Astronauts aboard the space shuttle Endeavour wrapped up a 16-day mission to prepare the International Space Station for its first six-member crew with a flawless touchdown at Nasa's backup landing site in California.

Double-sonic booms blasted through the Mojave Desert as Endeavour dipped below the speed of sound for the first time since its November 14 launch from the Kennedy Space Centre in Florida.

Nasa had hoped Endeavour would land there as well, but bad weather spurred flight controllers to divert the crew to the Edwards Air Force Base in California.

Commander Chris Ferguson steered the shuttle through a series of sweeping curves to burn off speed before gently guiding the 100-ton ship onto a runway at 4.25 p.m. EST (21.25 GMT) to complete Nasa's fourth and final mission of the year.

"Welcome back. That was a great way to finish a fantastic flight," astronaut Alan Poindexter radioed to the crew from Mission Control in Houston.

Mr Ferguson replied, "We're happy to be here in California."

The shuttle's California homecoming means it will be another day before returning space station flight engineer Greg Chamitoff is reunited with his wife and toddler twins, who were awaiting his arrival in Florida.

Mr Chamitoff, 46, had been in orbit for six months. He was replaced by rookie astronaut Sandra Magnus, who launched aboard Endeavour and who is to remain aboard the station with two other crew members until February.

Nasa prefers to land in Florida to save about a week and almost €1.6 million to ferry the shuttle cross-country.

Endeavour returned from an ambitious and successful home-improvement mission at the €79 billion space station, a project of 16 nations.

During 12 days at the orbital outpost, the crew installed a water recycling system that will enable Nasa to add three more astronauts to the space station's permanent live-aboard crew.

They also conducted four spacewalks to repair the station's power system and delivered two new bedrooms, a second toilet, exercise equipment, a small galley and other gear. A Russian cargo ship arrived at the station early Sunday with fuel, water and more supplies. Flight engineer Yury Lonchakov remotely guided the Progress vessel to a docking port after its automated system failed.

The Endeavour astronauts remained in California overnight and returned to the Johnson Space Centre in Houston yesterday.