Sunita Williams bid a tearful adieu to her Russian crew mates, her American replacement and the laboratory she called her home for the last six months as the Indian-American astronaut spent her last day in space.

As space shuttle Atlantis readies to return to earth with Williams after her six-month sojourn at the International Space Station [Images] on Tuesday, she said in an emotion-choked voice, "I am saddened to say goodbye."

"But that means progress is being made and it is time for the international space station to grow a little more," she told her fellow space travellers Fyodor Yurchikhin and Oleg Kotov and her American replacement Clay Anderson.

The shuttle is expected to undock from the International Space Station sometime Tuesday and land at Cape Canaveral in Florida [Images] on Thursday. The Russian and US flight controllers tested the ability of Russian navigation computers to handle attitude control of the ISS.

"Clay's going to finish it, or Clay, Fyodor and Oleg are going to finish it without me," Sunita said.

"Exploration is not necessary or always logical," she said in a halting voice.

"It is something that comes from inside each and every one of us. I believe we are all born with this curiosity called exploration. Human spaceflight is just one aspect of exploration, but it is the one that I have been intimately familiar with over the past six months."

She beamed down a recording of James Blunt's Goodbye My Lover, and dedicated a second song to Anderson: the Bee Gees' Stayin' Alive.

"You and the ISS will always be a part of me as you are part of so many who will pave the way for our future," she said.

Yurchikhin said, "We had real great time with Suni here. She did real great job in space. We would like to say Suni have a good trip and a safe trip."

Anderson thanked Williams saying she was 'very helpful and I know this is going to be a tough day for her, but I am very proud of her, she is a sister to me and I hope I can carry on and do half as well as she did while she was on orbit.'

Suni, as she is called, came to the station in December in her first space flight.

Sunita set a new space endurance record for female astronauts last Saturday, moving past the 188-day four-hour record set by Shannon Lucid during a stay aboard the old Mir space station.

However, mission managers may yet decide to add a day of docked operations for the Atlantis crew if the Russian navigation computers do not perform adequately during a test on Monday morning.

The astronauts on Friday fixed the ISS computers after a 48-hour systems breakdown by bypassing a power switch with a jumper cable.

Also, Atlantis' thermal blanket was fixed on Saturday, clearing the way for its return flight to earth after a 13-day mission that was extended by two days due to the damaged thermal protection system.

Undertaking the spacewalk on Sunday, Forrester and Swanson first prepared the Starboard 3 and 4 (S3/S4) truss segment of the ISS, which was installed on June 11.