Based in Colorado Springs, Colo., the multi-service military unit uses powerful radars to track thousands of pieces of space junk that range in size from large rocket bodies to golf ball-size chunks of debris.

Whizzing around the planet at 25 times the speed of sound, even a small piece of debris could puncture the hull of a space station module, triggering a catastrophic depressurization in the deadly vacuum of space.

A lesser debris hit could force a station crew to abandon ship, returning to Earth in a Russian Soyuz spacecraft that serves as an emergency lifeboat at the outpost.

Both the Soyuz and an unmanned Russian Progress cargo carrier at the station have onboard propulsion systems that could be used to move the outpost if need be.

But since Endeavour is docked at the complex, too, mission managers opted to use its steering thrusters to boost the station, thus saving limited fuel supplies aboard the Russian craft.

The orbital raising maneuver will delay Endeavour's planned station departure by about 40 minutes. The shuttle now is expected to pull away from the outpost at about 11:37 a.m. EST (1637 GMT).

Plans for a looping fly-around of the station will be curtailed, limiting an orbital photo opportunity.

Shuttles normally fly a full lap around the station so that astronauts can make a detailed photographic survey of the outpost's exterior. But the fuel exhausted on the re-boost maneuver will force Endeavour pilot Mark Kelly to head for home after just a quarter-lap.

Watching the departure from inside the station: Russian commander Yuri Onufrienko and two American flight engineers, Daniel Bursch and Carl Walz, the three of whom will live and work aboard the station until mid-May.

Taking their leave of the outpost: U.S. skipper Frank Culbertson and two cosmonaut colleagues, Vladimir Dezhurov and Mikhail Turin, a trio that fully expects to have mixed feelings after a four-month tour of duty there.

"It will be hard to leave it," Culbertson told reporters in a space-to-ground news conference earlier this week.

"But the most important thing is I feel very good -- as do my crewmates, I think -- about going home and seeing our families and our friends and getting a chance to reestablish our lives on Earth," he added. "This has been great. But it's time to move on."

Launched Aug. 10 aboard shuttle Discovery, Culbertson and his colleagues boarded the station two days later. They've spent the past 124 days conducting science experiments, taking delivery of a new Russian airlock and maintaining station systems.

Their replacements were launched Dec. 5 aboard Endeavour, and the joined crews have spent the better part of the past week unloading and then repacking an Italian moving van that was hauled up in the shuttle's cargo bay.

The pressurized freighter was launched with 3.5 tons of food, clothing, supplies and research equipment for Onufrienko and his crewmates. Another 2.1 tons of luggage, surplus gear, garbage and science experiment samples were packed into the cylindrical cargo carrier before its hatch was sealed up by the joined shuttle and station crews Friday afternoon.

Kelly then limbered up the shuttle's 50-foot (15-meter) robot arm and moved the Multipurpose Logistics Module, or MPLM, from a station berthing port to Endeavour's payload bay for the trip back home.

Running about an hour behind schedule, the stowage work -- which was completed at 5:44 p.m. EST (2244 GMT) -- represented the last major job scheduled for the shuttle crew during what has been a hectic, nine-day stay at the station.

"It's been a pleasure every minute," Gorie told engineers in Mission Control.

Originally scheduled to depart the station Friday, the Endeavour astronauts extended their stay a day so they could pitch in with some maintenance work aboard the outpost.

They helped refurbish an exercise treadmill in the station's Russian-built crew quarters earlier this week and also installed six new utility outlets around the complex. A leaky compressor on a Russian air conditioning system was replaced Friday.

Ground engineers, meanwhile, were watching over three crucial navigation units aboard the shuttle. One of them began acting erratically late Wednesday and the failure of another would force the shuttle crew to make an early return to Earth.

As it stands, Endeavour remains scheduled to land at Kennedy Space Center at 12:55 p.m. (1755 GMT) Monday. But the weather outlook is grim.

Meteorologists are calling for a chance of thunderstorms, high winds and low clouds -- conditions that could force the Endeavour crew to linger in low Earth orbit an extra day before making a landing attempt.

Clear weather, meanwhile, is expected at NASA's coastal Florida spaceport Tuesday, and the shuttle has enough food, supplies and electric power to remain in space until at least Wednesday.