Russian cosmonauts on the International Space Station (ISS) are engaged on a spacewalk to investigate a mysterious hole that caused a loss of air pressure in August.
The cause of the hole on the Soyuz MS-09 spacecraft, docked to the station, has not yet been established.
However, Russian officials have said they believe it was caused by a drill and may have been deliberate.
Oleg Kononeko and Sergei Prokopyev will spend about six hours on the spacewalk.
Nasa said that Kononenko will use a knife to cut through insulation and the protective covering of the Soyuz to locate the outside of the hole. Samples will be taken to be sent back to Earth.
The operation will be complicated because the spacecraft, unlike the ISS, is not designed to be repaired while in space and has no railings.
“There’s nothing, that’s the problem,” Kononenko said earlier.
Dmitry Rogozin, the head of Russia’s civilian space agency Roscosmos, described the spacewalk as “unprecedented in its complexity”.
The hole was discovered after crew members traced an air leak that was causing the minor loss of pressure on the ISS.
The Soyuz capsule had been used to deliver a new crew to the laboratory 400km (250 miles) above the Earth in June.
Crew members used tape to cover the hole and experts speculated that it could have been caused by the impact of a high-speed rocky fragment flying through space.
However, that theory was later ruled out.
Photos of the hole from inside the spacecraft circulated online.
A space industry source told Russia’s Tass state news agency that the spacecraft could have been damaged during testing at Baikonur Cosmodrome in Kazakhstan. The mistake might have then been covered up.
“Someone messed up and then got scared and sealed up the hole,” a source speculated, but then the sealant “dried up and fell off” when the Soyuz reached the ISS.
Sources quoted in Izvestia newspaper said that if sealant is found on the hull during the space walk then the hole was probably caused when the spacecraft was on the ground.
In September, Mr Rogozin said that a production defect was possible but “deliberate interference” had not been ruled out.