This is the code Matt Damon and NASA use to communicate in 'The Martian'

In "The Martian," astronaut Mark Watney is screwed.

He's alone on Mars after surviving a horrific dust storm.

In addition to a limited supply of food and water, Watney has no idea how he's going to make contact with another human being or how he'll get home.

The next mission to Mars isn't scheduled to arrive for another four years. NASA thinks he's dead, and, even when they figure out he is alive through satellite images, they still have no way to communicate with him.

The main communications dish was battered in the storm. The other three backup communications systems were located in the Mars Ascent Vehicle (MAV), which the rest of Watney's crew took with them when they left Mars, believing him dead from the storm.

So in order to gain contact with NASA, Watney digs up the Mars Pathfinder, the probe which NASA lost contact with in 1997.

He fixes it (woo!) and the probe is able to communicate with Earth. Through a camera on the Pathfinder, Watney is able to speak to NASA through messages he writes and shows to it.

While they find an easier means of communication later in the book and film, for their first contact they only have the Pathfinder to communicate through and that results in a few problems.

For instance, Watney has a limited amount of paper to write on. The larger problem? NASA can't communicate back. All they have is a camera on a platform which can rotate 360 degrees.

That's a start. The camera can point to things like "yes" and "no." It can even point to an alphabet. However, since the camera moves in a circle, it doesn't leave a whole lot of room for each of the English language's 26 letters.

Watney has to get creative.

So he decides to use a hexidecimal system, which has 16 characters, to communicate instead.

In the movie, Watney brings it up with little further explanation.

So, what is the hexidecimal system you see on screen?

The hexidecimal system is comprised of 16 symbols: 0-9 and A-F.

Typically it's used by programmers to condense and communicate large binary numbers easily.

It's also used to communicate data.

After putting signs for the 16 digits and numbers in a circle around Pathfinder, Watney combines the hexidecimal system with an ASCII table in both the book and movie. ASCII is a code that represents characters as numbers and is used to help display text onto computers among other devices.

An ASCII table contains 255 characters where each is assigned a number ranging from 0 to 255.

An easier way to think of it is by considering all the options on a smartphone keyboard. Now, imagine you only had 255 of those characters to choose from and each of those was assigned a number from 0-255. That's your ASCII table.

Essentially, the ASCII table can be used as a giant decoder.

Here's how it looks:

The important columns for "The Martian" are the ones labelled "Hex" and "Character."

When NASA points to a pair of hexadecimal digits, or hex digits, they can send any letter of the alphabet and character they want.

So if NASA wanted to say "STATUS" as they do in the book to Watney, they would point to the following pairs of letters and numbers: 53, 54, 41, 54, 55, 53.

Here are a few other strings of letters and numbers NASA sends to Watney in the books. Can you figure them out using the red and green columns?

1. 48 4F 57 41 4C 49 56 45
2. 43 52 4F 50 53 3F
3. 42 52 49 4E 47 53 4A 52 4E 52 4F 55 54

All done?

Keep scrolling for the answers.

How'd you do?

1. HOW ALIVE - NASA's way of asking Watney how he has survived for nearly 100 Mars days at this point.
2. CROPS? - NASA's wondering how in the world Watney was able to grow food on a planet where that's supposed to be impossible.
3. BRING SJRNR OUT - NASA'S telling Watney to bring the Sojourner rover out near the Pathfinder to try and figure out an easier means of communication.

As you can see, it's a rather time-consuming process which isn't ideal for lengthy conversation.Watney refers to their system as a Speak & Spell in the book.

I made it a little easier by giving you each character as two digits when Watney would really just be seeing everything as one character at a time. (In other words, HOW ALIVE would look like 484F57414C495645 instead of 48 4F 57 41 4C 49 56 45).

While effective at first for communication, it's not long before they find a simpler way later on to eventually type messages out to one another.