Discovery is scheduled to lift off at 19:49 GMT on Saturday amid concerns within the US space agency on the safety of the mission.

Saturday's launch is only the second shuttle mission since 2003, when seven astronauts died aboard the Columbia shuttle when it broke apart on re-entering the atmosphere.

Investigators said Columbia's break up was caused by a piece of insulating foam that broke away from the shuttle's external fuel tank and pierced the craft's heat shield.

Bryan O'Connor, Nasa's chief safety officer, and Chris Scolese, the chief engineer, had called for a six-month delay to the launch to redesign foam on the fuel tank.
 
But the two officials backed the launch after Nasa said the seven astronauts could abandon ship and take refuge in the international space station should the shuttle suffer irreparable damage during its launch.

Right decision

Michael Griffin, a Nasa administrator, acknowledged there was concern over the launch but insisted the flight was crucial to the sending of astronauts back to the Moon and eventually to Mars.

"Flying a shuttle is not without risk for many reasons," Griffin said on Friday at the Kennedy Space Center in Cape Canaveral, Florida.

"In fact, I worry that we spend so much time worrying about foam that we won't worry about other things which could get us."

The astronauts themselves said they were confident the decision to go ahead with the launch was the right one.

"It's not a democracy. We don't take a vote. We don't need 100% of the people to say it's OK," said astronaut Scott Kelly.

Nasa has installed more than 100 cameras to detect foam falling from the shuttle during launch.

Discovery's seven-member crew will test shuttle inspection and repair techniques, bring supplies to the international space station and deliver the European Space Agency's Thomas Reiter for a six-month stay aboard the orbiting outpost.