Moscow - Russian cosmonauts began 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.
Roscosmos space agency said the aim would be to discover whether the "small but dangerous" hole had been made on Earth or in space.
The two-millimetre cavity on the Soyuz spaceship docked at the ISS caused an air leak detected in August, two months after the craft's last voyage.
So far astronauts have only been able to examine the hole from inside the spacecraft.
Roscosmos chief Dmitry Rogozin said in October that an investigation had ruled out a manufacturing error. He had said earlier that Russia did not exclude "deliberate interference in space."
Russian media reported the investigation was probing the possibility US astronauts deliberately drilled the hole in order to get a sick colleague sent back home.
Russian officials later denied those reports.
The discovery of the hole was followed in October by the failure of a manned Soyuz launch, although the Russian and US astronauts returned safely to Earth.
Tuesday's spacewalk began at 1559 GMT and was set to last more than six hours.
Veteran cosmonauts Oleg Kononenko and Sergei Prokopyev left the ISS and Prokopyev operated a boom to move Kononenko safely from the ISS to work on the Soyuz spacecraft.
Once in position Kononenko was to use a knife to rip open the insulation and the debris shield protecting the spacecraft to look at the hole and scrape off samples
The samples will then be sent to Earth to "get at the truth" of the cavity's origins, the space agency said.
The cosmonaut was also to take photographs and film video, before putting new insulation over the area.
"It's a challenge. Sergei and I are accepting it," Kononenko said ahead of the spacewalk, which is his fourth and the second for Prokopyev.
Rogozin called the spacewalk "unprecedented in its complexity" on Twitter and Roscosmos said it would "enter the history of space exploration."
What makes it especially hard is that the Soyuz spacecraft, unlike the ISS, was not designed to be repaired in spacewalks and has no outside railings for astronauts to hold onto.
"There's nothing, that's the problem," Kononenko said.
The Soyuz spacecraft is used to ferry astronauts to and from the ISS. The hole is in a section that will not be used for the return journey to Earth on December 20.
The ISS is one of the few areas of Russia-US cooperation that remains unaffected by the slump in relations and Washington's sanctions.