The storm passed over the launch pad Monday evening where Atlantis was being prepared for a March 15 launch, said NASA spokesman Bill Johnson.

NASA technicians have decided to roll the shuttle back to the vehicle assembly building for further inspection and repair. The damage is concentrated in the upper third of the enormous external tank, a section which holds liquid oxygen propellant. If the tank itself can't be repaired, a new external fuel tank will be put into the Atlantis, delaying the launch to June.

Coincidentally, NASA managers on Tuesday began a two-day meeting at the Kennedy Space Center to determine whether there were any problems that would prevent Atlantis from launching next month.

In 1996, hail from a storm made 650 divots in space shuttle Discovery's external tank, forcing NASA to delay a launch and return the spacecraft to the Vehicle Assembly Building. Hail also hit the external tank of space shuttle Atlantis in 1990, causing minor damage.

The insulating foam on the external tank is of special concern to NASA since foam flew off space shuttle Columbia during liftoff in 2003 and struck the orbiter. The damage allowed fiery gases to penetrate Columbia during re-entry, killing the spacecraft's seven astronauts.

NASA redesigned the external tank, removing large amounts of foam, before last year's three successful shuttle missions. The space agency plans another design change to the tank before the shuttle program ends in 2010.