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I'm trying to work out what the surface gravitational field strength of an irregularly shaped body would be (for example Mars' moon Phobos). I know that for a sphere, any point outside it can consider all the mass to be at a point inside it, but for something that's potato shaped, how would you work out what g is?

For example, Phobos is a max of 27km long, and a min of 15km. Gravity would be weaker the further away you are from the centre, and stronger the closer you are. I'm presuming it's more complicated than just plugging the relevant radii in to Newton's Gravitational Equation, and probably involves some horrible integrals. Is there any simple equation for an ellipsoid which approximates irregularly shaped objects like these?

Any links to further reading material would be appreciated.

(For anyone interested, I'm trying to work out the best place to jump from a moon of Mars to the surface).

Thanks all

Don