Nov. 27, 2009  -- While many Americans are figuring out how much they can spend on Black Friday, flight controllers in mission control at the Johnson Space Center are crunching numbers of their own, calculating the odds of the International Space Station's getting hit by a piece of space junk.

At issue is an old rocket that could force the five astronauts on the orbiting station to change their orbit to avoid getting whacked by the debris. It's a Delta 2 rocket that launched a comet-sampling probe called Stardust from the Kennedy Space Center back on Feb. 7, 1999. The rocket is still up there, and ten years later, it has come back to haunt the International Space Station.

The threat of space junk is such a routine event that orbit crews are put on alert at least once a month to seek shelter or get ready to move out of harm's way.

Dr. Nicholas Johnson, orbital debris scientist at the Johnson Space Center in Houston, told ABC News earlier this year that even a small particle could disable a spacecraft.

"We try to design spacecraft to be able to withstand small impacts," he said. "The larger things, we can detect when they are coming, we do this for the space station every day. And if we see a large object coming close, we simply get out of the way."

The U.S. Space Surveillance Network, operated by the Department of Defense, tracks space junk. And it's a mess up there, with millions of pieces of debris orbiting Earth. Space junk has become such a part of the public psyche that the Air Force uses it as a recruiting tool in TV ads that run during sci-fi shows on broadcast and cable networks.

Imaginary Space Station Box Guides Planning

Flight controllers are running the numbers in order to make a decision by the end of the day to figure out if the old Delta rocket will come close enough to force them to boost the space station's orbit Saturday afternoon to avoid a close encounter early Saturday evening.

They use an imaginary box around the space station as a guideline in their decision making. It's shaped like a pizza box, roughly half a mile by 15 miles in size. An unplanned maneuver could change plans for the trip home for three space station crew members. Frank De Winne, Bob Thirsk and Roman Romanenko are scheduled to fly home Monday in a Russian Soyuz spacecraft. The Soyuz serves both as a ferry craft -- and a lifeboat if ever the station really does get hit.