Shuttle Launch Delayed

Nasa has called off today's launch of the space shuttle Discovery because of a faulty fuel tank sensor.

The shuttle's seven astronauts had arrived at the launch pad earlier in the day under a drizzle and began boarding the spacecraft for what had been planned as the first shuttle flight since the Columbia disaster two and a half years ago.

The mission had been intended to blast off at 6.51pm BST on a mission to bring supplies to the international space station and test new techniques for inspecting and patching the shuttle in orbit.

Apollo 13 astronaut Jim Lovell told Sky News that the problem was unlikely to cause permanent problems. "In every launch I've ever been associated with there have been a variety of tech issues at the last moment," he said.

The problems relate to the same part of the shuttle that caused the Columbia tragedy.

Discovery's fuel tank has been entirely redesigned since the February 2003 disaster, when a chunk of foam insulation fell off Columbia's fuel tank at liftoff and slammed into a reinforced carbon panel on the shuttle's wing, creating a hole that brought the spacecraft crashing down in pieces during its return to Earth.

Discovery's tank was carefully filled earlier in the day, and a team of inspectors made extra checks for any signs of ice caused by the super-cold liquid hydrogen and oxygen. Ice falling from the tank could be lethal if it hit the orbiter during liftoff.

Problems were apparent even before the discovery of the defect this evening. The chances of acceptable weather at launch time today had fallen to 40%, from 60 % yesterday, with threats of thunderstorms close to the Cape Canaveral launch site. The launch window was only five minutes long - an interval dictated by the position of the space station.

Nasa administrator Michael Griffin had earlier announced that he trusted the safety of the shuttle so much that he would ride it if he could. He said he members of the Columbia families yesterday and assured them that Nasa had learned lessons from that tragic flight.

Yesterday in an embarrassing setback for Nasa, a temporary window cover fell off the shuttle while it was on the launch pad and damaged thermal tiles near the tail. The problem was announced just two hours after Nasa declared Discovery ready to fly.

The mishap was a reminder of the very thing that doomed Columbia - damage to the spaceship's fragile thermal shield.

The plastic cover from one of Discovery's cockpit windows fell more than 60 feet and struck a bulge in the fuselage, said Stephanie Stilson, the Nasa manager in charge of Discovery's launch preparations. No one knows why the cover - held in place with tape and weighing less than 2lbs - fell off, she said. The covers are used to protect the windows while the shuttle is on the launch pad, then removed before liftoff. Two tiles on an aluminum panel were damaged, and the entire panel was replaced with a spare. Ms Stilson called it a minor repair job.