A little more than 15 years ago, astronaut Garrett Reisman was among a crew of seven who launched into orbit aboard Space Shuttle Endeavour. The shuttle remained attached to the Space Station for nearly two weeks, but when the orbiter departed, it left Reisman behind for an extended stay.

During his time at the station, Reisman would often pass through the Harmony module, which serves as a corridor connecting laboratory modules built by NASA and the European and Japanese space agencies. Sometimes, he would look up and see a small placard that said, “To CAM.” The arrow, however, pointed out into space.

“When I was up there on the space station, there was still the sign that says, ‘To CAM,'” Reisman said in an interview. “But there’s just a closed hatch. It was tragic. It was just kind of taunting me when I saw that because I think that could have been one of the most important scientific discoveries that we made.”