How big does a mass have to be before gravity

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Discussion Overview

The discussion revolves around the conditions under which a mass, such as a clump of rock, becomes spherical due to gravitational forces. It explores the relationship between mass, composition, and the resulting shape of astronomical objects, with a focus on theoretical and practical implications.

Discussion Character

  • Exploratory, Technical explanation, Debate/contested

Main Points Raised

  • One participant questions the minimum size a mass must reach for gravity to shape it into a sphere, specifically asking about clumps of rock.
  • Another participant suggests that understanding compressive strengths of minerals and densities is crucial for determining the size at which gravity overcomes structural integrity.
  • A reference to hydrostatic equilibrium is made, indicating that there is no clear threshold for when a mass becomes round.
  • Some participants argue that the transition size varies based on the composition of the object, with icy moons becoming spherical at smaller sizes compared to stony or differentiated asteroids.
  • There is mention of differing estimates for the size at which icy bodies become round, with a range of 400-600 km suggested, while stony bodies are noted to require larger sizes.

Areas of Agreement / Disagreement

Participants do not reach a consensus on a specific size threshold for when gravity causes a mass to become round. Multiple competing views regarding the influence of composition and size remain present in the discussion.

Contextual Notes

The discussion highlights limitations in defining a clear boundary for when gravitational forces dominate, emphasizing the dependence on material properties and composition.

RandyD123
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How big does a clump of rock or how big does the mass of an object have to be before gravity makes it round like a planet?
 
Space news on Phys.org
Sounds like a good exercise for learning/practicing what you know. Compressive strengths of minerals and mineral composites (rocks) run ~ 30kpsi, or 200MPa. Densities ~ 3000 kg/m3, or sp. gr. ~ 3. Pick your units. G = 6.67 x 10 -11Nm2/kg2. How large, volume or mass wise, can an accumulation of material get before surface structures crush under their own weights to less than some upper limit of radius you choose to allow for relief?
 
There shouldn't be a bright line - astronomical objects have different compositions, and their weight and strength depend on these compositions. But the transition region in the solar system seems to be in the 200-500 km ballpark. Icy moons tend to become round at lower radii than stony or differentiated asteroids.
 
The usual mentioned numbers for icy bodies is more like 400-600 km, as the Wiki ref says, while stony ones lies over that range. But YMMV.
 

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