The world is not entirely devoid of research in this field—medical databases contain over 200 citations in the literature relating to "partial gravity physiology." However, the only actual experiments that have been done reflect the poor level of attention that these fundamental questions have received.

Cosmos 782. This early biosatellite carried its mammalian payload in microgravity; however, it spun fruit flies, fish embryos, and carrot tissue cells at 0.6-g for the entire 20-day mission. Unfortunately, this data provides no information about mammalian systems.

Apollo. Astronauts on Apollo lunar missions experienced partial gravity (0.17-g) for up to 75 hours, but any useful data from these periods of adaptation was confounded by multiple days of microgravity exposure during their return to Earth.

Skylab, Salyut, Mir, Shuttle. These vehicles have all seen very limited centrifuge experimentation and produced little data useful for partial gravity research.

KC-135 Aircraft. The KC-135 "Vomit Comet" flies parabolic trajectories to reduce or eliminate the loading of Earth's gravity on its occupants. However, continuous exposures are limited to less than a minute, so no long-term effects can be examined.

Ground Unloading. Hindlimb suspensions, spring harnesses, tilted clinostats, and modified bed rest have all been used in attempts to simulate physiological unloading. However, these mechanisms provide very limited analogs to actual gravity reduction.

In order to replace our current guesses with robust data, we must make solid investigations of partial gravity physiology a scientific priority.