In space, they can hear you scream - if you're near a microphone - but whistling is another matter entirely.
Impatient for their spacewalk to begin Tuesday, NASA astronauts Mike Fossum and Ron Garan conducted a little science experiment of their own to see if they were at vacuum yet. They tried whistling.
"When you're down to vacuum you can't whistle anymore in a spacesuit," said mission commentator Rob Navias.
Judging from the perky whistling coming from airlock, the astronauts still had some time to spare before they could begin what was planned to be a 6.5 hour spacewalk.
Their primary job is to retrieve the station's 800-pound broken cooling pump and pack it on the shuttle to be returned to Earth.
The pump shut down last July, wiping out cooling to half the station. Space station astronauts made three emergency spacewalks to install a spare. NASA wants to analyze the broken unit to see if it needs to operate the system differently or make some design changes for the future.
Fossum and Garan are station astronauts helping out the short-handed shuttle Atlantis crew. Normally, shuttle astronauts do the spacewalking when they are visiting the station. The Atlantis crew, however, has just four astronauts, rather than the usual six or seven.
NASA pared down the crew size because it has no more shuttles available to bring the astronauts home in case their ship is too damaged to safely make it through the atmosphere for landing. Instead, the Atlantis crew will use the smaller Russian Soyuz capsules as lifeboats if needed.
The shuttle arrived in orbit in good shape after its thundering liftoff Friday from the Kennedy Space Center in Florida. Managers Tuesday cleared the shuttle's heat shield for re-entry, though the astronauts will make one more scan to check for orbital debris impacts before they head back to Earth.
The flight, the 135th and final of the 30-year-old space shuttle program, is due to end July 21.
On Monday, NASA extended the mission a day to give the crew more time to transfer over the five tons of cargo they've brought to the station and to pack up broken equipment and trash for the return trip home.
Image: At vacuum? Try whistling. Ron Garan (top) and Mike Fossum exit the International Space Station's Quest airlock to begin their spacewalk. Credit: NASA TV