Concern over the shuttle’s safety had been heightened after high drama caused by mysterious floating debris.

"Glad to be back. It was a great team effort, so I think assembly is off to a good start," commander Brent Jett said immediately after touchdown at Kennedy Space Center at 6.21 am [1021GMT] on Thursday.

The landing was a day later than planned because Nasa ordered up more inspections of the spacecraft's delicate skin to make sure it was safe to come home.

The fear was that one of the mysterious objects might have hit the shuttle.

"We've seen a new standard in Nasa vigilance," said Wayne Hale, shuttle programme manager.

After numerous cameras above and below, some of them manoeuvred robotically by the shuttle astronauts, Nasa proclaimed the spacecraft damage-free.

Safety fears

"We've seen a new standard in Nasa vigilance"

Wayne Hale, shuttle programme manager

The drama threatened to overshadow what had been a nearly flawless mission filled with strenuous spacewalks and rigorous robotics work that placed the international space station back on a path to completion after a 3½-year hiatus.

Nasa officials said their best guess was that the most worrisome object was a plastic filler placed in between thermal tiles that protect the shuttle from blasting heat. Four other pieces of debris, including a possible garbage bag, floated near the shuttle.

Atlantis' return avoided a near traffic jam at the space station, as a Russian Soyuz spacecraft arrived at the space station less than two days after Atlantis had departed.

It was the 21st landing in darkness of 114 successful landings.

The Atlantis mission was the first of 15 tightly scheduled flights needed to finish constructing the half-built space lab by 2010.

The shuttle delivered a 17½-tonne truss addition with two massive solar arrays that opened like gleaming golden wings. The solar panels will eventually provide a quarter of the station's power when it is finished in 2010.

In three highly choreographed spacewalks, astronauts hooked up cables, removed bolts and opened up a radiator over the solar arrays.

Nasa had described the 11-day schedule as one of the busiest and most challenging ever for a shuttle crew.