Mission accomplished, heat shield checked, shuttle ready to come home

HOUSTON -- Separated from the international space station Saturday, their heat shielding inspected one last time for damage and all their goals accomplished, the astronauts of the space shuttle Discovery were ready to come home.

NASA considers the flight an unblemished success, something the shuttle program hasn't had in nearly four years.

"It's sad to see the shuttle leave when it does, but I can't think of a better mission in recent history," said space station program manager Mike Suffredini. "Every objective that we went in with was completed and, in fact, a couple of extra things got done for us."

With just a few more last-minute radar data images remaining to be examined, NASA engineers could find no problems with the shuttle heat shield. They had used several methods to look for flaws during nearly two weeks.

An official, final "good to go" decision for landing is expected today. Discovery will try to land at a possibly cloudy and rainy Kennedy Space Center on Monday, at either 8:14 a.m. or 9:50 a.m.

Discovery must land by sometime Wednesday, and if it can't complete its flight Monday, NASA will consider the backup landing site at Edwards Air Force Base in California.

Early Saturday, Discovery's crew bid farewell to the international space station, taking pictures and leaving European Space Agency astronaut Thomas Reiter behind for a six-month stay.

Then, pilot Mark Kelly fired up the shuttle's steering jets, slowly backing Discovery away from the station as the two passed over the Pacific Ocean more than 220 miles below.

"Have a safe journey back, soft landing and we'll see you on the ground in a few months," space station flight engineer Jeff Williams radioed to Discovery.

Discovery commander Steve Lindsey responded: "Thanks, Jeff. We enjoyed it tremendously."

The undocking went "by the book," said chief flight director Tony Ceccacci.

After moving 45 miles away from the space station, the Discovery astronauts used the shuttle's 50-foot robotic arm and its new 50-foot extension boom to inspect the orbiter's right wing and nose cap -- the fourth precautionary examination of the 13-day mission. The shuttle stayed close enough to the space station that it could dock again if necessary to await a landing clearance from mission managers reviewing the inspection images.

Results of this final inspection were expected early Sunday. An earlier survey of the left wing, to check for any damage from orbital debris, turned up nothing of concern, said deputy shuttle program manager John Shannon.

There remained only one concern that could affect the astronauts' landing plans: a slow leak in one of the shuttle's three units that power hydraulic systems used for steering and braking.

There was no way of knowing whether the leak involved harmless nitrogen or flammable hydrazine, so the power unit with the leak will be turned on early Sunday as part of its normal testing and engineers will watch to see if the leak rate changes. If it does, NASA may burn off the hydrazine and shut the unit down before the landing to eliminate any fire hazard, Shannon said.

That scenario was unlikely, Shannon said. But if the power unit were shut down, the shuttle could land with just two power units for the first time in the spacecraft's history. The shuttle needs only one power unit to land.

Try as they might, NASA engineers could not find any damage to shuttle heat shields. The lack of apparent damage was a contrast from the last two shuttle missions.

It was a crack in Columbia's wing -- caused by a chunk of insulating foam that fell from its external fuel tank -- that allowed hot re-entry gases to burn into that shuttle, leading to its fiery disintegration and killing seven astronauts in 2003.

Last year's first post-Columbia flight, again involving Discovery, also had unexpected heat shield problems, including more foam hits.

This time the biggest piece of foam to come off Discovery's external fuel tank weighed less than an ounce and was no worry. The most noticeable blemish on Discovery's heat shield was a deposit of bird droppings that triggered more snickers than worry.