HOUSTON -- Separated from the International Space Station, the heat shielding inspected one last time for damage, and all mission goals accomplished, the crew of the space shuttle Discovery was ready yesterday to come home.
NASA considers the flight an unblemished success, something the shuttle program has not had in nearly four years.
With just a few more last-minute radar data images to be examined, NASA engineers could find no problems with the shuttle heat shield. They had used several methods to look for flaws over 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 tomorrow at either 9:14 or 10:50 a.m.
Discovery must land by sometime Wednesday, and if it cannot complete its flight tomorrow, NASA will consider the backup landing site at Edwards Air Force Base in California.
Early yesterday, 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."
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 so that it could dock again if necessary.
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 today as part of its normal testing and engineers will watch to see if the leak rate changes.
That outcome was unlikely, said John Shannon, the shuttle program's deputy manager . But if the power unit were shut down, the shuttle could land with just two power units for the first time in its 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 that allowed hot reentry gases to burn into that shuttle, leading to its fiery disintegration and killing seven astronauts in 2003.
Last year's first post-Columbia flight, involving Discovery, also had unexpected heat shield problems. 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 concern.
NASA is planning more shuttle missions to complete the space station, with the next one scheduled about Aug. 28.