NASA is under pressure to bring home the space shuttle Discovery with John Glenn and six crew-mates on time because of deteriorating weather at its landing sites, space agency officials said Friday.

"Saturday is the best day of all so we really will try to get down tomorrow," flight director Linda Ham told a news conference.The space agency is hoping to end the high-profile mission, which put U.S. Sen. John Glenn back into space after a 36-year hiatus, with a landing at the Kennedy Space Center in Florida today at 12:04 p.m. EST.

However, NASA has put on alert the shuttle's back-up landing strip in California, just in case. Twenty-two of the last 23 shuttle flights have ended in Florida, managers said.

Weather conditions at the Florida spaceport are expected to be fair for landing after the remnants of Hurricane Mitch stormed across the state earlier in the week. The only concern is the possibility of low clouds that could obscure the pilots' view of the runway.

The forecast also looked good at the shuttle's other landing strip, located at Edwards Air Force Base in California's Mojave Desert.

Discovery has two landing opportunities, 90 minutes apart, at each runway. The first California landing slot is at 1:35 p.m. EST.

On Sunday the picture looked much worse, forecasters warned, with more chances of thick clouds over the Florida runway. In California, rain and high winds were expected to rule out a landing there.

Discovery's crew of seven spent Friday testing the spaceship's landing systems and packing away more than 80 experiments that have kept them busy for eight days in the shuttle's laboratory compartment.