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.
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.
Roscosmos said Tuesday's spacewalk would be a six-hour mission carried out from 1605 GMT by veteran cosmonauts Oleg Kononenko and Sergei Prokopyev.
Kononenko will use a knife to rip open the insulation and protective covering of the spacecraft to look at the hole and scrape off samples.
These will then be sent to Earth to "get at the truth" of the cavity's origins, a video released by the space agency said.
"It's a challenge. Sergei and I are accepting it," Kononenko said.
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.
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