CAPE CANAVERAL, Fl. - Six astronauts set to ride NASA's Discovery orbiter to the International Space Station (ISS) are looking forward to launching spaceward later this year, though some issues remain to be resolved before a firm date can be set.

The STS-121 crew, currently slated to launch no earlier than this May, are poring over their spacecraft during a three-day inspection here at NASA's Kennedy Space Center (KSC). The close-up look comes as engineers evaluate work to reduce launch debris by eliminating a protective PAL ramp on shuttle fuel tanks.

"Right now, we are still holding to May, we're training for May," STS-121 commander Steven Lindsey told reporters, adding that additional wind tunnel tests of the tank modifications are expected in upcoming weeks. "The PAL ramp foam problem I think we have solved."

Discovery's fuel tank will be ready for shipment to KSC from NASA's Michoud Assembly Facility in New Orleans by Feb. 24, NASA shuttle officials said.

Lindsey and his crew - shuttle pilot Mark Kelly and mission specialists Michael Fossum, Lisa Nowak, Stephanie Wilson and Piers Sellers - are slated to spend 12 days testing orbiter inspection and repair techniques, as well as resupplying and repairing the ISS.

"It's frustrating to see delays in program but obviously we want to fly as safely as possible," Kelly said.

European Space Agency (ESA) astronaut Thomas Reiter, who will ride with the STS-121 crew and join the ISS crew in orbit, was not present at the orbiter inspection, known as a Crew Equipment Interface Test.

Returning to flight

Discovery's STS-121 mission is NASA's second orbiter flight following the Feb. 1, 2003 loss of seven astronauts aboard the space shuttle Columbia during reentry. Investigators later determined that damage from a chunk of insulation foam that fell from Columbia's tank during launch and struck the orbiter's left wing caused the accident.

NASA revamped shuttle external tanks and launched its first post-Columbia mission - STS-114 aboard Discovery - on July 26, 2005, but again saw unacceptably large pieces of foam fall pop free from the orbiter's tank during ascent. Engineers are removing the problematic PAL - or Protuberance Air Load - ramp for future shuttle flights.

"The program has never advertised that we would never lose any foam and we will lose foam on this flight, just like every other [flight]," Lindsey said. "The key is to make sure that the foam that we do lose is of a small enough size that if it hits the vehicle, and that's what we're working towards."

Busy mission

The six STS-121 astronauts have a busy mission ahead of them.

In addition to repeating orbiter heat shield inspection performed during the STS-114 flight, the astronauts will also place a spacewalking crewmember at the end of the shuttle's 50-foot (15-meter) orbital boom for the first time. The boom, which is attached to the end of the orbiter's own robotic arm, is designed to scan sensitive areas of the shuttle, including its heat-resistant, tile-lined belly.

If the boom can effectively be used as a work platform, it could allow astronauts to conduct underbelly repairs or operations during a flight to the Hubble Space Telescope - when the orbiter would not be docked at the space station - Lindsey said.

STS-121 spacewalkers Sellers and Fossum will also conduct three extravehicular activities, one of which will be completely dedicated to tile repair method tests that the STS-114 crew spent a mere hour on.

STS-114 mission specialists Soichi Noguchi and Steve Robinson tested an emittance wash for tile repair and black, NOAX material for cracks during one of their spacewalks. But they were limited to performing their experiments under extremely cold conditions, which led to bubbles and voids in the NOAX, Sellers said.

"We learned quite a lot from the [STS-114]," Sellers said, adding that the STS-121 NOAX test will be conducted under optimum, warm conditions. "We're dedicating the whole of EVA 3 to this, so we expect to give this a really good trial."

The rookie astronauts for STS-121 - Fossum, Nowak and Wilson - are also eager to reach orbit.

"We're very ready to go," Nowak said. "We're looking forward to having a great mission."