This research involves the development of a full 3-dimensional nonlinear model of a tethered aerostat, with the aim of conducting a preliminary feasibility assessment of Very High Altitude Tethered Balloons (VHATBs) under both steady-state and extreme operational conditions. A discretized lumped-parameter model is used to represent the tether. HATBs have a variety of potential applications ranging from telecommunications to solar power harvesting, but none have been practically implemented to date. The application considered in this research is Stratospheric Particle Injection for Climate Engineering for which the setup comprises of several unique features that are expected to have an impact on the system’s dynamics and therefore its technical feasibility. An operational altitude of 20km is recommended for this application, which makes it the largest scale tethered aerostat considered to date. A hollow pipe-like tether is required for the transportation of the high-pressure fluid to the stratosphere where it is expected to scatter radiation. In order to qualify for long-duration operation, HATBs must be able to withstand extreme weather conditions on top of the harsh steady-state winds the tether would be subjected to. In this research, a series of dynamic simulations are conducted to assess the system’s safety in the face of a range of potential 3-dimensional disturbances. The balloon’s altitude deviation and the tether’s maximum longitudinal stress relative to that of the system’s equilibrium are considered the critical parameters that are used to quantify the system’s response, as these are considered the primary risks of failure. The use of optimal control methods to minimise these is proposed and introduced into the dynamic simulations. Previous methods proposed for the altitude stabilization of tethered aerostats are not practical for the scales considered in this research, and so the horizontal motion of the ship the tether is mounted to is suggested as an alternative control input. A preliminary assessment of the potential use of this type of control to reduce failure risks in the face of extreme wind disturbances is provided. The closed-loop performance, practicality and robustness of the controller are considered in this analysis. The critical parameters are found to be reduced by ∼ 65% for in-plane disturbances and ∼ 90% for out of plane disturbances through the implementation of controllers that are deemed both practically feasible and sufficiently robust.