"The TPS (thermal protection system) is 100 percent clear for entry," a mission control official told Discovery commander Steven Lindsey from the Johnson Space Centre in Houston, Texas.
Astronauts on the shuttle had expressed confidence in their spacecraft after NASA engineers said problems found since last week's launch appear minor enough to withstand the rigours of landing.
In an email message to the shuttle crew, they said most blemishes found in post-takeoff inspections did not pose a danger to Discovery and they expected a few lingering safety issues to be declared safe as well.
"We've been told we have a very clean vehicle. I certainly have no concerns about returning," mission specialist Stephanie Wilson said in a news conference from space.
"I hope it comes out okay and I think it's going to," said flight commander Steve Lindsey. "To get a clean vehicle or a win, I think, is really exciting."
Gap fillers concerning
One lingering concern focuses on a couple of ceramic cloth "gap fillers" sticking out from between heat shield tiles.
NASA managers said one has been judged safe and the other is under study, but "it is expected that this will be cleared" upon final analysis.
The same was true for minor damage to protective thermal blankets on the spacecraft.
"Both of these items are very close to be cleared as safe for entry," the email said.
If it turns out that there is a problem of note, astronauts Piers Sellers and Michael Fossum could attempt to fix it while spacewalking this week ahead of landing on July 17.
They are scheduled to perform the second of three spacewalks later today.
The email message to the crew said marks that showed up on photos and sensor inspections of the shuttle's wing and nose areas turned out to be nothing worse than possible bird droppings.
A steady stream of good news about Discovery's condition has given NASA growing confidence that its $US1.3-billion ($A1.74 billion) safety upgrade of the shuttles following the 2003 Columbia disaster is a success.
This flight was viewed by many in the US space agency as a make-or-break mission for the troubled shuttle program, which is critical to finishing the half-completed $A134 billion space station.
The shuttle Columbia broke apart as it descended toward Florida on February 1, 2003 because of a hole punched in its wing heat shield at launch by insulating foam that shook loose from the fuel tank.
Hot gases entered the breach during its fiery descent through the atmosphere, which destroyed the spacecraft and killed the seven astronauts on board.