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Consider all of the naturally-occurring objects in the solar system : in the reference frame of the object's center (ie, ignoring the orbital velocity around the sun or, in the case of satellites, their planet) and considering an average point on their surface : which object has the fastest tangential velocity?

Note - I am not asking about the rotational velocity ($\omega$), but the tangential velocity, which for a solid spherical body would be simply $\vec v=\vec \omega\times r $. Since the solar system is typically not made of up homogeneous solid spheres, I can't just look up the tables of rotational velocities and radii of the objects.

I'm looking to be surprised, so everything goes : planets, their satellites and any of the other known objects (comets, asteroids, etc) - the only condition should be that this velocity should be more or less constant (I'm not sure if bodies in highly-elliptical orbits have severe changes in rotational velocity as they approach the sun).

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Hands down it's Jupiter. It has both the largest radius and faster rotational velocity of the planets, and smaller objects will not be able to compete with that huge radius. Jupiter's rotational velocity is approximately 13.4 km/s (with some variation due to wind currents, as it is a gas), which can be determined both from Doppler shifts of reflected sunlight, and also from watching the motion of equatorial cloud features. Saturn is only 10.9 km/s, as it has a longer rotation period and a smaller radius. None of the other planets can compete with Jupiter, and smaller objects would need prohibitively short rotation periods to have any chance. For example, the Earth would need to have a day that was about half an hour long to compete with Jupiter. Even a molecule that spins millions of times a second would need a size on the order of a cm to compete with Jupiter.

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    $\begingroup$ Hi @KenG. My intuition led me there too, however both you and I are short on facts. Is there a radial velocity curve of Jupiter out there? Does the upper atmosphere (or the lower layers) rotate in synch with the core? I know that we know the length of the Jovian day... but where is the proof that it is indeed the fastest? In order to accept your answer we'd need some of this proof ☺ $\endgroup$ – Bruce Becker Jan 1 '17 at 6:36
  • $\begingroup$ There is no exact rotation period because there are belts and zones that move at different speeds, but I don't think it's even a close call. Cloud features take about 10 hours to go completely around, that is less than half an Earth day on an object that is ten times Earth radius. It's not a controversial number at the necessary accuracy needed. Saturn's day is about 7% longer with a radius 20% less, so it's not competitive. Uranus and Neptune even less so. Jupiter's speed is over 10 km/s, you won't get that with a tidally locked moon or manmade satellite. $\endgroup$ – Ken G Jan 1 '17 at 7:37
  • $\begingroup$ Thanks. I'm convinced. Just woukd be nice to have a link to some reference data, for the other skeptics out there $\endgroup$ – Bruce Becker Jan 1 '17 at 7:43
  • $\begingroup$ One can also directly measure the Doppler shift of light bouncing off the limbs of Jupiter and Saturn at their rotational equators. The following link claims the results are that for Jupiter, the speed is 13.4 km/s, and for Saturn, 10.9 km/s. shelyak.com/dossier.php?id_dossier=16&lang=2 $\endgroup$ – Ken G Jan 1 '17 at 13:31
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    $\begingroup$ @WayfaringStranger I get it to only 2 km/s for the Sun. It has a period (a "day" to be absurd) of about 600 hours, almost a month by coincidence. $\endgroup$ – LocalFluff Jan 4 '17 at 18:56

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