Nasa on Friday postponed to March 2005 at the earliest the resumption of space shuttle launches stopped since the Columbia accident in February of 2003 which claimed the lives of seven astronauts.
"We changed the launch planning date to March 6 to April 18 of 2005," William Readdy, the National Aeronautics and Space Administration's associate administrator for space flights, told reporters on a telephone conference call.
Nasa had previously said shuttle flights would resume between October and September this year, but said it pushed that back to make further modifications to the program.
"We chose the Discovery as the return flight vehicle for the March time frame and then the Atlantis orbital vehicle will be in the queue behind that," Readdy said.
Nasa usually has one shuttle in flight readiness on the ground while another is in orbit, so a rescue mission could be launched in the event of an emergency. The fleet is down to three ships, Atlantis, Discovery and Endeavor.
A critical report in the wake of the Columbia shuttle disaster found a piece of insulating foam broke off one of shuttle's booster rockets during launch, striking the shuttle's heat shield and creating a fatal breach in the craft's left leading wing.
President George W. Bush announced in January the retirement of the shuttle programme by 2010. He has asked Nasa to develop a new space vehicle capable of enabling astronauts to establish a permanent base on the moon by the end of 2015.