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The term **innermost stable circular orbit** (**ISCO**) is used
for black holes and neutron stars:
an orbit at the distance from the center of the object (the **ISCO-radius**)
immediately beyond that for which general-relativity effects make such an orbit unstable.
The rotation of the host object affects this distance,
but for the simple case of no rotation:

r_{ISCO}= 6GM/c² = 3R_{S}

- r
_{ISCO}- ISCO-radius. - G - gravitational constant.
- M - mass of the object.
- c - speed of light.
- R
_{S}- Schwarzschild radius.

http://en.wikipedia.org/wiki/Innermost_stable_circular_orbit