Observations of Halley's comet by the Vega and Gioto spacecraft, as well as ground based studies in the 1980's showed two nuclear rotation periods of this potato-shaped comet nucleus. A 2.2 - 2.4 day period, and a 7.4 day period. Not enough is really known about other comets to unambiguously determine a rotation period, but their periods are most likely the same...to within an order of magnitude!!!!