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Last Updated: Tuesday, 4 July 2006, 04:40 GMT 05:40 UK
Discovery ruled safe for lift-off
Shuttle launch pad (Getty)
Strict rules govern when a shuttle is allowed to take off

The space shuttle Discovery will launch as planned on Tuesday despite the discovery of a defect on its fuel tank, Nasa officials have confirmed.

Engineers in Florida said a crack in the insulation foam lining the tank was not considered to be a serious problem and did not need to be repaired.

A small triangle of material was seen to flake away from the container during a routine check.

Lift-off from the Kennedy Space Center is expected at 1438 EDT (1838 GMT).

Two countdowns at the weekend had to be cancelled because of stormy weather around the space centre.

Ice build-up

Nasa's mission management team confirmed late on Monday that the launch would go ahead as planned, as all safety criteria had been met.

Officials said the damaged foam had been inspected and was found to be structurally intact, with no repair work needed.

"We've laid out the data. We've looked at it calmly. We're ready to go fly," said Bill Gerstenmaier, Nasa's associate administrator for space flight.

Shuttle crew (Getty)
Mission known as STS-121
Discovery's 32nd flight
18th orbiter flight to ISS
Lift-off: 1438 EDT, 4 July
Location: Kennedy Space Center, Launch Pad 39B
Objective: To test new safety equipment and procedures
Payload: Cargo bay has 12.75t of equipment and supplies
Crew: Lindsey, Kelly, Fossum, Nowak, Wilson, Sellers, Reiter

The flake fell from a cracked region of foam covering a bracket on the tank which holds a fuel pipe carrying liquid oxygen.

It is a region that is known to expand and contract during launch activities.

Engineers believe ice built up in the area after rainfall on Sunday, and that it crushed the piece of foam when the tank expanded on being drained after the cancelled launch.

Foam-shedding has been a persistent problem for the American shuttle programme, and was responsible for the catastrophic loss of the Columbia vehicle and its crew of seven in 2003.

On that occasion, a suitcase-sized chunk of material punched a hole in the ship's left wing during take-off, opening the orbiter to the destructive superheated gases experienced in a re-entry.

"If this would have happened in flight... would that have been an issue? The answer is 'no, absolutely'," John Shannon, deputy manager of the space shuttle program had said earlier on Monday.

"It is less than half the size that we think can cause damage to the orbiter."

European focus

Nasa has spent some $1.3bn over the past three years redesigning the external tank to try to prevent foam breakaway, and to make other safety upgrades to the remaining orbiters in its fleet.

Another vehicle loss would almost certainly shut down the shuttle programme, which is due to be retired anyway in 2010.

It would also leave the half-finished International Space Station (ISS) project in crisis. The orbiter fleet has been integral to its construction. Components such as Europe's $6.5bn Columbus science module were designed with the intention that they be carried into orbit by a space shuttle.

Columbus arrived at Kennedy last month to await its ride into space. It is booked on a shuttle launch for next year.

See where the foam problem occurred

The US space agency has until 19 July in the current launch window to get Discovery and its crew of seven astronauts airborne during daylight, as demanded by post-Columbia launch rules.

Beyond that date, the orbiter would have to wait for an August slot which is currently earmarked for a flight by the Atlantis shuttle.

The latest mission expects to spend just under two weeks at the ISS, delivering supplies and equipment.

It would also drop off German Thomas Reiter, who would become the first European Space Agency astronaut to experience an extended tour of six months on the orbiting platform.

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