Innermost stable circular orbit

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The innermost stable circular orbit (often called the ISCO) is the smallest marginally stable circular orbit in which a test particle can stably orbit a massive object in general relativity.^[1] The location of the ISCO, the ISCO-radius (${\displaystyle r_{\mathrm {isco} }}$), depends on the mass and angular momentum (spin) of the central object. The ISCO plays an important role in black hole accretion disks since it marks the inner edge of the disk.

The ISCO should not be confused with the Roche limit, the innermost point where a physical object can orbit before tidal forces break it up. The ISCO is concerned with theoretical test particles, not real objects. In general terms, the ISCO will be far closer to the central object than the Roche limit.