The Short Rotation Period of Nereid

We determine the period, p = 11.52± 0.14 h, and a light curve peak-to-peak amplitude, a = 0.029 ± 0.003 magnitudes, of the Neptunian irregular satellite Nereid. If the light curve variation is due to albedo variations across the surface, rather than solely to the shape of Nereid variations, the rotation period would be a factor of two shorter. In either case, such a rotation period and light curve amplitude, together with Nereid’s orbital period, p = 360.14 days, imply that Nereid is almost certainly in a regular rotation state, rather than the chaotic rotation state suggested by ???). Assuming that Nereid is perfectly spherical, the albedo variation is 3% across the observed surface. Assuming a uniform geometric albedo, the observed cross sectional area varies by 3%. We caution that the lightcurve found in this paper only sets limits on the combination of albedo and physical irregularity and that we cannot determine the orientation of Nereid’s spin axis from our data. Subject headings: planets and satellites: individual (Nereid) Visiting Astronomer, Cerro Tololo Inter-American Observatory. CTIO is operated by the Association of Universities for Research in Astronomy (AURA) Inc., under contract with the National Science Foundation.