"Fall is the earliest opportunity we'll have, one we need to be prepared for," NASA administrator Sean O'Keefe told a press conference, only two-and-a-half months after Columbia disintegrated as it re-entered Earth's atmosphere.

"We need to find out what caused this tragedy, fix it, and return to safe flight as expeditiously as possible," he said.

The final timetable for resumption of missions would depend on the speed with which investigators complete their inquiry into the causes, and time needed to implement any recommendations to come out of it.

That could delay the start of the new launch schedule until early next year, O'Keefe added.

His comments coincided with the release Thursday of investigators' preliminary recommendations to ensure the safety of future missions.

The Columbia Accident Investigation Board (CAIB) issued two recommendations, the first of which concerned the carbon-carbon reinforced thermal insulation tiles which protect the vessel, including the leading edges of its wings, from ferocious heat during re-entry.

The CAIB found that the current inspection regime was inadequate and recommended a more comprehensive plan to check the structural integrity of the insulation be put in place before the resumption of any shuttle flights.

The Board also called on NASA to reach agreement with the US military's National Imagery and Mapping Agency (NIMA) to make in-orbit imaging for each shuttle flight standard requirement.

"This recommendation was issued because of the board's finding that the full capabilities of the United States government to image the shuttle on orbit were not utilised," the CAIB said in a statement.

Suspicion over the cause of the Columbia disaster have focused on apparent excessive heating of the vessel's left wing during re-entry.

Invesigator's believe this may have come about after the heat-resistant tiles were damaged by pieces of insulating foam flying off the shuttle's external fuel tank just 81 seconds after lift-off on January 16.

Three days later, NASA technicians studying video footage of the launch noted that such damage may have occured, but mission controllers decided that it would not be necessary to ask NIMA to try and confirm the technicians' observations by photographing Columbia while it was in orbit.

O'Keefe stressed that NASA would follow the CAIB's recommendations to the letter.

"We will see them as imperative. We intend to act on every single one of them," he said.

Unlike the Challenger disaster in 1986, when investigators were able to hone in quickly on a single cause, the CAIB was likely conclude that a fatal chain of events led up to the loss of Columbia, O'Keefe added.

"They will likely conclude a combination of factors: hardware failure, process failure and bad judgement," he said.

Columbia disintigrated February 1 on re-entry to Earth's atmosphere, killing all seven astronauts on board just 16 minutes prior to scheduled touchdown.

The loss leaves NASA with only three of its original fleet of five vessels, and no new shuttles are due to come into service before 2010.

SPACE.WIRE