NASA officials are targeting a May 10 launch for the next space shuttle flight — the second to fly since the Columbia accident — but also acknowledging that much work remains to be done before the mission can be cleared for lift off.

Wayne Hale, NASA’s space shuttle program manager, told reporters Feb. 28 that he is optimistic the shuttle mission NASA designates as STS-121 will lift off in May based on ongoing work to prepare the Discovery orbiter and its fuel tank for flight.

“As of today I see no reason to say anything other than we’re progressing toward May,” Hale said during a press conference at NASA’s Kennedy Space Center . “There is a lot of work that needs to be done.”

An “aggressive” effort is under way to reach STS-121’s current launch window, which runs through May 22, though a significant, time-consuming glitch could push the flight into July, NASA officials said.

Much of the work remaining involves the verification of external tank modifications intended to prevent potentially harmful chunks of foam from separating during launch and striking the orbiter’s heat shield.

According to video from NASA’s first flight after the Columbia accident — STS-114 aboard Discovery — a total of 16 pieces of foam separated from the orbiter’s fuel tank during its July 26, 2005, launch, including a large chunk that fell from a protective ramp that was previously believed to be safe, according to a NASA report.

The problem was similar to one that caused the destruction that doomed the Space Shuttle Columbia and its seven-astronaut crew in early 2003. A briefcase-sized piece of foam fell from that orbiter’s tank at launch and breached the heat shield along its left wing, allowing hot atmospheric gases to enter the hole during re-entry Feb. 1, 2003.

Engineers were able to remove about 16 kilograms of foam in a pair of Protuberance Air Load (PAL) ramps from External Tank 119 for the STS-121 flight. But engineers are awaiting results from a string of ongoing wind tunnel tests to verify the fix will not compromise fuel tank integrity, since PAL ramps were designed to shield pressure and fuel lines from the air pressures at launch.

A study also is under way to determine how much material can be shaved from ice frost ramps, each covered in about 0.6 kilograms of foam, that run vertically along the external tank, Hale said.

Tim Wilson, NASA’s external tank tiger team lead for the agency’s Engineering and Safety Center, said his group is watching over the agency’s fuel tank preparations.

“Foam will still come off the tank after we’ve done all these mitigation efforts,” Hale stressed, adding that pieces the size of a matchbox or smaller are expected. “We believe the pieces will be small.”

If the Discovery’s STS-121 launch, commanded by veteran astronaut Steven Lindsey, goes as expected, NASA could launch two additional orbiters later this year, Hale added.

“It does depend on what happens on those flights,” Hale said. “I am very optimistic that, if we can fly in May or July, that we can get three flights up in this year.”

NASA launch director Michael Leinbach said there are other, non-foam-related challenges that need to be resolved before Discovery’s STS-121 flight.

Shuttle workers found that main engine seals used for Discovery’s three main engines are not as thick as specifications call for. While they passed the leak checks, there is an ongoing study to ensure their flight worthiness, Leinbach said. The seals could be replaced without detaching Discovery’s engines, he added.

Engineers also found a small metallic particle on a filter that leads into an engine valve. Analysis has shown it to be too small to clog up any plumbing in the engine if it shakes loose, but shuttle officials are concerned it could potentially cause the volatile liquid oxygen used during launch to ignite.

“We’ve tried to remove the particle and we can’t get it out,” Leinbach said, adding that engineers have attempted using a bore sight instrument to extract the particle through test ports.

Analysis is on going to determine whether the engine will have to be opened up — which could risk even more contamination — for cleaning, or if the particle can be left as is, he added.

Any extensive unplanned change or modification could push Discovery’s flight outside of its May launch window into July, where a liftoff could occur between July 1 and July 19.

“If we run into a big gotcha, we won’t have much time to resolve it,” Leinbach said. Under current plans there are no added contingency days in Discovery’s preflight schedule, he added. “Barring the big gotcha in processing, we’re confident we can make May.”

Leinbach said shuttle workers were eager to greet Discovery’s external tank March 1, which shipped out Feb. 25 from NASA’s Michoud Assembly Facility in New Orleans .

That tank, which was shipped to Kennedy by barge, would be the first to fly without a PAL ramp.

“We’re just really glad to get another piece of flight hardware here and to get on with the mission process,” Leinbach said.