The development of a flexible, high-fidelity, generic simulation of transatmospheric and interplanetary motion is described. The simulation incorporates aerodynamic and gravitational force modelling implemented in a Cartesian reference co-ordinate set. Propagation of the motion of a vehicle is carried out in a "working" reference frame whose origin is determined by the current gravitational sphere of influence. A semi-analytic model of planetary motion propagates the motion of the nine planets and six major moons, allowing simulation at any point within the solar system. Expansion and improvement of the model is facilitated through the vector formulation of the problem. The use and applicability of the method of matched asymptotic expansions is examined as a means of producing high quality trajectory predictions quickly and easily. Ballistic launch and entry trajectories are considered incorporating a velocity dependent model for the aerodynamic drag coefficient. Using the derived relations direct launch is considered as a low-cost means of transporting acceleration insensitive payloads to a space station in low Earth orbit. In addition, it is shown that the high quality trajectory predictions may be obtained using a simple spreadsheet package. Analytic modelling is also used as the basis of a highly robust, computationally efficient, controller design for autonomous aerocapture in the context of the lunar return problem. The validity of this approach to lunar return is examined and found to be of considerable potential in both its robustness and the potential improvements in payload mass-fraction available through the substantial fuel savings over direct return to Earth or propulsive return to a space station. The study shows that, using the derived control, the aerocapture manoeuvre can be successfully performed with existing material and technological capabilities.