Atmospheric pressure was slowly dropping, and the ISS crew spent the next few hours hunting down that leak.
Russian cosmonauts were to carry out a spacewalk Tuesday to examine a mystery hole in a Soyuz spacecraft docked on the International Space Station that a Moscow official suggested could have been deliberate sabotage.
Kononenko arrived at the ISS on a Soyuz MS-11 spacecraft last Tuesday, along with NASA's Anne McClain and the Canadian Space Agency's David Saint-Jacques, weeks after the previous launch had to be aborted shortly after takeoff.
The piece in question contained the mysterious drill hole identified in August as the source of an air leak and will be analyzed back on Earth. Kononenko and Sergei Prokopyev are inspecting a section where a mysterious leak appeared on August 30. As he vigorously cut into the spacecraft, small bits of insulation floated off into space.
The spacewalk was the fourth for Kononenko and the second for Prokopyev.
The two cosmonauts then used the same tools to cut into and peel away a thin metal orbital debris shield to expose the hole in the Soyuz MS-09's orbital compartment.
The spacewalkers reported seeing no drill marks around the black dot, like on the inside.
A Soyuz spacecraft docked to the International Space Station.
The samples, data and photographs taken from the leak site will be brought back to Earth and analyzed by Russian specialists. This part of the capsule will be jettisoned as usual before atmospheric re-entry, and so poses no risk for descent. Kononenko has now logged a total of 26 hours and 12 minutes working in the vacuum of space, including a 2008 spacewalk when he performed similar work to what he did on today, cutting into the insulation covering the Soyuz TMA-12 spacecraft to remove a pyrotechnic device. The extravehicular activity (EVA) was the 213th spacewalk in support of assembly and maintenance in the 20-year history of the space station.
Nasa said it was a "Eureka Moment", as the hole was found after several hours.
The Soyuz capsule had been used to deliver a new crew to the laboratory 400km (250 miles) above the Earth in June.