Today’s post is part of a special series here on Planet Pailly called Sciency Words. Each week, we take a closer look at an interesting science or science-related term to help us expand our scientific vocabularies together. Today’s term is:
HYPOGRAVITY
We have a pretty good idea how the human body operates in Earth-normal gravity (1 g). We also know a lot about how weightlessness (0 g) affects our bodies. But what about values of gravity between 0 and 1 g? According to this review from the journal Frontiers in Physiology, our knowledge about the human body in so-called hypogravity is shockingly limited.
The word hypogravity combines the word gravity with the prefix hypo-, which comes from a Greek word meaning “under” or “below.” It’s defined as an actual or perceived gravitational force greater than 0 g but less than 1 g. The term hypergravity (from a Greek word meaning “over” or “above”) is also used for gravitational forces greater than 1 g.
According to Google ngrams, the term started appearing in print during the 1950’s, which would coincide with the early days of the space program. My first encounter with this term was in an article titled “Medical Skills for an Interplanetary Trip: The Hostile Environment of Space and the Planet Mars,” which appears in this book about the Mars One program.
I can’t remember ever seeing this term prior to that article, which kind of surprised me at first. But based on my subsequent research, I think this term is used almost exclusively in the medical field, an area which I’m not well versed in.
According to that paper from Frontiers in Physiology, we know very little about what hypogravity does to us, medically speaking. Of course we do have the first hand accounts of those few astronauts who’ve walked on the Moon, as well as other records and archival footage from the Apollo program. Frontiers in Physiology also describes several ingenious ways scientists have learned to simulate hypogravity in the laboratory. And we have mathematical models to help us predict what hypogravity might do to us long term.
But still, our knowledge and experience with hypogravity “remains fragmentary,” as Frontiers in Physiology puts it. “Fragmentary” seems like just the right word, because old records, laboratory simulations, and computer models can only tell us so much. We have very little to go on here. Just bits and pieces. A few scattered data points.
The human body evolved in a 1 g environment. Prolonged weightlessness seems to do our bodies a lot of harm, from bone loss and muscle atrophy, to disrupting the balance of our internal fluids, to messing up our equilibrioception (a “sixth sense” most of us don’t realize we have until it’s taken away). I’d assume hypogravity does less harm than weightlessness. The question is: how much less?
I guess we won’t really know the answer until we start sending people to live on the Moon or Mars long term, and start finding out which health problems they do or do not develop.
Posted by J.S. Pailly 
