup$ – Tomáš Zato Jun 15 '16 at 12:48
  • $\begingroup$ See this question on how to hide in orbit. $\endgroup$ – JDługosz Jun 15 '16 at 15:42
  • $\begingroup$ Look at how I edited your formatting. You don’t need br tags; use blank lines for paragraphs; there is a toolbar button for headers. $\endgroup$ – JDługosz Jun 15 '16 at 15:44
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Only if you were to somehow be able to make the vessel immune to the friction of atmosphere or allow it to phase through matter. In the former, you'd probably have to continue to add some kind of propulsion.

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    $\begingroup$ No need to hide from anyone if you can phase through them :) $\endgroup$ – Diligent Key Presser Jun 15 '16 at 4:52
  • $\begingroup$ @DiligentKeyPresser If you're going to allow things like phasing through matter, why not just build your ship out of dark matter and let it orbit through anything? Boarding it might become difficult, though. $\endgroup$ – Ross Presser Jun 15 '16 at 15:21
  • $\begingroup$ Depends on what kind of tech those following have that involve phasing and phased matter too. :) $\endgroup$ – liljoshu Jun 17 '16 at 6:23
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Simply put, the difference between orbiting and flying is the friction caused by the atmosphere.

When you "orbit" you fall fast enough that you miss the ground, and slow enough that you don't go zooming off in a direction.

Flying uses the pressure of the atmosphere and the speed your going to generate lift.

If you were to go fast enough to miss the ground in the atmosphere, you would have so much friction with that atmosphere that you would slow down and hit the ground. The term is usually referred to as Air-Breaking. You can go fast enough to "skim" the atmosphere a little, then end up with a lower aspoapsis but if you stay in the atmosphere too long you just keep slowing down.

If you apply continuous thrust, then your flying. And your objective changes from going fast enough to miss the ground, to fast enough to generate lift.

So you can't orbit inside an atmosphere. You could, however, in theory, go from low orbit to high altitude flight, and then back to a low orbit after a while. It would be pretty hard on a space plane though, and waste a TON of fuel.

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    $\begingroup$ Braking, not breaking. Air-breaking sounds like breaking wind :P $\endgroup$ – Peter Cordes Jun 15 '16 at 15:01
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    $\begingroup$ @PeterCordes What's wrong with breaking wind instead of braking wind? ;-) $\endgroup$ – a CVn Jun 15 '16 at 15:07

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