NASA / Roscosmos

In November 2000, some 250 miles above Earth, a capsule carrying one American and two Russians docked to the International Space Station (ISS). A hatch leading to their new living quarters swung open, and the crew members floated in and got to work. They hooked up cables and computers for easy communication with the ground. They installed life-support systems to maintain breathable air. They activated the toilet. For the next four months, the ISS was their home.

Over the years, crews came and went, sourced first from the United States and Russia, and then from Japan, Germany, Italy, France, Canada, and other countries. New pressurized modules and other hardware arrived, growing the station in size and scope. So did science experiments spanning a myriad of fields, prepared by researchers eager to learn how stuff works in zero gravity.

One thing hasn’t changed in the past 18 years: There have always been people on board. When one crew departed, another remained inside, waving through the thick glass windows as it watched the capsule descend to Earth.

This fall, that guarantee seemed to be in jeopardy.

The trouble began back on Earth, at a launch facility in Kazakhstan for ferrying people to and from the ISS. On October 11, the American astronaut Nick Hague and the Russian cosmonaut Alexey Ovchinin wriggled into a small capsule atop a rocket and blasted off into the sky. Minutes into their flight, the launch vehicle’s computers detected a malfunction in the rocket and automatically triggered abort procedures. The crew capsule was shoved away from the rocket and parachuted safely to the ground.

Rescue teams scooped up the crew members, and Russia, which operates the launch system, opened an investigation into the incident, the first launch failure of a crewed Soyuz mission in 35 years. None of the world’s space travelers—from the United States, Russia, or elsewhere—would fly until the Soyuz system was deemed safe, officials said.

Three remained, however, on the ISS: Serena Auñón-Chancellor of the United States, Sergey Prokopyev of Russia, and Alexander Gerst of Germany. The trio arrived in June. They’re scheduled to leave in mid-December, when the Soyuz capsule responsible for transporting them reaches its time limit for remaining in space. Hague and Ovchinin were supposed to be on board to see them off.

The emergency landing created an unsettling question: What if the trio comes home before another crew goes up?

“We want to have people living and working there, so that would be a disappointment,” said Kjell Lindgren, a NASA astronaut who resided on the ISS in 2015 and part of the backup crew for future SpaceX missions to the station, in an interview. “It would be a very new thing for us not to have someone on the space station.”

It’s important to note that the ISS doesn’t depend on the presence of a crew to fly. Mission controllers on the ground can operate the station as it coasts through space, traveling at an average speed of 17,500 miles an hour. ISS systems are built to be redundant; a failure of one of several identical systems doesn’t signal a major catastrophe. If necessary, Russia can also deliver uncrewed Progress capsules to dock to the ISS and, as has been done in the past, fire their thrusters to elevate the station, keeping it in its usual orbit.

Roscosmos, the Russian space agency, completed its investigation of the incident last week. It concluded that the problem had originated with one of the engines on the Soyuz rocket and could be fixed for future launches. It has set the next crewed launch to the ISS for December 3, 10 days before the current crew is scheduled to depart. If the schedule sticks, the space agencies won’t have to face the possibility of an empty ISS.

But just in case, before Russia finished its investigation, NASA spent several weeks preparing for the possibility of leaving the ISS unoccupied. The space agency has a “de-crew” document for this scenario, which instructs the departing astronauts to make sure systems are running fine, install backups, and top off science experiments. “We primarily focus on what actions the crew would need to take in order to leave the station in an optimal configuration for the ground-control team until we are able to return crew onboard,” said Kenny Todd, the station’s mission-operations-integration manager.

But NASA’s protocols don’t specify exactly how long the ISS could theoretically operate without a crew. “I don’t believe we have a defined time limit,” Todd said.

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.

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.

López-Alegría points out that the ISS was younger then. “I do think the systems are more redundant because the systems are more complete now,” he said.

But if a similar outage were to occur while the ISS was empty, mission control would be flying the station blind. Officials wouldn’t know, for example, about a small change in breathable air that suggested a leak was afoot.

When I asked Lindgren whether there were any hidden upsides to an unoccupied ISS, he mentioned an autonomous physics experiment in the Japanese segment of the station. The experiment is designed to study the Marangoni effect, in which differences in surface tension drive the motion of liquids. On Earth, the phenomenon creates problems for semiconductors, degrades heat-radiation devices in computers, and, most delightfully, produces the tiny streaks on your glass of wine as you swirl it around.

On the ISS, the experiment involves building tiny bridges out of liquid suspended in microgravity. “Those bridges will break up even with the slightest vibration, so they’re often done at night when the crew is asleep,” Lindgren said. “This would actually be an opportunity where it would be fairly quiet.”

There could also be a hygiene advantage. Astronauts on board the ISS spend several hours each weekend on household chores like wiping down surfaces and vacuuming filters. But most of the debris comes from the crew itself, in the form of clothing fibers, hair, and skin. So the ISS may actually be cleaner without any human inhabitants.

“If there was nobody there to do that cleaning, that would also suppose that there’s nobody there to make the mess,” Lindgren said.

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