By Dr David Whitehouse
BBC News Online science editor
The next space shuttle mission could be in March or April of next year, says orbiter director Michael Kostelnik.
Kostelnik sets his sights on 2005
The delay is because of ongoing work into air flow around the shuttle's huge external fuel tank.
The next mission will be the first since shuttle Columbia was destroyed while re-entering the atmosphere in February 2003.
New rules will limit the launch of the orbiter to daylight hours when it can be observed during flight.
Michael Kostelnik said the biggest problem was finding ways to make sure the foam insulation on the shuttle's external fuel tank did not fall off during launch.
It was a suitcase-sized chunk of this foam that is thought to have fractured protective panelling on Columbia's left wing 82 seconds into the orbiter's mission last year.
The US space agency (Nasa) was unaware of how extensive the damage was until the shuttle was destroyed during its return to Earth 16 days later, killing all seven crew members on board.
Following an investigation into the cause of the accident, the agency pledged to implement all the recommendations of the investigation board.
New rules will govern launches
It had hoped to complete the work by September with shuttle Atlantis resuming servicing and construction of the International Space Station.
In addition to the technical difficulties of preparing the shuttles for return to flight, Nasa's options for future launches are now restricted to the daytime to enable ground cameras to have a clear view of the ascent.
This is because investigators said the lack of information about the debris strike contributed to the false conclusion that the foam impact posed no risk to Columbia or its crew.
Kostelnik said a launch opportunity in March and April 2005 was when the shuttle would likely be ready for lift-off.
Nasa administrator Sean O'Keefe had already told a Congressional science committee that launching this year was "doubtful".