Not a duplicate of Why does Mark use Hexadecimal to communicate?

ASCII can encode a question mark. Its code in hexadecimal is 3F. Since a question mark only appears once per question and most questions would be fairly long (i.e. many characters) why would Mark Watney waste precision (17 positions vs 16) to save on one code (?, length 1, infrequently used vs 3F, length 2)? Also, it would seem that controlling the rotation in 16ths would be much easier than 17ths as 17 is a prime number whereas 16 positions can be constructed by repeatedly halving the perimeter of a circle. Was this simply dumbing it down for the audience?

First result for "ascii table" on Google:


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I see no reason why Mark's ASCII table wouldn't have been at least as comprehensive as this. Unless in the movie universe ASCII is substantially different.

  • 1
    Data vs metadata. Any useful communications protocol requires framing bits. Nov 14, 2015 at 15:45
  • @LightnessRacesinOrbit How do you know where/when the framing bits start and end? Wouldn't you need framing bits for them, too? I'm being facetious (ooh! a-e-i-o-u!), but why couldn't they have used pre-existing ASCII codes to signal End Of Text (03), End Of Transmission (04), etc. That's why these codes were built into ASCII in the first place! It seems as though they took a protocol that could already do everything they needed and then reinvented the wheel!
    – CJ Dennis
    Nov 15, 2015 at 0:51
  • Because it wasn't a "they": it was one guy, a farmer. He did the best he could. Once they'd bootstrapped communications with basic character sequences (and the naive "question mark" approach to provide some level of control), they moved everything on to the proper Pathfinder platform at which point their tapping on a keyboard would have been powered by a proper protocol with all the niceties you just named! Nov 15, 2015 at 2:14
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    He had an ASCII table; he did not have access to Wikipedia. Nov 15, 2015 at 12:50
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    The characters he used are not in question. The manner in which he used them is. Are you sure you're not really nitpicking here now? What problem are you trying to solve, at this point? Nov 16, 2015 at 1:55

1 Answer 1


It's not supposed to be a question mark card (though that is how the film depicts it).

It's a spot for Mark to post his questions to the NASA team.

From the book, Sol 97, he begins discussing how he's going to send messages:

Time to make an alphabet. But I can't just use the letters A through Z. Twenty-six letters plus my question card would be twenty-seven cards around the lander. Each one would only get 13 degrees of arc.

Then later in the same chapter:

So, I'll make cards for 0 through 9, and A through F. That makes 16 cards to place around the camera, plus the question card.

So, it's a dedicated place for them to aim the camera when they're waiting for a response or to show that they've finished sending their message.

  • Surely movement vs no movement would suffice?
    – CJ Dennis
    Nov 14, 2015 at 6:03
  • @CJDennis I'm not saying I think it's necessary... Just quoting the explanation from the book. It does make it very clearly an "end transmission" so Mark knows that the entire message was received.
    – Catija
    Nov 14, 2015 at 6:08
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    @Izkata OK, that makes a bit more sense. But Mark had full freedom of movement, so he could have just stuck his question in front of the sign that the camera was pointing at when it stopped, e.g. the 3, if NASA were using 03 to signal the end of their information.
    – CJ Dennis
    Nov 14, 2015 at 7:32
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    There are 32 control codes defined by ASCII, but in practice it's quite rare to use more than four of them: backspace, horizontal tab, line feed and escape. Windows computers use a fifth, carriage return, because they use carriage return followed by line feed to indicate a new line rather than just a line feed alone. It's perfectly reasonable for him to ignore all of them entirely.
    – db48x
    Nov 14, 2015 at 16:00
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    @Kevin In the movie he doesn't find a Unicode/UTF-8 table (which would be huge!), he finds an ASCII table, which suffices for almost all English communication. The movie also shows that he is already familiar with some computer science. He knows about hexadecimal already and goes searching for an ASCII table, neither of which he would know about if he was "only" a botanist.
    – CJ Dennis
    Nov 15, 2015 at 2:42

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