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NASA briefly placed astronauts aboard the International Space Station (ISS) on emergency standby after a long-running air leak in the station’s Russian section appeared to worsen during repair work. The alert was issued on June 5 while Russian cosmonauts were trying to fix leaks inside a transfer tunnel connected to the Zvezda service module, a critical part of the Russian segment that supports life-support systems, communications and station control. As a precaution, NASA instructed five crew members to move into a docked SpaceX Crew Dragon spacecraft, put on emergency gear and prepare for a possible evacuation if station pressure began falling unexpectedly.

Notably, the leak has been a concern for years. Engineers first detected pressure losses in the area around 2019, and since then both NASA and Russia’s space agency, Roscosmos, have carried out multiple inspections and repair attempts. The affected section, known as the PrK transfer tunnel, is attached to the Zvezda module and connects to a docking port used by visiting spacecraft. Small cracks and tiny openings have repeatedly allowed station air to escape into space. Earlier this year, officials believed repair efforts had stabilized the problem, but new signs of air loss were detected again after a recent Russian cargo mission visited the station.

Several reports indicate that the rate of air loss increased from around one pound of atmosphere per day to about two pounds per day. While this is still a relatively small amount compared with the total air inside the ISS, an increasing leak rate can suggest that existing cracks are growing or that parts of the structure are slowly weakening. In spaceflight, a leak that is becoming larger is often considered more dangerous than a small leak that remains stable.

Meanwhile, during the repair operation, Russian specialists reportedly identified two possible leak locations. One was sealed, but work on the second area was paused after engineers reviewed pressure readings and decided more analysis was needed. NASA’s decision to place astronauts inside their return spacecraft was mainly a precaution. If the repair work had accidentally worsened the leak or caused a sudden pressure drop, the crew would have been ready to leave the station immediately. However, after about one to two hours of monitoring, the repair operation was suspended, and the astronauts were allowed to return to normal duties.

At the time of the incident, seven people were living aboard the ISS. The astronauts affected by the standby order included members of the Crew-12 mission and NASA astronaut Chris Williams. Throughout the event, life-support systems continued operating normally, and station pressure remained within safe limits. No one was in immediate danger. The procedure activated by NASA is known as a ‘safe haven’ protocol, in which astronauts shelter inside their docked spacecraft so they can quickly undock and return to Earth if necessary. Despite more than 25 years of continuous human presence aboard the ISS since November 2000, the station has never undergone a full emergency evacuation.

The incident also highlights a growing challenge facing the ISS as it ages. The Zvezda module entered orbit in 2000 and is now more than two decades old. The station itself, about the size of a football field, circles Earth at an altitude of around 400km and travels at about 28,000km per hour. As the ISS moves toward its planned retirement around 2030, engineers are increasingly dealing with issues linked to ageing hardware, metal fatigue and long-term wear in the harsh environment of space.

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