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By Irene Klotz

CAPE CANAVERAL, Fla., March 26 (Reuters) - The U.S. space shuttle Endeavour returned to Earth on Wednesday, capping a milestone flight that brought Japan fully into the International Space Station partnership with the delivery of the first part of its research laboratory.

Clouds at the Kennedy Space Center in Florida prompted NASA to bypass Endeavour's first landing opportunity and nearly the second, but conditions stabilized and the wheels touched down at 8:39 p.m. EDT (0039 GMT Thursday), ending NASA's 122nd shuttle mission in darkness, just as it began 16 days ago.

With commander Dominic Gorie in control, Endeavour crossed Florida from the west, heading toward the Atlantic coast. Double sonic booms shattered the evening silence as the ship's speed dipped beneath the sound barrier for the first time since it blasted off in the predawn hours of March 11.

Gorie looped over the ocean and then nosed the 100-ton ship onto a concrete runway between canals in the Florida marshland.

"Welcome home Endeavour," said astronaut Jim Dutton from Mission Control in Houston. "Congrats to the entire crew .... on a very successful mission."

"Thanks Jim," replied Gorie. "It was a super rewarding mission, exciting from the start to the ending."

Endeavour dropped off at the space station a storage room for Japan's Kibo lab, as well as a Canadian robot to help astronauts maintain the $100 billion outpost.

In the crew cabin upon its return was space station flight engineer Leopold Eyharts, a French astronaut who spent seven weeks aloft to set up a European lab, called Columbus, which was delivered during the last shuttle mission in February.

NASA hopes to complete three more missions to the space station this year and a servicing call to the Hubble Space Telescope as it whittles down an 11-flight manifest that must be finished by the time the shuttles are retired in 2010.

The Endeavour astronauts stayed at the station for 12 days, longer than any previous crew, and conducted five spacewalks to install Kibo's storage room, assemble the massive Dextre maintenance robot and prepare the complex for the next wave of construction.

NASA astronaut Garrett Reisman replaced Eyharts during the mission and will stay aboard until shuttle Discovery reaches the station in late May with the main part of Kibo.

Endeavour's departure cleared the way for the space station's next visitor.

Europe's debut cargo carrier, the Jules Verne, is scheduled to undertake two days of practice rendezvous maneuvers before berthing April 3. A Russian Soyuz rocket with two more new crew members is due to arrive the following week. (Editing by Jim Loney and Michael Christie)

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