Discovery separates from the international space station 250 miles above Earth. Photo: NASA TV/Spaceflight Now

The shuttle Discovery undocked from the international space station today, leaving a fresh three-man crew behind and bringing the lab's departing Expedition Two crew one step closer to Earth after five-and-a-half months in space.

With shuttle pilot Frederick Sturckow at the controls, Discovery undocked from the station at 10:52 a.m. as the two spacecraft sailed 250 miles above the south Atlantic Ocean.

"Thanks a lot, guys, see you later," new station commander Frank Culbertson radioed the shuttle fliers.

"Thanks, Frank. Have a great mission," replied shuttle skipper Scott "Doc" Horowitz.

After looping around the station for a final bit of photo documentation, Sturckow fired Discovery's maneuvering jets shortly after noon to depart the area for good. If all goes well, Discovery will land at the Kennedy Space Center around 12:46 p.m. Wednesday.

Earlier today, floating in the Destiny laboratory module, Horowitz, Sturckow, Daniel Barry and Patrick Forrester looked on as Culbertson and his Expedition Three crewmates - Vladimir Dezhurov and Mikhail Tyurin - made their formal goodbyes.

"It's time to say goodbye station and good luck new crew," said outgoing station commander Yury Usachev. "I'd like to say thanks to all the people who supported our mission in Moscow and Houston and Huntsville. Thank you very much and we'll see you soon."

The ten astronauts gather in the international space station's Destiny module for farewell ceremony before undocking. Photo: NASA TV/Spaceflight Now

Usachev and his Expedition Two crewmates, Susan Helms and James Voss, were launched to the station on March 8. When they land Wednesday at the Kennedy Space Center, they will have logged 167 days five hours and seven minutes off the planet.

"This is a very important day for everybody in here," Culbertson said. "It's a very proud day for Scott, Rick, Dan and Pat as they finish a very successful shuttle mission. And it's a very emotional day for Yury, Jim and Susan as they're leaving what's been their home for over five months. It's a very significant and challenging day for myself, Vladimir and Mikhail as we begin our new mission.

"We wish you soft landings," Culbertson said to Horowitz. "Yury, Jim and Susan, I know it's a tough day, I know it's hard to say goodbye. But we really, really are proud of all you have done. We will do our best to keep up the good tradition you have started and to maintain just as high a standard of excellence if we can. Thank you very much for what you've done."

Said Horowitz: "We wish them a great journey as they continue in the exploration of space that will sometime take mankind out through the solar system and beyond."

At today's mission status briefing, NASA managers declared Discovery's mission a complete success.

"As this mission's launch package manager, I am ecstatic," said Sharon Castle, who managed the logistics transfer work at the Johnson Space Center in Houston.

The astronauts transferred 10 bags, or 993 pounds, of water from the shuttle to the station, along with some 9,700 pounds of equipment and supplies. Another 3,800 pounds of equipment and trash were moved from the station to Discovery for return to Earth.

The international space station as seen by Discovery during a flyaround maneuver performed after undocking. Photo: NASA TV/Spaceflight Now

"If you look at how much we transferred to the ISS, including water, it was about 10,600 pounds," Castle said. "And if you include all the hardware we transferred back and forth it was about 14,400 pounds. Of that, we had about 900 pounds of food that we took up, about 900 pounds of clothing and other provisions.

"We took systems spares and we took seven payloads in support of Increment Three," she added. "And down from ISS we are taking empty food containers that we will recycle, clothes that we will also recycle, about 900 pounds of Russian hardware that's going to be refurbished and six payloads along with their data for the scientists on the ground."

James Van Laak, manager of operations and integration for the station program, praised the combined crews and engineers on the ground for the "very successful completion of this first mission in Phase Three of the space station assembly program."

"It was a real demonstration of what the mature phase of ISS-shuttle operations is going to look like in that we accomplished a crew rotation, we transferred the logistics items that we needed to keep the crew alive, to support continuing research and to keep spares on board so we could maintain the station itself.

"We performed maintenance of critical hardware and software and of course, we performed EVA operations," he said. "We did complete the 25th (shuttle-based) EVA of the assembly of the space station, and that represents approximately 16 percent or so of what was forecast for assembly EVA operations."

Veteran shuttle flight director Wayne Hale said the forecast for Wednesday's landing is generally favorable. But depending on the progress of tropical storm Chantal, mission managers may opt to staff Edwards Air Force Base, Calif., Wednesday as a backup.