Hello, Curiosity

On Sol 32 (Sept. 7, 2012) the Curiosity rover used a camera located on its arm to obtain this self portrait. The image of the top of Curiosity's Remote Sensing Mast, showing the Mastcam and Chemcam cameras, was acquired by the Mars Hand Lens Imager (MAHLI). The angle of the frame reflects the position of the MAHLI camera on the arm when the image was taken. The image was acquired while MAHLI's clear dust cover was closed.

That's from NASA's description of this great Curiosity self-portrait.

What really stuck out to me, though, was the use of "Sol 32". Sol is what you call a "day" on Mars. We use a different word because the length of time is also a bit different. One Martian Sol is equal to 24 hours, 39 minutes, and 35.244 seconds. Sol 32 isn't, itself, a date, but simply a record tracking the number of sols that Curiosity has been on Mars—starting with Sol 0, which was August 6th. Every mission to Mars since Viking has kept its own sol count, so you can't really use these sol dates to keep track of history except as it relates to a specific mission.

There have, however, been proposals for a standardized Martian calendar system with a starting point that all dates progress from. NASA includes a Mars Sol Date on its Mars24 Martian clock app. In this case, the count begins on Earth date December 29, 1873 at noon Greenwich Mean Time and MSD represents the number of sols that have happened since then.

Why December 29, 1873? The Mars24 explainer just says that this date was chosen because it precedes all the really good, detailed observations of how time worked on Mars—how fast the planet was spinning, how often it went around the Sun, what the orbits of its moons were like ... that kind of thing. In 1877, the orbit of Mars took it particularly close to Earth, allowing humans—and their increasingly good quality telescopes—to get a really nice view of the planet.

That still doesn't exactly explain the 1873 date, though. But, according to Wikipedia, it's also the birthday of Carl Lampland, an American astronomer. Among other achievements, Lampland would calculate temperatures on the Martian surface, finding a large difference between soltime temperatures, and those at night. That data gave scientists their first clue that Mars had a particularly thin atmosphere, compared to our own.



    1. If you read the whole explanation at the NASA link, you’d note that this was not a cutesy photo op. That is a fun side-effect of checking on the dust shield on the camera that looks like an eye. 

    2. Who pissed in your grape nuts? It’s not as if you can even waste “signal”. The signaling medium (light, and some vacuum to put the light into) doesn’t run out as long as Curiosity has the ability to recharge itself.

  1. I think they’re missing a real opportunity if they mark Sol from some completely arbitrary date. It means that Sol 0 means nothing, and there is no landmark in time to mark B- and A-.

    Just because there aren’t detailed observations of Mars before this arbitrary date, doesn’t mean scientists aren’t going to be referring to stuff that happened on Mars millions of years Before X. And if people ever land on Mars, don’t they want the date to be x years after something important, like the date a man-made object forst landed on Mars?

    Their chosen date and explanation is worse than the Unix timestamp. Indeed, it sounds like it was chosen by software engineers rather than anyone with any sense of history.

  2. A martian calendar is another solution to a problem that doesn’t exist yet. When earth humans are on the martian surface and need to track time for themselves, then the moment their ancestors landed would make a reasonable sol 0.

    Suddenly I’m curious about when the Mars Reconnaissance Orbiter was programmed to take a picture of Curiosity landing. Was it tod to trigger the shutter relative to its own launch? 

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