Shuttle Discovery and seven astronauts will soon begin preparing for a planned 7:05 p.m. landing at Kennedy Space Center, a process that will start with a maneuver to avoid a piece of debris dislodged during the mission's third spacewalk. The unidentified debris apparently came from either the shuttle or the station Saturday during a seven-hour spacewalk that deployed a cargo carrying device, wired cables and replaced gyroscope sensors, a circuit breaker and Global Positioning System antennas.
"Exactly what it is is not known," NASA TV commentator Pat Ryan said this morning.
The relatives speeds of the object and orbiter are not greatly different he said, but enough to cause concern.
As a result, Discovery commander Rick "C.J." Sturckow and pilot Kevin Ford will fire Orbital Maneuvering System engines just after noon, less than an hour after the crew wakes up, to create safe clearance for Discovery.
Flight controllers will be closely watching the weather in Central Florida throughout the day. A chance of thunderstorms within 30 nautical miles of the shuttle landing strip threatens to keep Discovery aloft another day or two, but managers hope they'll dissipate at landing time.
If the conditions don't co-operate for the first landing opportunity after 202 orbits, a second chance comes one orbit later for a 8:42 p.m. touchdown.
For the second time in as many flights, Discovery is outfitted with a small "speed bump" on its belly as part of an experiment to better understand the effects of heated airflow as the orbiter descends at many times the speed of sound.
A 0.35-inch bump was placed on a heat-shielding tile under Discovery's left wing, which is slightly larger than the bump flown during the shuttle's STS-119 mission in March. The bump creates turbulence that increases heating on a controlled section of tiles behind it.
Engineers say the data will help understand the risks posed by loose gap fillers between shuttle tiles, and provide information that would help design future heat shields, such as the Constellation program's Orion capsule.
A Navy plane (left) will fly beneath the orbiter as it descends at Mach 18, helping to measure temperatures on the orbiter's heat shields. 
In the model shown at left, the heating produced by the speed bump in March is the smaller red area on the left wing. The more ominous looking red area on the right wing was part of normal re-entry conditions and no threat to the crew, engineers said.
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