At least one design element of the Mars Curiosity rover--its wheels--combine function and whimsy, as they contain an inside joke.
Curiosity's wheels are multifunction. Their primary use case--obviously--is locomotion. However, NASA uses visual evidence to estimate distances and scales on Mars, so Curiosity's wheel tracks are designed to aid in this effort. A series of irregular holes placed in each of Curiosity's six wheels create a repeating pattern as the rover drives across the Martian desert.
By observing the placement of these tracks, NASA scientists can confirm that the rover's odometers are working properly and use the visual evidence to judge distances in photo data that Curiosity sends home. But so long as you're punching holes in a rover's wheels to create an identifiable pattern, you may as well have some fun with it.
The aforementioned holes are placed in between the horizontal treads of the rover wheels, so the only variation engineers had was width of the holes between the treads--the height was fixed. Thus, the pattern is of long and short holes or, for you cryptography geeks, long and short data signals. There's a venerable if not always well-recognized encoding standard for long and short signals--Morse Code. Each wheel on Curiosity devotes three treads to the odometric encoding, which gave Curiosity's designers three letters to play with.
Every time Curiosity's wheels complete a revolution, they print the Morse Code for JPL on the Martian surface in honor of Caltech's Jet Propulsion Laboratory, which oversees the Mars Science Laboratory project.
That's not just some clever cryptographic Curiosity-encoding; it's an ingenious interplanetary instance of inside joke-worthy Geek Trivia.
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