US space shuttle Endeavour undocked from the International Space Station (ISS), Sunday, and began its journey back to earth, marking the end of its 16-day mission of upgrading the orbiting space laboratory.

Endeavour undocked at 11:55 pm (EDT) after its 6-member crew - five US astronauts and Italian Roberto Vittori - bid farewell to three colleague on board the ISS.

While Shuttle commander Mark Kelly (husband of Rep. Gabrielle Giffords) joked that they're going to leave these guys to some peace and quiet and not disturb their space station any more, the station's skipper Russian Andrey Borisenko wished the astronauts a soft landing.

During its 12 days at the station, Endeavour's crew completed four spacewalks to complete construction of the U.S.-side of the $100 billion outpost, a project of 16 nations that is being assembled in orbit since 1998, delivered the station's premier science experiment - a $2 billion Alpha Magnetic Spectrometer particle detector - and a pallet of spare parts intended to tide over the orbital outpost after the shuttle program ends.

The astronauts also worked on some of the critical life-support systems inside the ISS, in an effort to leave the orbiting station in the best possible shape for the shuttle-less years ahead.

The space shuttle's poignant 2-day journey back to earth, after spending 12 days at the station, is going to be a memorable one as NASA plans to retire its three-ship fleet. It will land on Earth on June 1 and will be displayed at the California Science Center in Los Angeles.

Space shuttle Atlantis will launch on July 8, carrying a year's worth of supplies for ISS - a contingency plan in case the commercial companies hired to take over supply runs to the station encounter delays with their new vehicles.

It will be the final shuttle mission before the U.S. ends its 30-year shuttle program, dogged by 2 accidents - the Challenger accident (1986) and the Columbia breakup during re-entry (2003) - that left the nation mourning the deaths of 14 men and women and questioning the inherent risk of its space program.

The end of the program will save NASA $4 billion in annual operating expenses. The space agency now plans to develop new vehicles that can travel beyond the the ISS orbit.