About four hours into a spacewalk on Tuesday, Russian astronauts pulled out a knife and began cutting into a spacecraft docked at the International Space Station.

Soon they began slashing with vigor through the outer insulation, with flurries of debris flying into space. Other times, they used what looked like hedge shears to cut first through the insulation that was the spacecraft’s outer layer, and then into an aluminum micrometeoroid shield beneath that.

“Don't forget to be careful,” Russian flight controllers in Moscow repeatedly warned the astronauts, protected from the vacuum of space by nothing but a spacesuit. “And don't forget about the sharp edges. That's our main concern.”

Finally, the astronauts, Oleg Kononenko and Sergey Prokopyev, found what they were looking for: a tiny, sealed hole in the spacecraft’s hull.