The Shape of Nereid's Phase Function Changed From 1998 To 1999 and 2000

We present well-sampled light curves of Nereid for 1999 and 2000 which show symmetric shapes centered on opposition as is characteristic of a large opposition surge. Surprisingly, the phase functions for 1999 and 2000 are significantly different from the well-measured phase function for the year 1998, with the 1999 and 2000 curves being more peaked at low phase angles and substantially brighter at high phase angles. We know of neither precedent nor explanation for this mystery on Nereid.