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Nereid is a small irregular moon of Neptune that displays large-, moderate-,and small-amplitude photometric variations on both fast and slow time scales.The central mystery of Nereid is now to explain the physical mechanism of theseunique brightness changes and why they change with time. To characterizeNereid's variability, we have been using the SMARTS telescopes on Cerro Tololofor synoptic monitoring from 1999 to 2006. We present a well-sampledphotometric time series of 493 magnitudes on 246 nights mostly in the V-band.In combination with our earlier data (for 774 magnitudes over 362 nights), our20-year data set is the most comprehensive for any small icy body in our SolarSystem. Our yearly light curves show that Nereid displays various types ofbehaviors: large amplitude brightenings and fadings (1987 to 1990);moderate-amplitude variation about the average phase curve (1993-1997, 2003,2005), moderate-amplitude variation and systematically brighter by roughlyone-quarter magnitude throughout the entire season (2004); and nearly constantlight curves superimposed on a surprisingly large-amplitude opposition surge(1998, 1999, 2000, 2006). Other than in 2004, Nereid's variations were closelycentered around a constant phase curve that is well fit with a Hapke model forthe coherent backscattering opposition surge mechanism with angular scale of0.7+-0.1 degrees. In our entire data set from 1987-2006, we find no significantperiodicity. We propose that the year-to-year changes in the variability ofNereid are caused by forced precession (caused by tidal forces from Neptune) onthe spin axis of a nonspherical Nereid, such that cross-sectional areas andaverage albedos change as viewed from Earth.

Nereid: Light Curve for 1999-2006 and a Scenario for its Variations