Arenstorf orbits are closed trajectories of the restricted three-body problem. That is, two bodies of masses m and 1-m moving in a circular rotation, and a third body of negligible mass moving in the same plane.
The computation of these orbits is very sensitive to small errors and are a good test for the accuracy of numerical methods for solving Ordinary Differential Equations.
The simulation displays the results of solving the equations with a fixed 4-th order Runge-Kutta method, and an adaptive-step Runge-Kutta-Fehlberg. The results are much better due to the adaptive nature of the RKF method.