Since August 22nd, when it took its first test drive, the car-sized Curiosity Rover has been happily driving hither-and-yon around Mars. It turns out, though, that it’s been doing something else too: leaving tracks in Morse Code. The tracks spell “JPL,” the initials of NASA’s Jet Propulsion Laboratory, and they help the rover calculate how far it’s travelled through a process called “visual odometry.”
Visual odometry solves a slippery problem: Although Curiosity knows how many times its treads have turned, it doesn’t necessarily know how far it’s travelled, because the treads can slip on especially loose terrain. The patterns in the soil give it a way of measuring how far it’s travelled visually. Matt Heverly, the lead “rover driver,” explains it this way:
Imagine standing in front of a picket fence, and then closing your eyes and shifting to the side. When you open your eyes, you wouldn’t be able to tell how many pickets you passed. If you had one picket that was a different shape though, you could always use that picket as your reference…. With Curiosity, it’s a similar problem in featureless terrain like sand dunes. The hole pattern in the wheels gives us one ‘big picket’ to look at.
Opportunity and Spirit, the other Mars rovers, also use visual odometry, but only Curiosity actually actually writes something on the Martian surface. More at the Jet Propulsion Laboratory.
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