When I first bought these covers, I thought, not unreasonably, that an otolith was some kind of rocket, like the nice pink-and-white striped one pictured in two of the cachets below. But it turns out that the otolith is a structure in the inner ear that has something to do with balance, which, in turn, has something to do with how we react to gravity. So, if you're NASA and want to study whether or not weightlessness causes disorientation, what do you do? You send two bullfrogs into space, of course!
On November 9, 1970, two bullfrogs were launched into space from Wallops Island, Virginia. These brave astronauts had monitors surgically implanted in their ears--the frog's otolith is structurally similar to that in humans--and the experiment was a success! The data showed that the frogs were disoriented for the first three days but, according to the insert in one of the covers, "on the 4th day they calmed down as though recondiled to the unusual lack of gravity and behaved toward the end of the experiment almost as they had done before the launching."
And the fate of the frogs? Did they return to earth for a ticker-tape parade? Alas, no. They orbited the earth for a week, but they were never recovered.
The frogs have a Facebook page; as yet, I'm the only fan.