Light Curves of 20-100 km Kuiper Belt Objects Using theHubble Space Telescope

We report high precision photometry of three small and one larger Kuiper BeltObjects (KBOs) obtained with the Advanced Camera for Surveys onboard the HubbleSpace Telescope (ACS/HST). The three small bodies are the smallest KBOs forwhich lightcurve measurements are available. 2003 BF91 has a diameter of 20kilometers (assuming 10% albedo) and a 1.09 magnitude, 9.1-hour lightcurve thatis feasibly explained by the rotation of an elongated, coherent body that issupported by material strength and best imagined as an icy outer Solar Systemanalog to asteroid (243) Ida. Two other small KBOs, 2003 BG91 and 2003 BH91(diameters 31 and 18 km, with albedo 10%), exhibit an unremarkable lightcurveand no detectable photometric variation, respectively. For the larger KBO 2000FV53 (116 km diameter, assuming 10% albedo) we strongly detect a non-sinusoidalperiodic (7.5 hours) brightness variation with a very small amplitude (0.07mag). This KBO may be nearly spherical, a result that might not be unusual inthe Kuiper Belt but would be remarkable among outer Solar System satellites ofsimilar size. We carry out a study of possible physical states and bulkdensities under the assumptions of both fluid equilibrium and finite, non-zerointernal friction. The densities for the these KBOs are likely to be in therange 1--2 g/cm3, and a plausible solution for 2000 FV53 is a rubble pile ofthis density that is held slightly out of the minimum-energy shape by internalfriction among constituent blocks that are relatively small. Our interpretationof 2000 FV53 as a pulverized but essentially primordial object and 2003 BF91 asa collisional fragment is consistent with models of collisional timescales inthe outer Solar System. We compile all published KBO lightcurve data andcompare our results to the larger population. [abridged]Comment: AJ, in press. Tables 1-4 will be electronic only in published version but appear here in full. Figures 1,3,5 in colo