New York โ€” Engineers continued to work feverishly to repair the hail-damaged fuel tank of NASAโ€™s Atlantis shuttle in time for a mid-May launch, though space agency officials are still unsure whether a replacement might be required.

โ€œWe still donโ€™t know which tank weโ€™ll use for STS-117,โ€ William Gerstenmaier, NASAโ€™s associate administrator for space operations, told a senate subcommittee March 28 of Atlantisโ€™ mission. โ€œWe can still potentially make the May launch window if we use this tank, if we donโ€™t weโ€™ll be in the June time frame.โ€

NASA officials have said Atlantis could launch between mid -May and May 21 if the shuttleโ€™s fuel tank repairs, and a subsequent analysis, go well. Replacing the tank with a new one would push the spaceflight to no earlier than June 8.

NASA is expected to set a new launch target for Atlantisโ€™ STS-117 mission to the international space station (ISS) April 10.

The shuttle and its six-astronaut crew were preparing for a planned March 15 liftoff from NASAโ€™s Kennedy Space Center, Cape Canaveral, Fla., when a severe storm battered the orbiterโ€™s foam-covered fuel tank with golf-ball-sized hail Feb. 26.

The resulting damage left thousands of dings in the vital foam insulation on Atlantisโ€™ external tank, which engineers have been sanding smooth and repairing inside NASAโ€™s Vehicle Assembly Building at Kennedy . Of the more than 2,500 dings in the fuel tank, some 1,600 may require repair, NASA officials have said.

โ€œItโ€™s going to be very challenging,โ€ George Diller, a NASA spokesman at Kennedy , said of the time-consuming repair work. โ€œThereโ€™s a lot of work to do, but itโ€™s achievable.โ€

A new shuttle fuel tank, originally built for NASAโ€™s STS-118 shuttle mission but which could be used for STS-117, will be shipped April 8 or April 9 from the agencyโ€™s New Orleans-based Michoud Assembly Facility, NASA officials said. It will arrive two days earlier than expected at Kennedy, they added.

โ€œWeโ€™re expecting the next tank on [April] 8 if they donโ€™t run into weather that slows them down en route,โ€ Diller said. โ€œHowever, that is not going to speed the decision process any โ€ฆ the tenth is still decision day.โ€

Gerstenmaier said it will be the repair workโ€™s progress that will dictate when shuttle mission managers will target Atlantisโ€™ launch date.

โ€œWeโ€™re letting the work kind of drive the activity,โ€ Gerstenmaier said. โ€œWeโ€™re not picking the launch date and then forcing the work to fit into that launch date.โ€

Commanded by veteran shuttle flyer Rick Sturckow, NASAโ€™s STS-117 mission is the first of several ISS construction flights scheduled for 2007. The 11-day flight features three planned spacewalks to install two new trusses and solar arrays to the stationโ€™s starboard side.