Orbital Motion
For more details on this topic, see Schwarzschild geodesics.A particle orbiting in the Schwarzschild metric can have a stable circular orbit with . Circular orbits with between and are unstable, and no circular orbits exist for . The circular orbit of minimum radius corresponds to an orbital velocity approaching the speed of light. It is possible for a particle to have a constant value of between and, but only if some force acts to keep it there.
Noncircular orbits, such as Mercury's, dwell longer at small radii than would be expected classically. This can be seen as a less extreme version of the more dramatic case in which a particle passes through the event horizon and dwells inside it forever. Intermediate between the case of Mercury and the case of an object falling past the event horizon, there are exotic possibilities such as "knife-edge" orbits, in which the satellite can be made to execute an arbitrarily large number of nearly circular orbits, after which it flies back outward.
Read more about this topic: Schwarzschild Metric
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