Dmitry Olegovich Rogozin - the head of Russia's state space corporation, Roscosmos - said the issue was an air leak due to a tiny fracture on the Russian-manned Soyuz MS-09 spacecraft, which docked at the International Space Station in early June.
They eventually discovered that the leak was coming from a 2 millimeter hole in the Russian segment of the ISS, the upper section of the Soyuz MS-09 spacecraft - which is now attached to the Rassvet module.
However, NASA ground control realized that wasn't exactly ideal, according to the Telegraph, explaining that a thumb isn't "the best remedy" for a hole in one of the most expensive, important pieces of space infrastructure.
"As flight controllers monitored their data, the decision was made to allow the Expedition 56 crew to sleep since they were in no danger", NASA said in a statement.
Mission controllers in Houston, Texas, and Russia's capital, Moscow, say the six-strong crew are in no danger.
NASA said "the leak has been isolated to a hole about 2 millimeters in diameter".
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On Thursday (US time), the crew taped over the hole, slowing the leak.
Possibly, the perpetrator was a micrometeroid or space debris, but there's also a chance it was simply a weak spot in the Soyuz skin.
Additional leak checks will be performed once the patching is finished.
He said the fracture was found on the Soyuz ship that brought astronauts to the ISS in June for a six-month mission and is now docked with the space station.
But for now, the makeshift repairs have stabilised the situation, officials said. Flight controllers in Houston are continuing to monitor station's cabin pressure in the wake of the fix. This is a section of the Soyuz that does not return to Earth. But the decision reportedly caused some tension aboard the station, with a source at the Space Center in Houston telling Interfax that the ISS commander, USA astronaut Andrew Feustel, disagreed with the plan.