The two men aboard the international space station heard a strange metallic sound again Friday, four months after being startled by it the first time.
Cosmonaut Alexander Kaleri was talking to flight controllers in Moscow when he heard a loud drumlike noise coming from the instrument panel of the station’s Russian-built living quarters.
Kaleri and astronaut Michael Foale first heard the mystery noise — described as a flapping sheet of metal — back in late November. Neither the crewmen nor flight controllers were ever able to identify the sound.
Damage or 'tin-canning'?
Some engineers suspected that space junk may have damaged something on the exterior. But NASA sources told MSNBC.com two months ago that the noise may have a more benign origin: "tin-canning," or thermal buckling of the station's hull.
Such a phenomenon could arise when direct sunlight, plus sunlight reflected by the station's solar arrays, heat up a section of the station significantly more than usual, the sources said. They said the effect was observed aboard the Skylab space station in the 1970s.
It was not immediately clear whether there was a connection between last November's noise and the sound heard Friday morning. But Kaleri said the noise came from about the same place as before and sounded the same.
“I had the headset on, so I didn’t hear it very clearly. But it sounded sort of like a drum. It sounds sort of like a sheet of something being bent,” the cosmonaut reported.
Russian flight controllers told Kaleri that they would try to figure out where the noise was coming from, and speculated that perhaps one of the systems inside the station was the source of the problem, rather than something on the outside.
NASA officials, however, said all systems appeared to be operating properly.
“It’s very strange,” Russian Mission Control said. “I doubt that it would be a coincidence that you’re hearing the same thing coming from the same place.”
Tracking down the cause
During a spacewalk in February, Kaleri and Foale were supposed to check the exterior of the space station where the noise originated last November. But Kaleri’s spacesuit overheated and became damp, and the spacewalk had to be cut short, so the men did not have time to inspect the area.
Kaleri and Foale’s six-month space station mission is almost over. Their replacements are due to arrive in another 2½ weeks.
NBC News space analyst James Oberg said flight controllers should continue trying to track down the definitive cause of the noise.
"One 'funny' is an anomaly," he said. "Two is a pattern. We'll see what develops. It could be entirely normal and nothing to worry about. But in the post-Columbia era, NASA must prove this, not just assume it."