Astronauts told to return to International Space Station after sheltering over air leak repairs

The leak that has put the ISS crew on alert today is not new - it has been one of the most persistent and troubling problems in the station's history, and despite years of attempts, engineers have never fully solved it.

The problem is in the PrK - a small tunnel that connects a docking port to the Russian Zvezda service module. Microscopic structural cracks in its walls have been slowly releasing air into the vacuum of space.

The Russian Space Agency, Roscosmos, first reported the leak in September 2019. Over time it worsened, eventually doubling to around nearly one kilogram of air lost per day — prompting Nasa to classify it as the station's highest-level safety risk.

Earlier this year, engineers thought they had finally made progress. After multiple inspections and sealant applications, Nasa reported in January that pressure readings suggested a stable configuration had been reached - though there remained uncertainty about whether the leak had truly been sealed or whether air was simply escaping elsewhere.

Those doubts proved well founded. On 1 May, as Russian cosmonauts unloaded cargo from the Progress 95 supply spacecraft, sensors detected a fresh pressure drop. The leak had returned.

By Monday this week it had escalated once again to a kilogram of air per day, prompting Roscosmos to abandon its mend-and-make-do approach and go for a proper repair today.

The reason five crew members sheltered in the Dragon spacecraft is straightforward: the repair requires the cosmonauts to work inside the very section that is losing pressure.

If the cracks widen suddenly, the rate of air loss could accelerate faster than ground teams could compensate for. Having the rest of the crew suited up and inside a sealed, self-contained spacecraft means they can undock and be home within hours if the situation deteriorates.