When we look at larger objects (the Moon, Mars, Earth), they are pulled into spherical shapes by their own gravity.

The question I have relates to smaller objects... those close to the threshold at which gravity pulls them into spherical shapes. Early in their evolution, they are irregular shapes. As more material accretes, something happens - perhaps somewhere between the size of Vesta and Ceres - even while the surface gravity is tiny compared to Earth - the accretion process appears to produce a spherical body. Why doesn't a body need to get much bigger - say Moon sized, for that to happen? What is going on that is already taking hold at that size?

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    $\begingroup$ Conjecture: Once enough micrometeorites have hit the body, the 'pointy bits' are mostly worn off, some of results of those collisions collects as dust in the shallows to fill them in. $\endgroup$ – Andrew Thompson Jun 21 '15 at 8:14
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    $\begingroup$ Perhaps you are over-estimating the amount of gravitational force needed to pull the body into a spherical shape? $\endgroup$ – jamesqf Jun 21 '15 at 18:47
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    $\begingroup$ spaceanswers.com/deep-space/… $\endgroup$ – PearsonArtPhoto Jun 22 '15 at 0:59

The size required to do this is a function of the materials that make up the object. There are a number of articles on the internet that explain what it takes. The bottom line is, the stronger the material, the more of it it takes to make the object spherical. Imagine if there was a mountain 500 miles high on Earth. It would require tremendous strength to stand up to the pressure from the rest of the planet trying to pull it back down, and in fact, this wouldn't work for very long. Basically, the mountain will flatten itself out, spreading it's pressure over a larger area.

Owing to the above, every planet (Or asteroid, etc) has a maximum height for mountains. Further physics has an article that explains this, for a purely vertical block. For a triangular shaped block, the distance is about twice as much. Mount Everest is close to the maximum theoretical height for a mountain on Earth, although slightly higher is possible. Smaller planets allow for higher mountains. Eventually, one reaches a point where it doesn't matter how high a mountain is, it won't make a difference. That is the point where an object is considered near spherical.

It turns out that the size where one can't have mountains larger than the object turns out to be size at which an object will be spherical. Ceres, for instance, could theoretically have a mountain 177 km high, which is well below its size. Vesta, on the other hand, could support a mountain 234 km high, which is basically the same size as its radius of 260 km. Thus, Vesta is nearly spherical, but can have some significant deviations, as can be observed in the images taken by Dawn.

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