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

“We still don’t know which tank we’ll use for STS-117,” William Gerstenmaier, NASA’s associate administrator for space operations, told a senate subcommittee March 28 of Atlantis’ mission. “We can still potentially make the May launch window if we use this tank, if we don’t we’ll be in the June time frame.”

NASA officials have said Atlantis could launch between mid -May and May 21 if the shuttle’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 Atlantis’ 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 NASA’s Kennedy Space Center, Cape Canaveral, Fla., when a severe storm battered the orbiter’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 Atlantis’ external tank, which engineers have been sanding smooth and repairing inside NASA’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.

“It’s going to be very challenging,” George Diller, a NASA spokesman at Kennedy , said of the time-consuming repair work. “There’s a lot of work to do, but it’s achievable.”

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

“We’re expecting the next tank on [April] 8 if they don’t run into weather that slows them down en route,” Diller said. “However, that is not going to speed the decision process any … the tenth is still decision day.”

Gerstenmaier said it will be the repair work’s progress that will dictate when shuttle mission managers will target Atlantis’ launch date.

“We’re letting the work kind of drive the activity,” Gerstenmaier said. “We’re not picking the launch date and then forcing the work to fit into that launch date.”

Commanded by veteran shuttle flyer Rick Sturckow, NASA’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 station’s starboard side.