= Kerr ISCO spin limits
{c}
For equatorial test-particle circular orbits in the extremal-spin limit, the prograde <Kerr black hole> <innermost stable circular orbit> has Boyer-Lindquist radius tending to $GM/c^2$ and specific <energy> tending to $1/\sqrt3$. The retrograde limit has radius $9GM/c^2$ and specific <energy> $5/(3\sqrt3)$. The prograde radius approaching the horizon coordinate does not make the massive limiting orbit identical to the null horizon generator.<Rotating Black Holes: Locally Nonrotating Frames, Energy Extraction, and Scalar Synchrotron Radiation by James M. Bardeen and others>[§II, Eqs. 2.20–2.23, pp. 351–353]{ref}
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