Nasa managers cleared Discovery to return home today, planning for a possible touchdown in New Mexico for only the second time in space shuttle history because of bad weather on both coasts.
As the crew woke up to Christmas music this morning, they still did not know where the spacecraft would touch down.
Shortly after noon, they continued to press on with landing preparation activities, including closing the payload bay doors. Nasa managers wanted the astronauts to be ready in the unlikely case that the unfavourable weather conditions at Kennedy Space Centre in Florida improve in time for the first landing opportunity tonight.
Discovery needs to be on the ground tomorrow or it could run out of the fuel that powers its electrical system. Nasa normally has more time for the landing, but the astronauts spent an extra day at the international space station this week to work on a stubborn solar array. Rain and clouds were forecast during the shuttle's first landing opportunity at Kennedy, and crosswinds were expected at Nasa's next-best option, Edwards.
Another option, White Sands, has not been used for a shuttle landing in 24 years, and in that landing, sand on the runway contaminated the orbiter, and the brakes were damaged. Normally, it is not equipped to service the shuttles, either.
Nasa managers hoped the weather would clear at one of the favoured sites by the first landing opportunity, but they shipped a crane to White Sands anyway, along with equipment that purges gases and cools and heats the shuttle on the ground, thruster plugs and 60 workers from the Kennedy Space Centre.
"As we get closer, we'll have much more certainty on what we're really faced with," said entry director Norm Knight, who will direct the landing. Nasa has seven more opportunities to land the shuttle on Saturday.
Discovery originally had been scheduled to land on Thursday, but the flight was extended to allow a fourth spacewalk to fold up an accordion-like solar array on the space station.