Hello. In my book with examples of astronomy, I found this: Calculate, around which the known celestial bodies in the solar system (with a diameter greater than 1000 km) do a probe one complete cycle fastest (without the use of engines). Other movements of these bodies is negligible. If cycle must be fast, it must be a celestial body with small diameter (but greater than 1000 km). It is Tethys (1060 km) or something smaller? It is true that the circulation period is equal to: 2 * π * (radius of celestial body + height probe above the surface) / speed the probe It is good? Sorry for my bad English.