Inspired by this thread, what’s the most spherical object in our solar system? Most large bodies are oblate spheroids, so which is the least oblate?
If it makes things easier, let’s say things that have a mean radius of greater than 10km (or 100km, if that makes things easier).
As for how to define “most spherical”, I guess we can use sphericity.
Thanks. That was fast.
Ok, let’s go with “what’s the most spherical astronomical object in our solar system that isn’t the sun?”
And to make it slightly more interesting, let’s divide that into objects that are greater than 100km in radius and smaller than 100km in radius.
Probably Venus, though Mercury is close
…erm WAG: Puck
Where does Europa rank? I’d think it’d be rather high, given it’s covered with a world-spanning ocean. I have seen, but cannot link, due to this friggin tablet, sites that mention it is basically flat except for a few rapidly eroding impact craters.
I don’t think the main factor is topography (mountains, oceans, etc.), but rather overall shape. In other words, most “spherical” planets and moons are oblate enough that the topography is relatively unimportant for this question.
I recall reading back in the 1980’s that NASA had launched a satellite containing some small glass spheres (maybe a couple inches in diameter) for a study about the age or shape of the universe, or something like that. Anyway, they were said to be the most spherical objects ever created; the claim was that if you expanded one to the size of the earth, the deviation from perfect sphericity would have been no more than a few inches.
Since they’ve been into space, does that make them astronomical objects?
They’ve built really spherical ones in 2004 for Gravity Probe B. Perfect within 40 atoms! Maybe that’s the one you are thinking of.
This one looks like the current record holder.
Cool! I think they need to clarify that “if the balls were blown up to Earth size, the biggest ‘mountain’ would be four feet high,” they mean IF the Earth were a sphere (but retained its topography) rather than oblate.
I’m not sure that clarification adds anything. The Earth-size glass would be much less oblate than the real Earth, as well as having smaller topography. But the Earth doesn’t need to be a perfect sphere to imagine blowing a small sphere up to ‘Earth size.’
Of course, a nitpicker would start asking how you compare their sizes, referencing the polar diameter of the Earth or the equatorial diameter or whatever. 
Actually, Europa is probably prolate, rather than oblate, since it’s mainly distorted by the tides from Jupiter.
WAG is Venus…large object, doesn’t rotate fast.