Time-Resolved Photometry of Kuiper Belt Objects: Rotations, Shapes and Phase Functions

We present a systematic investigation of the rotational lightcurves of trans-Neptunian objects based on extensive optical data from Mauna Kea. Four of 13 objects (corresponding to 31%) in our sample ((33128) 1998 BU48, 2000 GN171, (20000) Varuna and 1999 KR16) were found to exhibit lightcurves with peak-to-peak range > 0.15 magnitude. In a larger sample obtained by combining our data with reliably determined lightcurves from the literature, 7 of 22 objects (32%) display significant (> 0.15 magnitude range) lightcurves. About 23% of the sampled objects have lightcurve ranges > 0.4 magnitudes. Curiously, the objects are very large (> 250 km diameter, assuming an albedo of 0.04) and, in the absence of rotation, should be near spherical due to self compression. We propose that the large amplitude, short period objects are rotationally distorted, low density rubble piles. Statistically, the trans-Neptunian objects are less spherical than their main-belt asteroid counterparts, indicating a higher specific angular momentum perhaps resulting from the formation epoch. In addition to the rotational lightcurves, we measured phase darkening for 7 Kuiper Belt objects in the 0 to 2 degree phase angle range. Unlike Pluto, the measured values show steep slopes and moderate opposition surge indicating backscatter from low albedo porous surface materials.