Floating weightless in the new airlock

It has taken three spacewalks to install the new 'front door' or Joint Airlock Module (JAM), which will enable astronauts in the International Space Station to work outside their craft even when a space shuttle is not alongside.

Until now, space walks have only been possible by leaving the station through airlocks on the shuttle docked beside it.

The ISS has not had a proper airlock of its own to allow astronauts safely to make the transition from a pressurised environment to the vacuum of space.

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

The JAM will make the presence of a shuttle unnecessary.

What is more, its universal fittings mean it will be possible for both American and Russian crew members to use it.

In the shuttle's airlock, the communications system and connections for oxygen and coolant will only accept American suits. The $164m JAM will eventually accept Russian connections as well.

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.

First spacewalk from 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, has been anchored to the ISS with the help of the station's advanced Canadarm2, which was added to the platform in April.

The airlock's installation will complete Phase Two of the space station's construction, which began in 1998 with the arrival of the Russian-built Zarya base block in orbit.

The five Atlantis shuttle astronauts are expected to return to Earth on 24 July.