One, reminiscent of a Hollywood blockbuster, envisages Discovery damaged beyond repair. It would be emptied of its cargo and its seven crew and left to plunge into the atmosphere and disintegrate.

Back on Earth, the shuttle Atlantis would be prepared for an emergency mission to rescue the crew left stranded at the International Space Station.

Four astronauts -- -- Commander Steve Lindsey, co-pilot Mark Kelly and mission specialists Mike Fossum and British-born Piers Sellers -- have been training for six months to stage a rescue mission.

NASA expressed confidence Wednesday that debris seen falling off Discovery during its blastoff Tuesday would not cause major problems for the shuttle -- the first mission since Columbia broke up on February 1, 2003 killing the seven crew.

Discovery began its ascent on Tuesday, small pieces of tile broke off, NASA officials said.

Columbia broke apart as it re-entered Earth's atmosphere, killing all crew members on board. A piece of insulating foam had broken off an external tank on liftoff, piercing Columbia's wing and allowing superheated gases to penetrate the shuttle.

The Discovery emergency plan is dubbed "Safe Haven."

As part of the plan, Discovery is carrying excess supplies and equipment that would be transferred to the space station in the event that the shuttle is deemed critically damaged, NASA said.

"Once the shuttle consumables are depleted, the unmanned shuttle will be remotely commanded to undock by mission control in Houston and burn up in the Earth's atmosphere," NASA noted.

At the same time, Atlantis "will become the rescue vehicle, and work will focus on it launching and arriving at the station before consumables run out," the US space agency added.

The seven Discovery astronauts and the two people on the space station could spend more than a month in orbit in the station, which is normally designed to accommodate three people at a time.

Still, the rescue mission itself is not without its risks. Indeed, in this last-resort scenario, Atlantis's launch would have to be rushed, with no time to conduct the customary battery of tests before liftoff.

Upon its return, the shuttle would carry 11 astronauts, in another first for the US space agency.

Such a mission would leave NASA with just two shuttles, eliminating the rotation of vessels from mission to mission, in effect dooming the US space program until the shuttles' successors roll out -- in around 2010.

But NASA is also confident it will not come to that. "The probability is so low that we are just covering ourselves with belt and suspenders," shuttle deputy program manager Wayne Hale commented recently.

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