kessler is a Python package for simulation-based inference and machine learning for space collision avoidance and assessment. It is named in honor of NASA scientist Donald J. Kessler known for his studies regarding space debris and proposing the Kessler syndrome. The package can parse, analyze, and plot conjunction data messages (CDMs), offers tools for training recurrent neural networks to predict CDMs, and includes a simulator to generate synthetic CDMs and perform inference over observed data.