Shuttle astronauts on their way home

The multinational crew of the space shuttle Endeavour is preparing to return to Earth on Friday, following a marathon mission to the International Space Station.

There was only a slight chance that wet weather near NASA's Kennedy Space Centre runway in Florida might force the seven-member crew to spend an extra day in orbit, according to forecasters.

"It's time to come back." said Endeavour commander Mark Polanski, as the astronauts prepared the ship for its high speed descent.

"We have accomplished our primary objectives successfully."

Endeavour has two opportunities to touchdown Friday: at 10.48am (1248 AEST Saturday) and 12.22pm (0222 AEST Saturday).

Touchdown would end a long sojourn, during which the astronauts installed the final piece of Japan's Kibo science laboratory and delivered spare parts crucial to the space station's future operations.

The flight also saw a record number of astronauts aboard the orbital outpost, representing Europe, Japan, Canada Russia and the United States.

Inspections of the shuttle's fragile heat shielding revealed no major damage after the long flight, according to Bryan Lunney, the NASA flight director, who will supervise landing operations from Mission Control.

"The vehicle is in great shape," he said.

Kibo, the largest and best equipped of the space station's major experiment facilities, required three shuttle missions over 16 months to assemble.

Endeavour's crew carried out the last of the construction by joining a new external experiment platform to Japan's primary science enclosure and separate stowage module.

The long shuttle flight was punctuated by moments of drama and some humour.

Four of the astronauts conducted a record-tying five spacewalks, including one outing that had to be cut short when one of the astronauts registered higher than expected readings of carbon dioxide inside his space suit, in what proved to be a temporary setback.

The spacewalkers went on to replace a half-dozen batteries in the station's oldest power storage module, ensuring electricity for crucial life support systems and science experiments as the orbital outpost ages.

American Tim Kopra, who launched aboard Endeavour, will stay at the space outpost at least until early September. He replaced Koichi Wakata, Japan's first long-duration astronaut.

Wakata, who had flown in space twice previously, returns to Earth aboard Endeavour after 138 days in space and a novel experiment in which he wore a single pair of underwear made from a bacteria resistant fabric for a month.

"My space station crew mates never complained," he told an interviewer on Thursday.

Among the snags encountered during the mission was a temporary breakdown by one of the station's two toilets soon after Endeavour docked.

Later, an American device that cleanses the station's breathing air of carbon dioxide faltered as well. The untimely mishap threatened to force the shuttle astronauts to depart the station early until Mission Control devised a temporary solution.

Efforts to repair the air purifier were still under way aboard the station on Thursday.

Endeavour launched July 15 after five previous attempts to start the mission were scuttled for weather and technical reasons. Plans to lift off in June were stalled by an evasive hydrogen leak and daily thunderstorms at the Florida shuttleport.

As the seven shuttle astronauts finally roared into orbit, Endeavour's underside heat shielding was sprayed by breakaway fragments of foam insulation from a region of the external fuel tank that previously had not been considered a source of debris.

Mission managers determined the damage was minor after two days of external inspections carried out by the astronauts.

However, the unexpected loss of foam prompted a round of testing on the shuttle Discovery's fuel tank.

Discovery's 11-day mission to deliver scientific equipment and other supplies to the space station has been delayed until late August by an unplanned round of tests to check the bonding on the foam insulation.

The shuttle's fuel tank foam has been a source of concern for NASA since the 2003 loss of the shuttle Columbia.

The death of Columbia's seven astronauts was blamed on undetected heat shield damage from a large piece of the foam that jackets the big tank.

NASA's shuttle fleet is facing retirement in late 2010, and Endeavour's mission reduces the number of remaining flights to seven.

Many of the missions that remain will haul equipment and other supplies to the station to extend activities well beyond the shuttle's retirement.

The final flight, currently scheduled for launch on September 16, 2010, will deliver the Alpha Magnetic Spectrometer, an external observatory designed to measure cosmic radiation, as well as reveal levels of antimatter and dark matter.