"The cosmonauts spent hours using knives, scissors and what looked like garden shears to cut away at the insulation around the spacecraft's orbital module, to peek at the damaged area", NPR's Geoff Brumfiel reported.
Two Russian cosmonauts - Oleg Kononenko and Sergey Prokopyev - on their 6th hour of spacewalk took samples to solve the mystery of a hole in their capsule docked at International Space Station (ISS).
The puncture has been sealed since August, halting the oxygen leak.
What makes it especially hard is that the Soyuz spacecraft, unlike the ISS, was not created to be repaired in spacewalks and has no outside railings for astronauts to hold onto.
The crew slowed the leak with a piece of Kapton silicon tape, and later patched the hole with gauze and epoxy. Prokopyev controlled the booms' motion from the opposite end, moving Kononenko into place, before shimmying up the second boom himself. As he vigorously cut into the spacecraft, small bits of insulation floated off into space.
A Soyuz spacecraft docked to the International Space Station.
And so on Tuesday the 2-millimeter hole led to a foot-long gash, as Kononenko sawed through the debris shield and insulation, looking for clues.
So far astronauts have only been able to examine the hole from inside the spacecraft.
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But as they've investigated the hole in the Soyuz capsule, astronauts can't figure out what created it.
Kononenko used a pair of forceps and a swab to collect samples of the dark epoxy.
That section of the capsule will be jettisoned as usual before re-entry, and so poses no risk for descent.
"The active participation of worldwide partners, primarily NASA, in the investigation is not expected, although we have called for a joint investigation and been ready to provide available materials: photos, videos, documents, samples of materials taken during the spacewalk", the source said.
Tuesday's spacewalk began at 10:59 a.m. EST (1559 GMT) and ended at 6:44 p.m. EST (2344 GMT) when the hatch to the Pirs docking compartment was closed.
Back in August, astronauts noticed a slight drop in pressure on the International Space Station.
The insulation proved harder to remove than expected.
The Soyuz capsule is due to transport three of the station's crew back to Earth next Wednesday.