SPARTA direct simulation Monte Carlo (DSMC) simulator. SPARTA is a parallel DSMC code for performing simulations of low-density gases in 2d or 3d. Particles advect through a hierarchical Cartesian grid that overlays the simulation box. The grid is used to group particles by grid cell for purposes of performing collisions and chemistry. Physical objects with triangulated surfaces can be embedded in the grid, creating cut and split grid cells. The grid is also used to efficiently find particle/surface collisions. SPARTA runs on single processors or in parallel using message-passing techniques and a spatial-decomposition of the simulation domain. The code is designed to be easy to modify or extend with new functionality. SPARTA is distributed as an open source code under the terms of the GPL, or sometimes (by request) under the terms of the GNU Lesser General Public License (LGPL). The current version can be downloaded here. SPARTA was developed at Sandia National Laboratories, a US Department of Energy (DOE) laboratory. The chief authors are Steve Plimpton and Michael Gallis who can be contacted at sjplimp at sandia.gov and magalli at sandia.gov respectively. Funding for SPARTA development has come from the DOE.