The Endeavour looked for a break in the clouds to return to Earth after a successful two-week mission to install a node with a seven-window observation deck on the International Space Station.
Mission Control lined up a series of landing opportunities at NASA's Kennedy Space Center in Florida at 10:20 pm and 11:55 pm and two more later at Edwards Air Force Base in California.
However, predictions of rain showers and low clouds at both Kennedy and Edwards threatened to delay a homecoming for the six shuttle astronauts.
"Tomorrow is going to be a very difficult weather day," predicted NASA flight director Norm Knight, who will make the decision to land from Mission Control. "It's going to go down to the wire."
As Endeavour and its six-member crew prepared to touch down, NASA was focused on the shuttle's looming retirement and trying to ensure long-running efforts to assemble the International Space Station reach the finish line.
Four remaining missions are intended to bring the 12-year effort by the United States and its partners in Russia, Europe, Japan and Canada to assemble and outfit the sprawling orbital laboratory to a close by late September.
The Endeavour astronauts successfully delivered and installed Tranquility, the last of the habitable modules contributed by the United States to the space station.
Working with the five astronauts on the station, the shuttle crew outfitted the new module with critical life support systems, exercise equipment and a roomy observation deck.
They also revived the station's water recovery system, the hardware the recovers drinking water from urine and moisture in the breathing air.
The new observation dome surrounds a control post for the robot arm that will play a crucial role in docking the commercial cargo capsules NASA will rely on to deliver supplies after the shuttle's retirement.
It also offers those living aboard the station unprecedented views of the Earth below.
"Arguably, mankind has been after this view for centuries, this perspective, this view of the world," said Endeavour commander George Zamka after his crew completed the installation of Tranquility with three textbook spacewalks.
"We finally have it, and we will take advantage and enjoy it."
President Barack Obama doted over the new Earthly panoramas as he spoke with Zamka and his crew, Terry Virts, Kay Hire, Steve Robinson, Bob Behnken and Nicholas Patrick last week.
Endeavour lifted off in the pre-dawn of February 8 with the bus-sized enclosure that pushed the total mass of the space station to over one million pounds, or 90 percent of its final weight and 98 percent of the volume at full assembly.
Tranquility, which was named in honor of the lunar landing site explored by Apollo 11 astronauts Neil Armstrong and Buzz Aldrin, was built in Italy under the supervision of the European Space Agency.
The Europeans provided the module to NASA in exchange for the 2008 shuttle launch that delivered their Columbus research module to the station.
"It's an amazing facility, and it's very rewarding for all of us to be here at this stage, as we transition from assembly to full utilization," said NASA astronaut Jeff Williams, the station's current commander.
"We all look forward to reaping the benefit of the utilization over the next 10 years or so."
However, the station experienced a succession of command and control computer problems early Sunday, leaving Williams' crew unable to communicate with the Earth for an hour, a reminder of the station's remote status. Mission Control was investigating.
The remaining shuttle flights are intended to stock the station with research gear and spare parts to keep the outpost operating well after the final mission, which is set for late September.
Just days before Endeavour's departure, Obama presented Congress with a plan to overhaul NASA, including the controversial cancellation of the Constellation lunar exploration program.
The next shuttle mission, which will be flown by a crew of seven US and Japanese astronauts aboard Discovery, is tentatively scheduled to lift off on April 5 Discovery will deliver science gear to the station's US science module.
Endeavour's final flight is planned for a July 29 launching. Astronauts will deliver the Alpha Magnetic Spectrometer, an external station observatory for studies of cosmic rays, dark matter and anti-matter.
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