There are secret messages left aboard the International Space StationS

In a practice that's reassuringly similar to roommate-behavior here on Earth, astronauts aboard the International Space Station have been known to leave eachother notes – little messages, requests and reminders to one another, posted inside (and, occasionally, outside) the spacecraft.

Above: A diagram of a space toilet, with a relevant note on bathroom etiquette. For an explanation, see below.

Take this message, for example, which appears on an adapter recently installed on the ISS’s Canadarm 2.

There are secret messages left aboard the International Space StationS

It reads: “Installed by your friendly Expedition 36 Crew.”

A photo of the adapter was recently captured by ESA astronaut Luca Parmitano, which he later tweeted. The snapshot got Universe Today's Nancy Atkinson thinking: are there any other messages posted in, on, or around humanity's orbital outpost? To find out, she spoke with NASA astronaut Tom Marshburn, who returned from a 5-month stint aboard the ISS earlier this year. His answer: Oh, yes. There are many.

According the Marshburn, notes are kind of a tradition aboard the ISS, left by everyone from the engineers who designed the spacecraft's components to the astronauts who call it home. My favorite example from Atkinson's writeup is this little gem on bathroom etiquette:

“Everyone has to urinate into a funnel that goes into a hose,” Marshburn explained. “We are pretty good about cleaning ourselves up in the bathroom, but some crewmembers have not been so good about cleaning up the equipment because written in Sharpie on the wall in the bathroom is, ‘Blessed are those who wipe the funnel.’ That’s just a good little reminder.”

Also common aboard the spacecraft are instruments, books, and other little mementos:

There are secret messages left aboard the International Space Station

“There is a four-inch version of Gort, the robot figure from the movie “The Day the Earth Stood Still” stuck on the wall where we gather in the Destiny Lab for our daily planning conferences,” Marshburn said. “He sometimes gets unstuck and floats around the ISS, so whenever we find him wandering around, we stick him back up on the wall. He’s kind of ubiquitous.”

Read more about the notes, toys and assorted tidbits that have accumulated aboard the ISS over the years in Atkinson's fantastic post, over at Universe Today.