Faced with dismal weather in Florida on Saturday, the Atlantis astronauts were ordered to back out of landing preparations and to remain in orbit a second extra day in a row.
Their next chance to land comes Sunday.
In Washington, meanwhile, the White House announced that former shuttle commander Charles F. Bolden Jr. will be nominated as NASA's next administrator, along with Lori Garver as his deputy, ending four months of speculation.
"These talented individuals will help put NASA on course to boldly push the boundaries of science, aeronautics and exploration in the 21st century and ensure the long-term vibrancy of America's space program," President Obama said in a statement.
On Sunday, the astronauts will have two opportunities to land at Kennedy Space Center in Florida and two at Edwards Air Force Base in Southern California. For the first Florida opportunity, the astronauts would fire Atlantis' twin braking rockets at 8:58 a.m. EDT, setting up a landing around 10:11 a.m.
The astronauts had hoped to land Friday at the Kennedy Space Center, but rain, lightning, and low clouds forced a one-day delay in hopes of better conditions. Kennedy was socked in again Saturday, with low clouds and rain showers near the shuttle landing strip. The forecast for Sunday is marginal, with clouds and rain expected.