Human space exploration is entering a new era through the design of missions that venture further from earth for longer durations, such as missions to the Moon and Mars. Mission success during long duration exploration missions (LDEM) requires astronauts to flawlessly execute exceptionally complex skills across a multitude of tasks, presenting new challenges for training. This thesis investigates virtual reality (VR) as a potential platform to support critical skill training and maintenance during LDEM. First, a VR Entry, Descent, and Landing (EDL) training scenario was created to support the investigational aims of this research. EDL represents a highly complex task that requires extensive preflight training and carries extreme consequences of failure if skill is not properly maintained over time. A collection of design objectives drove the development of the training scenario, including complex subtask integration, algorithm facilitation of adaptive difficulty, a sense of immersion and presence, allowing for repeated training without predictability or boredom, and providing feedback to support learning. The EDL task was created in a modular format that allowed for presentation in immersive VR, non-immersive screen, and physical mockup modalities, as well as with static or adaptive difficulty. In the second aim, the EDL task was then investigated in a collection of between-subjects experiments. In the first experiment, results showed promising trends that adaptive VR training supports higher skill gain and less plateau in learning as well as higher average performance when transitioning from virtual to physical task performance. In the second experiment, facilitation with an adaptive algorithm was found to significantly improve task mastery as opposed to self-led difficulty selection or static difficulty training. Next, simplified corollary task training was found to support retention of skill only if the cognitive demands of that simplified task were appropriately mapped to those of the operational task. And finally, just-in-time training was found to be an effect method to mitigate skill loss while preventing the risks of negative training. The work presented in this thesis found that VR is a valuable tool for the maintenance of mission-critical skill during LDEM. However, skill maintenance is best supported through design of VR training that utilizes adaptive algorithms to facilitate learning, presents content with cognitive demands that align to the tasks they seek to train, and when refresher training is administered in a JIT cadence. [The dissertation citations contained here are published with the permission of ProQuest LLC. Further reproduction is prohibited without permission. Copies of dissertations may be obtained by Telephone (800) 1-800-521-0600. Web page: http://www.proquest.com/en-US/products/dissertations/individuals.shtml.]