The crewmembers of the U.S. space shuttle Discovery are packing up their gear before undocking from the International Space Station Saturday.  A huge Italian cargo container that ferried up tons of supplies is being returned to the shuttle before Discovery backs away from the space laboratory and begins its flight back to Earth.  The space agency NASA says the mission has been a success despite several tricky technical problems.

Discovery is bringing back a mountain of trash accumulated on the space station during the two years the shuttle was grounded after the Columbia tragedy of 2003. The astronauts' mission of resupplying the outpost and conducting three spacewalks to renovate its systems has been carried out.

Lead flight director Paul Hill says the unexpected technical problems Discovery experienced did not shake the mission team's confidence.

"There are still very high spirits in how successfully this mission has gone, the great condition of this orbiter, and the good job this crew has done for us in orbit," said Mr. Hill.

But the technical problems of this long-awaited flight raised fears about Discovery's ability to withstand the scorching heat of the plunge through the atmosphere back to Earth.

 More hard insulation foam came off the external fuel tank during launch and hit the orbiter, just as it did to doom Columbia.  As a result, NASA has grounded further shuttle flights again until it solves the problem.

The foam apparently damaged Discovery's fragile heat shield.  An unprecedented photographic, radar, and laser survey of the craft in orbit found chips in ceramic tiles on the underside and a torn piece of a thermal blanket below the cockpit.  But after exhaustive analyses, NASA declared the shuttle intact and safe for re-entry.

A problem unrelated to the foam debris occurred when two thin strips of fabric wedged between the tiles to cushion them during takeoff popped up a few centimeters. Engineers worried that resulting changes in airflow around them at supersonic speeds of re-entry would increase the heat load on the orbiter. So an astronaut embarked on the first shuttle repair spacewalk to pluck them out.

Paul Hill says mission managers are not debating any further technical issues.

"Deorbit is not perfectly risk free, but the vehicle is in pristine condition," explained Mr. Hill.  "So we're not worried about any systems performing for deorbit. Our big risk now, aside from the inherent risk of deorbiting through a fireball, would be weather."

Discovery is scheduled to land at the NASA launch site in Florida on Monday. If weather is bad, Discovery has enough fuel and supplies to remain in orbit two extra days. Or flight controllers can order it to touch down at a desert location at an air base on the other side of the continent in California.

Although Discovery is considered ready for landing, Mr. Hill says he will be more at ease when the mission is over.

"All the contingency cases that we were worried about are behind us now," Mr. Hill said.  "But it is still real flying in space with real risks and we'll be more relieved at wheel stop when we see the crew step off the vehicle."