Program: GN-2012B-Q-85

We propose to use GMOS-N in queue mode to measure the rotational light curve of the binary object Sila-Nunam, a member of the Kuiper Belt that likely formed insitu beyond Neptune. This binary was recently discovered to show mutual events - one body eclipsing the other every half orbital period of 6.25 days. Such events present a rare opportunity to determine the size, density, and composition of a primitive solar-system body, largely unaltered since the time of solar-system formation. Our primary goal is to make the first precise measurement of the shape, period, and phase of the full rotational light curve. This will determine if the rotations of Sila and Nunam are tidally locked to their mutual orbit. This is essential to understanding the physical parameters of the binary revealed by ongoing and proposed eclipse observations with other telescopes. A secondary goal is to measure the color dependence of the light curve. This will determine if the target bodies have heterogenous composition, and test for the exchange of meteroid impact ejecta predicted for close-separation binaries. Our total request is 6.5 h, spread over 13 consecutive, non-photometric grey/dark nights , with a minimum requirement of 3.9h.