NASA gave the green light for a shuttle liftoff Tuesday despite worries about a piece of foam that popped off Discovery's external fuel tank while the spacecraft sat on the launch pad. "We're go to continue with the launch countdown," said Bill Gerstenmaier, NASA associate administrator. The 7.62 cm triangular piece of foam that appeared to come from a 12.7 cm-long crack late Sunday or early Monday is far smaller than the foam chunk that brought down Columbia, killing seven astronauts in 2003. "I dont think we're taking any additional risk than we did in our original assessment" in going ahead with a launch, said Gerstenmaier. A scrub was called Sunday afternoon because of stormy weather, the second launch delay in two days. The back-to-back delays cost NASA an estimated $2 million (â‚¬1.6 million) in overtime pay and fuel costs. NASA hopes to extend the 12-day mission by an extra day to squeeze in a spacewalk that will test new techniques for repairing damage to the shuttle's thermal skin. By replenishing the shuttle's on-board fuel now, the astronauts will have a better chance of getting that additional day in orbit. "It's a strong desire to get that 13th day," said John Shannon, deputy manager of the space shuttle program. Tuesday's weather forecast also was better than Sunday or Monday, with only a 40 percent chance that storms at launch time would prevent lift off, said U.S. Air Force 1st Lt. Kaleb Nordgren, a shuttle weather forecaster. The forecast worsened a bit Wednesday but improved again on Thursday. NASA planned to make launch attempts on Tuesday and Wednesday before taking a break on Thursday. "Local weather can threaten the ability to get to Tuesday," said Michael Leinbach, shuttle launch director, when Monday turned out stormy. "It's a very, very tight plan to get all that work done in two days." Once in orbit, Discovery's crew will test shuttle-inspection techniques, deliver supplies to the space station and drop off German astronaut Thomas Reiter for a six-month stay. The decision to launch was made by NASA Administrator Michael Griffin despite the concerns of two top agency managers who wanted additional repairs to the foam insulation on the external fuel tank. Columbia was brought down by a chunk of flyaway foam, and a piece broke off Discovery's redesigned tank last July, barely missing the shuttle.