Atlantis is due home Friday. Photo: NASA TV/Spaceflight Now

The Atlantis astronauts tested the shuttle's navigation and maneuvering systems today, conducted a final few atmospheric observations for an Air Force experiment and stowed equipment in preparation for landing Friday at the Kennedy Space Center.

With forecasters predicting ideal conditions, commander Jeffrey Ashby plans to fire Atlantis' twin braking rockets at 10:36:15 a.m. Friday to set up a landing on runway 33 at the Kennedy Space Center at 11:44:06 a.m.

"Today was a busy day for the crew of Atlantis," said entry flight director John Shannon. "They're preparing to come home tomorrow. As part of that, the pilot and commander went through a check of all the critical entry systems, navigational systems, hydraulic systems and all the reaction control jets. Everything checked out fine, we had no problems at all. The vehicle looks like it's in great shape to come home tomorrow."

Shannon said the forecast was so good, in fact, mission managers opted not to staff the shuttle's backup landing site - Edwards Air Force Base, Calif. - Friday or Saturday. Atlantis has enough on-board supplies to remain in orbit until Tuesday in a worst-case scenario. But no one expects it to come to that.

"The weather at Kennedy, I'm happy to report, looks really good," Shannon said. "The forecast right now is a 'go' forecast. There is a forecast for some scattered clouds at 2,500 feet, which is not a violation, also the winds will be coming almost out of the due north at 10 to 16 knots. That would mean our preference would be KSC (runway) 33, the runway that runs from the southeast to the northwest. So right now it looks good. The vehicle's in good shape ... and the forecast is currently go."

This morning, Ashby and pilot Pamela Melroy checked out the shuttle's re-entry systems, firing up one of the ship's hydraulic power plants, testing the movement of aerosurfaces and making sure cockpit instrumentation was working properly. They also test fired Atlantis' small steering jets in a routine pre-entry check. As Shannon said, everything went smoothly.

Their crewmates, meanwhile - Sandra Magnus, Piers Sellers, David Wolf and cosmonaut Fyodor Yurchikhin - spent the day tidying up the shuttle's cabin, stowing loose gear, laptop computers and local area network equipment. All of the astronauts put in a final round of exercise and all participated in a final set of round-robin media interviews.

All agreed that saying farewell to the crew of the international space station Wednesday - commander Valery Korzun, Sergei Treschev and Peggy Whitson - was a particularly poignant moment, bringing tears to the eyes of several fliers.

"Even test pilots cry when a great thing is coming to an end," Melroy said today. "But it was especially moving to see Sandy and Peggy, who are such close friends. We practically had to pry them apart like two pieces of Velcro stuck together. And I think that had an effect on all of us, too."

For the space station's crew, now 137 days into a planned 167-day voyage, having company was a definite morale booster.

"For them, it was a little bit like having house guests," Melroy said. "They were probably really happy to see us after having been alone up there for awhile and then they were probably happy to see us go. We tried to be good house guests, we tried to be clean and tidy and to be everything we could be to be polite to them.

"But the most important thing, their commander, Valery, said to us one night as we were eating dinner together in the service module, he said 'thank you. Thank you, this is what we need, the human contact and all the crew together sitting and having some social time.' So we could tell that was really important for them, as well as for us."

This afternoon, Ashby and Melroy were scheduled to take turns flying a sophisticated shuttle landing simulator to sharpen their piloting skills before the real thing on Friday.

Atlantis' re-entry will begin 249 miles above the central Indian Ocean. Flying upside down and backwards, Ashby and Melroy will fire the shuttle's twin orbital maneuvering system rockets for one minute 58 seconds starting at 10:36:15 a.m., slowing the ship by 152 mph. That will lower the far side of the shuttle's orbit deep into the atmosphere above the Kennedy Space Center. After a 36-minute freefall, Atlantis will hit the discernible atmosphere at an altitude of 399,200 feet above the south Pacific Ocean.

From there, Atlantis' ground track will carry the astronauts above Central America and the western tip of Cuba before crossing the west coast of Florida south of Tampa. Taking over manual control at an altitude of 50,000 feet above the Kennedy Space Center, Ashby will guide Atlantis through a sweeping 287-degree right overhead turn to line up on runway 33. Touchdown is expected around 11:44:06 a.m. to close out a 4.5-million-mile voyage spanning 170 complete orbits since blastoff Oct. 7.

"It's been a smashing success from all our points of view and all good things come to an end," Sellers said. "At least for a while."