The 115th shuttle mission is only the second since the Columbia 2003 disaster.
NASA cleared five areas on Discovery's heat shield that had been of potential concern, Steve Poulos, the orbiter program manager, told reporters at the Johnson Space Centre in Houston, Texas.
He said analysts will further examine two other spots: an insulating
blanket on Discovery's windows and a gap filler protruding from between thermal tiles in the shuttle's underside.
Last year, in the first shuttle mission since the Columbia tragedy, an astronaut conducted a spacewalk to remove protuberances in the shuttle's underside.
NASA officials said a similar spacewalk could take place if necessary to remove the gap filler, which is located in the back of the shuttle.
In a spacewalk planned before the shuttle's launch, two astronauts have tested a boom extension added to Discovery's robotic arm as a possible work platform that would enable them to get to hard-to-reach parts.
Poulos said the elongated robotic arm would have to be used to remove the gap filler.
"The only way to access that location is using the boom," he said.
Columbia disintegrated as it returned to Earth in February 2003 after its heat shield was pierced by foam insulation that shed from its external fuel tank during liftoff.