Launched in 1971, the spent upper stage of a Soviet SL-8 rocket was spotted flying in a highly elliptical orbit, the perigee of which put it on a potential collision course with the station.

Officials with U.S. Space Command at first warned NASA that the solid-fueled stage -- a 20-foot-long (six-meter-long) metal cylinder -- could pass within one-third-of-a-mile (a half-kilometer) of the station.

Based in Colorado Springs, Colo., the multi-service military unit -- which uses high-powered radar to track satellites and space junk -- revised that early predication. But NASA mission managers decided to boost the station out of harm's way anyway.

Hurtling 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 propulsion systems that could have been used to move the outpost. Mission managers, however, opted to employ the shuttle's thrusters to save limited fuel supplies aboard the Russian craft.

Endeavour commander Dom Gorie and pilot Mark Kelly carried out the orbit-raising maneuver about 10 a.m. EST (1500 GMT), pushing the 140-ton station up about six-tenths of a mile (0.96 kilometers).

And while that move might seem small, it propelled the station on a path that will create a 40-mile (64-kilometer) gap between the craft by the time the rocket stage makes its closest approach to the outpost Sunday afternoon.

The evasive action, meanwhile, prompted a delay in the shuttle's departure from the station and also exhausted fuel that otherwise would have been used to conduct a full lap-and-a-quarter "fly-around" of the station.

Camera-wielding shuttle crews routinely carry out detailed photographic surveys of the station's exterior while circling the outpost, and the looping swings normally serve as training exercises for shuttle pilots, often giving them their first "stick time" with a winged spaceship.

That in fact was the case for Kelly, a rookie astronaut who took full control of an orbiting shuttle for the first time Saturday, backing the ship away from the station at 12:28 p.m. EST (1728 GMT) -- or about 90 minutes later than originally scheduled.

Plans for his inaugural flyaround had been cut to just a quarter-lap, but some fast re-planning effectively enabled mission managers to offer Kelly a chance to fly a half-lap around the outpost.

The shuttle crew, however, had to agree to a couple of concessions in order to save enough fuel for an expanded loop.

"As you know, life is full of choices. So we're going to give you a choice," astronaut Shannon Lucid told the shuttle crew from NASA's Mission Control Center in Houston.

"If you are willing to delay undocking, if you are willing to give up one hour of off-duty time this afternoon, and if this evening you are willing to sleep heads up -- in other words, no Earth viewing during crew sleep -- then we think we can give you a half a lap for a flyaround," she said.

"Merry Christmas, Mark."

"Thanks, Shannon. I really appreciate all the hard work you guys did to give me the half-lap flyaround," Kelly replied. "Since it will be in the sun, that's certainly the best part."

"And it's not only daylight, it's over the top," Lucid added.

With Kelly slowly backing Endeavour off to a point about 450 feet (136.5 meters) from the station, shuttle cameras beamed back pictures of the outpost gleaming against jet black space, a forest of radiators, robot arms, radio antennas and solar arrays jutting out from its core.

Other pictures were captured as the shuttle cruised toward a point directly above the station, which was passing over the Pacific Ocean, South America, the Caribbean Sea and the southeastern United States at the time.

"Great job flying, Mark. Looks great," station flight engineer Daniel Bursch said over a ship-to-ship radio link.

"He's having a great time," Gorie replied. "Flying over Florida was a treat, watching you guys soar over the Earth below."

Kelly ultimately conducted a final separation burn, propelling the shuttle, its four astronauts and the returning station crew away from the vicinity of the outpost.

"Fair winds and following seas, my friends," Gorie said.

"It was great flying with you guys," Bursch replied. "We'll miss all of you, and hopefully it'll seem like yesterday when we get home."

The shuttle's departure came about three-and-a-half hours after hatches between Endeavour and the station were closed for a final time.

Launched Dec. 5, the shuttle ferried a new crew -- Yuri Onufrienko, Carl Walz and Bursch -- up to the station along with about 3.5 tons of food, clothing, supplies and research equipment.

Known as the Expedition Four crew, the trio plans to stay at the station until mid-May, conducting 65 U.S. and Russian science experiments and overseeing the delivery of a key station truss segment.

Culbertson and his two Russian colleagues, meanwhile, presented the new station residents with a few going away gifts, candy canes and a Christmas tree.

"This is a small tree that you can hang up and help you count down the days until Christmas," the veteran U.S. astronaut said. "It has a little souvenir for each day in December until the 25th, and hopefully (it) will help you get in the spirit."

The Expedition Three crew, meanwhile, will be home in plenty of time for the holidays.

With Culbertson, Dezhurov and Turin in tow, Endeavour is scheduled to land at Kennedy Space Center at 12:55 p.m. EST (1755 GMT) Monday.

High winds, thunderstorms and low-level clouds could force mission managers to keep the shuttle aloft an extra 24 hours, but weather here at NASA's coastal Florida spaceport is expected to clear by Tuesday.