Abstract
We determine the period, p = 11.52 ± 0.14 hr, and a light-curve peak-to-peak amplitude, a = 0.029 ± 0.003 mag, 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 2 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 in work of Schaefer & Schaefer and of Dobrovolskis. 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 light curve found in this Letter 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.