A Conceptual Design and Concept of Operations for Intermittent Short-Radius Centrifugation for Artificial Gravity

Existing methods to mitigate spaceflight-induced deconditioning employ a suite of countermeasures that are only partially effective and largely inefficient; several countermeasures are required to maintain physiological function during flight and ensure astronaut health upon return.  To fill the void in this piecemeal approach, we propose optimized conceptual design parameters for short-radius, intermittent artificial gravity (AG). An integrated, comprehensive countermeasure, AG has the potential to mitigate deconditioning in multiple physiological systems concurrently.  However, there has historically existed a lack of effective and feasible baseline architecture for AG implementation.
This pre-existing gap was narrowed with the completion of three Specific Aims. In the first two aims, we investigated human acclimation to the “Coriolis” cross-coupled (CC) illusion, a tilting or tumbling sensation experienced after executing head movements during centrifugation.  The human tolerability concerns associated with the CC illusion have historically limited AG spin rates to approximately 4-6 rotations per minute (RPM), thus preventing the implementation of a more technologically and financially feasible short-radius approach.  We performed a series of 7 experiments to update these previously accepted tolerable spin rate limits.  In these investigations, we found that subjects can rapidly and completely acclimate to the CC illusion, exhibiting near-linear acclimation with rates averaging 1.5 RPM/session (SD: 1.0 RPM/session) across all studies (n = 60).  Further, individual studies found acclimation to be similar across different rotation and head tilt axes, that this acclimation was retained to some degree but did not transfer well to new head tilt axes, and finally, that a standardized acclimation protocol was slightly less effective and less tolerable than a personalized, incremental approach.  While there were large differences in individual acclimation in all of these investigations, we found that acclimation rate could be predicted by each individual’s vestibular perceptual roll rotation threshold.  Additional analysis was performed to develop a human tolerability metric to inform expected acclimation of a general population. 
The acclimation investigations suggest that the majority of humans (50-75%) can acclimate to spin rates as high as 20-30 RPM. Specific Aim 3 sought to narrow the now wider design envelope for short-radius, intermittent centrifugation by utilizing existing bedrest investigation data to inform which centrifuge parameters would result in maximal mitigation of bedrest-induced deconditioning.  It was found that higher loading levels, longer sessions, and the addition of exercise during centrifugation resulted in superior levels of mitigation.  It is recommended that future operational AG conceptual designs administer at least 1g loading at the subjects’ heart for at least 60 minutes each day.  Additionally, the findings on how this loading should be administered (i.e., all at once in one continuous session or divided into several shorter sessions throughout the day) was mixed, though the latter approach may have some slight benefit in terms of protocol tolerability.  Finally, additional research should be conducted to identify optimized exercise protocols that can be combined with AG to maximize efficacy of the countermeasure. 
The human tolerability metric and recommended centrifuge parameters provide a data-driven approach towards development of a tolerable and feasible short-radius, intermittent AG conceptual design.  This solution can provide astronauts with a comprehensive countermeasure to mitigate spaceflight-induced deconditioning, enabling future long-duration human space exploration.