# Equation help - Orbit

## Main Question or Discussion Point

Hi all,

I have a gravitationally bound rubble pile asteroid. This asteroid reaches and exceeds 'critical' spin where centripetal force exceeds gravitational force and surface material is launched off the surface.

At what minimum initial velocity would the surface material need to be to achieve a circular orbit? Just looking for the equation or a reference site.

Cheers

David

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tiny-tim
Homework Helper
Hi David!

As soon as it exceeds critical spin, any loose rock (on the top of the highest "mountain", say) will go into circular orbit (hopefully just missing the mountain on its return!).

(In practice, it'll collide will other rocks, and either crash, or get thrown out into some random elliptical orbit, where it will continue to collide with other rocks and eventually they'll all settle down to some regular pattern.)

I though this to, but then I might have confused myself. ie for my object, at Critical spin, the surface of the primary is travelling at 2.28 m/s. BUT escape velocity is 3.28 m/s. At the surface Fg = 0.00706 m/s^2. So when I spin fast enough for Fc = 0.00706, The surface object is travelling at 2.28 m/s and will effectively just be weightless on the surface where it is, higher velocities will actually launch it.

BUT, if the object is now disconnected from the parent, Fg will still be acting on it, the more distant the object gets from the parent the slower it's orbital velocity and thus like a projectile it should return to the surface so I am assuming this all has something to do with Angular Momentum BUT I am having trouble connecting all the dots.

Cheers

David

tiny-tim