A worsening air leak aboard the International Space Station (ISS) led astronauts to take shelter and prepare for a possible evacuation for roughly two hours on Friday. A “save-haven” order was issued at 1.04pm GMT while Russian astronauts attempted to fix a fracture on the Zvezda service module's transfer tunnel, which had previously suffered from cracking, Nasa said.Five members were told by Nasa’s mission control to put on their spacesuits and take refuge in their docked spacecraft in case they needed to escape quickly.The order was later reversed and the astronauts ​were told they ⁠could return to the station as Nasa and its Russian counterparts from Roscosmos examined the rate of leaking air.Roscosmos said on Friday that its ‌experts had detected two leaks aboard the ISS but that there was no immediate threat to the crew.What happened on board?The ISS is a football field-size orbital laboratory where ​astronauts live ⁠and work in space.It was the first space station developed and crewed through international cooperation and is the largest spacecraft ever constructed.The research station is orbiting around 250 miles above Earth, and assembly began in 1998 with the launch of Russia's Zarya module.It has hosted astronauts since 2000, which is the longest continuous human presence in space.The International Space Station and the docked space shuttle Endeavour orbit Earth in 2011 (Getty Images)However it is showing the signs of use and the cracks responsible for air leaks have persisted intermittently for around six years. As such, Nasa is planning for its replacement.The air leaks have been relatively minor in recent months but escalated ‌on Friday from a pound of air per day to two pounds, a senior Nasa official who asked not to be named told Reuters.Fractures in the Zvezda service module had been an ongoing concern, said Nasa.Announcing the precautionary measure, Nasa spokesman Bethany Stevens said in a post on X: “The Zvezda service module transfer tunnel, known as PrK, has suffered from cracks and leaks for some time, and has been mitigated by Roscosmos as much as possible to date.“The cracks have always been a concern that Nasa watches very closely.A view of Earth from the Cupola on the earth-facing side of the International Space Station (NASA)“Nasa and Roscosmos have been working to determine the root cause of the cracks, and Roscosmos manages the issue through operational mitigation measures and periodic partial-repair efforts.“Following new leaks, Roscosmos has elected to proceed with a more extensive repair operation on Friday 5 June.“Out of an abundance of caution, Nasa has directed all four of the agency’s SpaceX Crew-12 members and Nasa astronaut Chris Williams to assume an elevated safety posture in the Dragon spacecraft while the repair is under way.“We continue to work with our Russian counterparts, along with the rest of the international community that supports the space station, to arrive at a more permanent resolution.”Nasa reversed that order roughly two hours later.In a later update, Ms Stevens added: “Roscosmos has paused Friday’s structural repair efforts inside the Zvezda service module transfer tunnel, known as PrK, as more measurements and data is assessed.“Given this development, Nasa has instructed the crew members inside the Dragon spacecraft to end the safe haven procedures and return to planned operations aboard the International Space Station.“We look forward to working with Roscosmos on a collaborative approach to address the leaks.Roscosmos said its experts had found two leaks aboard the ISS but steps had been taken to address these.In a statement, the agency said: “The situation does not threaten the safety of the crew and onboard systems – the pressure on board the ISS is stable and maintained at the calculated level.”Who was on board?Crew-12 members, Russian cosmonaut Andrey Fedyaev, NASA astronauts Jack Hathaway, Jessica Meir, and ESA astronaut Sophie Adenot walk out of the Operations & Checkout Building at the Kennedy Space Center before transport to Launch Complex 40, ahead of their launch to the International Space Station (Reuters)The ISS is currently home to seven astronauts from two missions, including the four astronauts from Nasa’s Crew-12 team — Nasa’s Jessica Meir and Jack Hathaway, European Space Agency astronaut Sophie Adenot, and Roscosmos cosmonaut Andrey Fedyaev — ⁠who arrived in February.The other crew of one US astronaut, Christopher Williams, and two cosmonauts, Sergey Kud-Sverchkov and Sergei Mikayev, arrived in November. Kud-Sverchkov and Mikayev, who did not execute evacuation procedures, were planning to use a saw to reach an area where they believe they can access the crack leaking air, the Nasa official told Reuters.Nasa officials disagreed with this method, they said, prompting mission control in Houston to order safe-haven procedures.What happens next?Astronauts have ‌never had to evacuate the ISS in its 27 year ⁠history.Safe-haven orders are rare, though pieces of space debris that risk colliding ​with the ISS and smaller changes in air leak rates have triggered the process in recent years.Nasa and Russia's space agency Roscosmos, the station's two primary operators, have debated for months over the cause and potential fixes ⁠of small air leaks aboard Russia's Zvezda service module, a key structure of the ISS, Legislation is before the US Congress that would extend the planned life of the space station for two years, ⁠until 2032, to give companies more time to develop a replacement.In the future, the US plans to transition its operations in low Earth orbit to “commercially-owned and -operated destinations”, which it says will allow for continued access to research and technology development.