ExESS (Extraterrestrial Exospheres and Surfaces Simulations) is the project title under which numerical models and simulations are created to analyze exospheres and surface of extraterrestrial bodies in our solar system. While several models of the exospheres and surfaces individually exist, their interaction with one another is the focus of ExESS.

While not all of the bodies in our solar system can be modeled this way, those with a collisionless atmosphere, also called a surface-bounded exosphere, can be. Several targets like this exist, the most prominent one being Earth's Moon. Others include Mercury and asteroids like Ceres. 

The model is written entirely in the high-performance computer language Julia and is continuously updated and extended to cover more targets as well as to include further physical and geochemical mechanisms. The Code is published on GitHub.