Although the station can be operated remotely, there’s no substitute for having people on board. Astronauts conduct repairs inside and outside the station, replace aging hardware, and perform regular checks of life-support systems. Flight controllers can track the status and health of virtually every piece of the station, but astronauts are their eyes and ears. They know far more about what’s going on, especially during emergencies.

One night in August, as the crew slept, flight controllers on Earth noticed that the air pressure on the ISS had dropped slightly—a sign of a leak somewhere on the station. The air wasn’t escaping quickly, so flight controllers decided not to wake up the slumbering crew, which consisted at the time of three Americans, two Russians, and one German.

When the crew woke up the next morning, it was instructed to scour the station to find the source of the leak. The crew members found it inside a Soyuz capsule docked to the ISS, in a section that burns up during the re-entry to Earth—a two millimeter hole they’d never seen before. The crew plugged the hole with sealant and gauze. It took photographs and video footage of the scene and sent them to Earth, where an investigation was launched into the cause.

Russian officials are still working on it. They have ruled out an impact with micrometeoroids, space rocks that travel at thousands of miles an hour and can easily cut through metal. They believe someone drilled the hole, but they don’t know who, why, or even when.

Read: What the heck happened on the International Space Station?

In this emergency situation, mission controllers could talk to the crew and walk it through patching up the mysterious hole. But what would happen if the ground couldn’t communicate with the station?

In 2007, Michael López-Alegría, a NASA astronaut, and his crewmates woke up one weekend to find that half of the lights were off in the ISS. As they floated through the station, they realized half of everything was down, from the systems that regulate the temperature to the scrubbers that remove carbon dioxide from the air so it doesn’t become toxic. On top of that, communications were down. Mission control didn’t know what was happening on board, and vice versa.

The crew turned to the printed manuals stowed away on board, looking for fixes. Within two to three hours, it had restored communications with the ground, and flight controllers guided the astronauts through rebooting the rest.

“That was the type of thing that would be problematic if you don’t have crew on board,” said López-Alegría, who retired from NASA in 2012, after three flights on the Space Shuttle and six months on the ISS.

Even without people on board, the ISS is a chatty environment. The station transmits data back to Earth constantly, relaying thousands of signals about its tranche of systems and its condition. López-Alegría said that during the communications blackout, this information didn’t make it to the ground.