The United States space shuttle Atlantis is on its way back to Earth after eight days docked to the International Space Station ISS).

During their joint flight, the five Atlantis astronauts and three space station crew installed a new $164 million air lock, bringing to an end the first assembly phase of space station construction.

We accomplished a lot of work and I think it's primarily because these two crews came together as one crew.

Shuttle commander Steven Lindsey

During a farewell ceremony on board the ISS, Yuri Usachev, the Russian commander of the space station, presented each of the Atlantis astronauts with a silver medal.

"Thank you very much, and have a soft, safe landing,"' he said.

The spacecraft parted company high above the North Atlantic after the two crews posed for photographs and exchanged hugs.

Astronaut Jim Voss sets the timer on his camera for a group shot

"It's hard for us to leave," said shuttle commander Steven Lindsey. "We accomplished a lot of work and I think it's primarily because these two crews came together as one crew."

The Atlantis astronauts were due to land in Florida early on Tuesday, but the landing was delayed by nearly 24 hours because of bad weather in the vicinity of Cape Canaveral.

The installation of the new air lock marked an important step forward in the development of the ISS.

Before this, it had only been possible to carry out space walks by leaving the station through air-locks on the space shuttle docked beside it.

Now, astronauts living on the space station will be able to work outside the craft even when a shuttle is not there.

Airlock spec

Material - Aluminium Length - 5.5 metres Diameter - 4 metres Weight - 6,064 kilograms Volume - 34 cubic metres Cost - $164 million, including gas tanks

US astronauts Michael Gernhardt and James Reilly made use of the air lock for the first time on Saturday as they floated out into space to hang a high-pressure gas tank on the side of the door.

Three spacewalks were required to install the new 'front door' or Joint Airlock Module (JAM).

Its universal fittings mean it will eventually be possible for both American and Russian crew members to use it.

Open house

"Once the airlock is installed, the hatches between a visiting space shuttle and the space station can remain open," said Mike Hawes, Nasa Deputy Associate Administrator for the Space Station.

Floating weightless in the new airlock

"This is important because in previous missions we've had to open and close the hatches several times to maintain correct pressure during spacewalks.

"Now, we'll be able to keep those doors open, providing greater efficiency during joint orbital activities."

The JAM, which measures four metres (13 feet) across and 5.5 metres (18 feet) in length, was anchored to the ISS with the help of the station's advanced Canadarm2, which was added to the platform in April.