The next opportunity to fly Discovery on one of the shuttles' final cargo runs to the International Space Station likely will be in February, said NASA space flight chief Bill Gerstenmaier.

Until then, engineers plan tests to see if they can replicate the cracking that appeared following Discovery's November 5 launch attempt.

"We've hit a point where there is no obvious answer to what occurred," shuttle program manager John Shannon told reporters.

With data analysis running dry, NASA will press ahead with two tests, one with tank components at the manufacturing site in New Orleans, and the other with shuttle Discovery at the launch pad at Kennedy Space Center in Florida.

"I have strong confidence this is a solvable problem," Shannon said.

NASA had planned to fly Discovery in November to deliver a storage room, spare parts and a prototype humanoid robot to the station and then launch Endeavour in February with the $2 billion Alpha Magnetic Spectrometer particle detector.

It also hoped to win congressional funding for a bonus mission, aboard Atlantis, over the summer.

That flight is needed to stash a year's worth of food, water and other supplies aboard the station, a $100 billion project of 16 nations that has been under construction since 1998.