By Jeff Franks

HOUSTON, June 21 (Reuters) - Weather forecasts were not encouraging, but space shuttle Atlantis was scheduled to return to Earth on Thursday in an afternoon landing at Kennedy Space Center in Florida.

The shuttle's seven astronauts completed preparations on Wednesday night for the trip home, then rested ahead of the planned fiery descent through the atmosphere.

Atlantis spent most of its 13-day mission at the International Space Station, where the crew installed a 17-ton metal truss that included solar power panels to generate additional electricity for the half-finished complex.

They were due to land in Florida at 1:55 p.m. EDT/1755 GMT. NASA officials said forecasts called for low clouds and possible thunderstorms in the area, which could force them to wave off Atlantis for a day.

Kennedy will be their only possible landing spot on Thursday, but Edwards Air Force Base in California becomes an option on Friday if Florida weather stays bad, said flight director Norm Knight.

"Obviously, I would like clear skies, unlimited visibility and little wind, but unfortunately those are not the cards we're typically dealt," he said at Johnson Space Center.

Atlantis has enough supplies to stay in space until Sunday if necessary, Knight said.

NASA managers cleared Atlantis for landing after two inspections during the flight found no damage to the orbiter's heat shield.

During one of four spacewalks performed by Atlantis crew, astronaut Danny Olivas patched up a torn thermal blanket that protects an area near the shuttle's tail from heat.

NASA engineers believe they underestimated how much heat the underlying shuttle layers had experienced during launch on June 8.

"When they modeled it, they made a mistake," Deputy shuttle program manager John Shannon said of the original analysis. "Still, the engineering and safety teams believe there's absolutely no risk at all during re-entry."

Heat shield problems are a major NASA concern since Columbia broke up while returning to Earth in 2003, killing the seven astronauts on board. The accident was blamed on an undetected crack in the heat shield.

The Atlantis mission was the first of four shuttle flights scheduled this year. The addition of the solar panels and other projects by the shuttle crew prepared the $100 billion space station, the project of 16 nations, for upcoming station additions from Europe and Japan.

Their work was overshadowed for a time last week when computers that keep the space station properly positioned crashed and required an improvised rewiring to revive.

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