Light curves of 20-100 km Kuiper Belt objects using the Hubble Space Telescope

We report high-precision photometry of three small and one larger Kuiper Belt objects (KBOs) obtained with the Advanced Camera for Surveys on board the Hubble Space Telescope. The three small bodies are the smallest KBOs for which light-curve measurements are available. The object 2003 BF ₉₁ has a diameter of 20 km (assuming 10% albedo) and a 1.09 mag, 9.1 hr light curve that is feasibly explained by the rotation of an elongated, coherent body supported by material strength and best imagined as an icy outer solar system analog to asteroid (243) Ida. Two other small KBOs, 2003 BG ₉₁ and 2003 BH ₉₁ (diameters 31 and 18 km, with albedo 10%), exhibit an unremarkable light curve and no detectable photometric variation, respectively. For the larger KBO 2000 FV ₅₃ (116 km diameter, assuming 10% albedo) we strongly detect a nonsinusoidal periodic (7.5 hr) brightness variation with a very small amplitude (0.07 mag). This KBO may be nearly spherical, a result that might not be unusual in the Kuiper Belt but would be remarkable among outer solar system satellites of similar size. Light curves may be caused by variations in albedo or shape, and we carry out a study of possible physical states and bulk densities under the assumptions of both fluid equilibrium and finite, nonzero internal friction. Under most assumptions, the densities for these KBOs are likely in the range 1-2 g cm ^-3, and a plausible solution for 2000 FV ₅₃ is a rubble pile of this density that is held slightly out of the minimum-energy shape by internal friction among constituent blocks that are relatively small. Our interpretation of 2000 FV ₅₃ as a pulverized but essentially primordial object and 2003 BF ₉₁ as a collisional fragment is consistent with models of collisional timescales in the outer solar system. We compile all published KBO lightcurve data to date and compare our results to the larger population.