By Irene Mona Klotz
at the Kennedy Space Center, Florida
The US space agency (Nasa) has not fully met Columbia accident investigators' requirements to safely return the shuttle fleet to flight.
The panel contains a number of former astronauts
That is the assessment of an oversight panel that has been monitoring the agency for two years.
Despite the finding, panel members said they had no qualms about Nasa launching shuttle Discovery next month on the agency's first mission since the 2003 Columbia disaster.
"We feel that it is a safe vehicle to fly," said Joseph Cuzzupoli, a member of the oversight panel headed by former astronauts Thomas Stafford and Richard Covey.
Nasa is scheduled to make a decision about Discovery's launch date during a two-day meeting at the Kennedy Space Center, which begins on Wednesday. The agency has targeted the launch for between 13 July and 31 July.
The Stafford-Covey panel determined that Nasa has met or exceeded 12 of the 15 recommendations made by the Columbia Accident Investigation Board (CAIB) for the safe resumption of shuttle flights.
The space agency fell short in three areas, however: eliminating debris from the shuttle external tank; making the orbiter more resilient to debris impacts; and developing an operational heat-shield repair technique that can be used by a shuttle crew in orbit.
The Columbia accident was triggered by a piece of foam insulation that slid off the external fuel tank during lift-off and smashed into the ship's wing. The damage had no effect on the shuttle in orbit, but once the crew attempted to return through the atmosphere for landing, superheated gases lashed into the broken wing.
Within minutes, Columbia was torn apart and seven astronauts aboard the ship were killed.
Since the accident, Nasa's primary focus has been to eliminate debris sources from the shuttle's external tank.
While the foam issue is well understood, the agency recently began identifying the hazards of ice, which can build up on the outside of the tank during fuelling prior to lift-off and then break off during launch.
One point of contention was the CAIB's ruling that Nasa should eliminate all sources of external tank debris - a standard that some oversight panel members said was impossible to attain.
Another reason why Nasa fell short of the meeting the CAIB's intent was because of the president's directive to retire the fleet in 2010, making some improvements, such as safer heat-shield panels, a moot point as they would not be ready much before the fleet would be retired.
"While we're saying they may not have fully met the intent of CAIB, we're also saying they have made significant progress toward reducing the likelihood that any of these bad events will happen," said panel member James Adamson, also a former astronaut.
The panel found Nasa most lacking in the requirement to develop on-orbit repair techniques for the shuttle's heat shield. While five experimental processes will be tested during Discovery's flight, none are considered to be useable in case of a breach in the shuttle's heat shield.
Nasa aims to launch Discovery as early as 13 July
If Discovery was too damaged to return to Earth, the crew would probably be ordered to stay aboard the International Space Station until a rescue mission could be launched.
"They're well on their way to coming up with the capabilities and getting them certified," said panel member and former astronaut Kathryn Thornton.
"It's just that at this time, on this day, I don't think that we can say that they're there."