The payload module is responsible for maintaining a healthy, comfortable, and experimentally sound environment for the animal payload, collecting and storing experimental data, and selectively transmitting data and telemetry to mission control via the communications sytem. The responsibilities of the payload team are fourfold:

Environmental Management. The habitat will provide a comfortable living environment for our mice to live. The interior structure has been specially designed to avoid injury and minimize undue stress to the animals. An automated food and water supply will provide free access to nutrition and fluids, while preventing leakage and controlling pathogens. One-way air circulation will disperse contaminants, preventing buildup, and transfer contaminated air the air revitalization loop; this processing system will replace oxygen and nitrogen lost through leakage and metabolism, and remove carbon dioxide, trace contaminants, and particulates from the air stream. Below the cabin, a waste management system will receive wastes from the living area, treat them to deter decomposition, and store them for later analysis. Overall, the payload module will manage external disturbances to stabilize the environment, controlling noise, vibration, temperature, and humidity within the living area.

Experimental Control. In order to conduct valid experiments, the payload module must ensure homogeneity between the different habitat environments, including thermal and vibrational disturbances. Furthermore, it must provide a mechanism to duplicate as many environmental variables as possible in ground controls; sensor arrays will thus provide environmental data telemetry to mission control, whereupon ground control modules will simulate the actual spaceflight environment in control experimental groups. Some parameters, of course, cannot be duplicated: in particular, Coriolis and centripetal accelerations caused by the rotating environment will be much greater in flight than in Earth-gravity controls. However, the payload module will seek to minimize the effects of these confounding variables.

Data Collection. Though the mission's most revealing studies will be conducted after the payload has returned to Earth, onboard data collection is of supreme importance to fulfilling scientific objectives. In-flight data will enable us to watch the process of adaptation to partial gravity over time, a crucial piece of the physiological puzzle, as well as monitoring animal health and welfare during the mission. In addition, data downlinked in flight may be our sole repository of results in case of reentry failure. Data systems will be designed to support the primary science objectives: bone, muscle, and neurovestibular adaptations. Some possibilities include: (1) video footage, providing data on general health, behavioral, and vestibular studies; (2) urine analysis, tracing biomarkers of bone and protein metabolism; and (3) locomotion analysis, using existing video algorithms to study muscular and motor control changes by observing animal movement.

Spacecraft Integration. Clearly, the mass, bulk, and energy consumption of the payload drives the magnitude of all other orbital systems. Hence, minimizing the impact of payload systems is key to keeping the mission within the desired constraints. The payload must also provide structural and electronic interfaces to the rest of the spacecraft system. In particular, these include: a separation system for reentry jettison from the carrier bus; a symmetric and predictable mass distribution to allow for stable reentry; data transfer to and from the command, control, and communications systems; and structural integration with the reentry vehicle.

The Mars Gravity mission will attempt things never done before in the harsh environment of outer space. The following are some of its more ambitious challenges.

Wild Accelerations. The systems within the Mars Gravity Payload Module must protect its occupants from a highly variable acceleration profile, shown in the graph to the right. Systems must be optimized for partial gravity, but function in one-gravity (pre-launch, recovery) and micro-gravity (orbital transition periods). In addition, they must endure accelerations on the order of 10 Earth gravities during launch and reentry phases, and a land impact shock of perhaps even greater magnitude. Furthermore, as seen in the three configuration diagrams, primary accelerations will point in different directions during different phases of the mission. Adding Coriolis and centripetal effects from the rotational environment, the design environment becomes even more complex.

Mission Duration. The Mars Gravity flight experiment requires a mission duration far longer than that of any previous biosatellites. Such a long-duration mission creates several new challenges, including deterioration of bulk consumables, habitat durability against animal destruction, data compression, and most importantly, contaminant control and waste management. Current prototyping efforts are finding ways to extend the normal laboratory habitat-change interval from a few days to a few weeks.