Russian cosmonauts stationed at the International Space Station engaged on a spacewalk on Tuesday to investigate a mysterious hole in a visiting spacecraft that caused cabin air to leak out of the ISS four months ago.

Mysterious Hole

Back in August, ISS crew members noticed a slight drop in pressure on the orbiting laboratory and eventually traced this to a hole on the inside of the habitation module of the Soyuz spacecraft, which delivered a new crew to the orbiting laboratory in June and is currently docked at the space station.

The problem was fortunately immediately fixed after astronauts Sergey Prokopyev and Alexander Gerst had patched the 0.07-inch-wide hole using material soaked in an epoxy sealant.

It was initially thought that a micrometeorite has collided with the spacecraft and created the hole. However, Russian officials later said that based on its shape, the hole appears to have been drilled, leading to speculations that someone may have intentionally created the hole.

The leading theory now is that the hole was made during the manufacturing of Soyuz. A worker may have accidentally drilled a hole in the wrong spot and attempted to cover this up before the spacecraft launched.

No formal conclusion, however, has been made so Russia decided to conduct the spacewalk to shed light on the origins of the mysterious hole.

Unprecedented Spacewalk

Cosmonauts Oleg Kononenko and Sergei Prokopyev spent seven hours and 45 minutes on what the Russian described as an unprecedented spacewalk on Tuesday.

They used knives and other tools to cut through insulation and protective covering the Soyuz capsule to locate the outside of the mysterious hole. Prokopyev and Kononenko spotted the tiny hole in the external hull of the capsule more than five hours into the spacewalk.

The cosmonauts took samples, which will be brought back to Earth to find clues to what caused the hole.

"Kononenko and Prokopyev will unseal thermal insulation and the meteorite shield, inspect and make a photo of the hole in the habitation module. They will also scrape off samples from the hole's surface and send them aboard the same Soyuz MS-09 spacecraft to Earth on December 20 for further studies," the Russian news agency TASS reported.

The objective of the spacewalk is to solve the mystery on who or what caused the hole in the Soyuz spacecraft. The spacewalkers nonetheless reported seeing no drill marks around the hole.