By Irene Klotz
Kennedy Space Center, Florida
British-born astronaut Piers Sellers will play a vital role in the next space shuttle flight.
Once again, Sellers will go "walking" in space
The mission will test the success of changes made after the 2003 Columbia disaster and a troubled test-flight in July 2005.
The US space agency (Nasa) needs to get the shuttle fleet back into regular flight service to complete assembly of the half-built International Space Station.
Laboratories, including Europe's Columbus module, and other components must be launched before the space shuttles are retired in 2010.
The gear was designed to fly only on the American orbiters.
Sellers, 50, originally from Crowborough in Sussex, is the lead spacewalker for Nasa's follow-on test-flight, to launch in May aboard shuttle Discovery.
He and members of the crew were at the Kennedy Space Center on Friday for training.
"We're looking forward to getting this mission off soon," Commander Steven Lindsey told reporters after introducing his crew.
The main purpose of the flight is to assess changes to the shuttle's external fuel tank intended to stem the shedding of debris during launch.
Falling foam insulation from the tank was responsible for the damage that downed Columbia as it attempted to return to Earth on 1 February 2003.
The problem reoccurred during Discovery's return-to-flight mission last year, though the debris did not hit the shuttle.
Nasa ordered another tank re-design, which will be tested during the launch of Sellers and his crewmates on what is expected to be a 12-day mission.
"I feel pretty confident that the decisions we're making are going in the right direction," Lindsey said.
"But this is a test-flight... and part of the purpose of this test-flight is to test the changes we've made in the tank. We won't know for sure [they work] until we fly."
The crew also will test materials and procedures to fix the shuttle's heat shield while the ship is in orbit. Nasa implemented the safety upgrade following the Columbia accident.
A hole in one of Columbia's wing panels, caused by the debris impact during launch, triggered the break-up of the ship and the deaths of seven astronauts.
While Nasa has devoted most of its attention to preventing debris through tank redesigns, several on-orbit inspection and repair techniques were also developed after the accident in case damage should occur during launch.
Initial tests during Discovery's first post-Columbia flight had mixed results, Piers Sellers said, prompting Nasa to expand the testing program for the upcoming mission.
Sellers and astronaut Michael Fossum plan to make three spacewalks during Discovery's flight, one of which will be devoted to testing materials to repair the shuttle's heat-resistant ceramic tiles and its carbon wing panels.
Sellers and Fossum also plan to test if the shuttle's 50ft-long (15m) robot arm coupled with a new 50ft-long inspection boom are steady enough to serve as a work platform in case astronauts have to make repairs on the ship's belly.
"It's essentially one big stick," Sellers said. "We're going to stand on the end of that and do a whole lot of variations."
Another spacewalk will be devoted to fixing the station's mobile transporter, which serves as a mobile base for the station's robotic crane.
In addition to making repairs and delivering cargo to the station, the Discovery astronauts will transport a new crewmember to the space station.
Germany's Thomas Reiter will become the first European astronaut to live aboard the outpost.
Nasa and its space station partners agreed to trim the number of live-aboard station astronauts from three to two after the Columbia accident and subsequent grounding of the shuttle fleet to save on supplies.
Since the accident, the station has been dependent on the smaller Russian crew and cargo ships for transport.
Reiter's assignment on the station will bring the outpost back to a three-member crew. Nasa still hopes to double the crew size to six once station assembly is complete.
In addition to Sellers, Lindsey, Fossum and Reiter, the Discovery crew includes pilot Mark Kelly, flight engineer Lisa Nowak and astronaut Stephanie Wilson.
Sellers and Kelly will be making their second spaceflights; Lindsey will be on his fourth flight; and Fossum, Nowak and Wilson are rookies.
Reiter, 47, previously made a long-duration flight on the Russian Mir space station.