The Smith Cloud: Past, Present, and Future

The Smith Cloud is a prominent high-velocity HI cloud with a well constrained distance that allows us to derive many of its physical properties. It contains several million solar masses of neutral and ionized gas, and is on a collision course with the Milky Way disk (Lockman et al. 2008, ApJ, 679, L2; Hill et al. 2009, ApJ 703, 1832; Hill et al. 2013, ApJ, 777, 55; Fox et al. 2016, ApJ, 816, L11). We have analyzed new 21cm HI data from the Green Bank Telescope (GBT) that cover hundreds of square-degrees around the Smith Cloud. They reveal previously unknown components of the Cloud that stretch out over a wide area. In all, the Cloud appears to extend more than 40 degrees across the sky, spanning Galactic latitudes from -40 degrees all the way through the Galactic plane to positive latitudes, with a continuity of velocity. We will discuss the results of fitting trajectories to the Cloud components that establish limits on its total velocity and trajectory through the Milky Way halo: it's past, present and its future.