The Aerobraking Trajectory Simulator is a Python-based modeling and simulation tool developed to assess flight performance for the aerobraking mission set. The tool is open-source for the benefit of the entire aerospace community. The simulator can be used to model an entire aerobraking campaign, a single orbit, or a single atmospheric passage around Earth, Mars, and Venus, while providing estimates of important flight performance parameters, including dynamic pressure, heat rate, and energy depletion rate. The computational efficiency of the tool also enables uncertainty analysis through Monte Carlo techniques. The built-in modularity of the tool enables selection of physical models from a range of built-in options, including a method to efficiently incorporate complex atmospheric models, such as Mars-GRAM 2010. The development of the simulation is discussed including modeling assumptions. Example trajectory analyses are presented to illustrate the effectiveness of the tool, including, an Odysseylike mission with and without the use of trajectory control and a blunted-cone ballistic flight.