Discovery shuttle cleared for return to earth

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 at 3:56pm EST (8.56pm Irish time).

Other landing opportunities were one and a half hours later at all three of Nasa's landing sites: Kennedy, Edwards Air Force Base in California's Mojave Desert and White Sands Space Harbor in New Mexico. Additional opportunities at Edwards and White Sands were 1½ hours after that. Nasa managers were considering a last try at 8:36pm EST (1.36am Irish time on Saturday) at Edwards.

Discovery needs to be on the ground Saturday 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.

The third 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.

Thursday afternoon, after another inspection and more analysis of the shuttle's heat shields, the space agency pronounced Discovery safe to return. Shuttles are routinely inspected in flight now for any debris damage of the sort that doomed Columbia in 2003.

During the 25 years of the shuttle programme, there have been 63 landings at Kennedy, 50 at Edwards and just one at White Sands.

Even though the White Sands runway regularly is used for practice landings by astronauts, Nasa does not like to use it for the real event. It could take as long as two months to get the shuttle back to Florida from New Mexico, compared to a week from Edwards, threatening NASA's ability to get Discovery ready to fly again next October.

Flight controllers in Houston, trying their hand at holiday songwriting, sent the Discovery crew in their daily messages lyrics to their version of the song, Let it Snow.

"Oh, the weather at KSC is frightful. But at White Sands, it's so delightful. And since we have to land. Land White Sands. Land White Sands. Land White Sands," it said.