CAPE CANAVERAL – Future space shuttle missions were grounded again Wednesday by the same problem that doomed shuttle Columbia and its seven astronauts — a problem NASA claimed was fixed after spending more than $1 billion.

At least two dangerous chunks of foam insulation flew off Discovery’s newly designed external fuel tank during blastoff Tuesday, one of them about three feet in length and 10 inches wide. Only pure luck kept it from hitting Discovery and possibly causing another accident, mission managers said.

Discovery and its seven astronauts were in no immediate danger, but the dramatic and unexpected development raised new questions about the troubled shuttle program and about NASA’s ability to resolve the issue responsible for the Columbia catastrophe.

”Until we fix this, we’re not ready to go fly again,” said Bill Parsons, NASA’s shuttle program manager. “You can say that means we’re grounded.”