Workers examine the very top of the external tank. Credit: NASA-KSC

Senior NASA managers, including agency Administrator Mike Griffin, plan to meet late next week to assess the shuttle Atlantis's hail-damaged external tank, what will be needed to repair it and when the shuttle, originally scheduled for launch March 15, might be able to take off on a space station assembly mission.

Program managers continue to hold out hope the damage can be fixed before the current shuttle launch window closes May 21. But repairs near the tip of the tank, which experiences the most extreme heating and buffeting during ascent, will require engineers to spray on new foam insulation. That is considered a "non-standard" repair and it will require a detailed engineering analysis to ensure the new foam can stand up to the aerodynamic and thermal rigors of launch, at least until the shuttle is out of the dense lower atmosphere.

"They are still hopeful they can develop flight rationale that will say this is a safe tank," said a NASA official familiar with the ongoing assessment.

Shuttle Program Manager Wayne Hale today cleared tank workers to begin repairing damage that can be fixed by so-called "sand-and-blend" techniques or filling in isolated dings and pits, both considered "standard" repairs. No spraying will be attempted until engineers practice with a mockup.

Engineers are hopeful the tank can be fixed during a somewhat arbitrarily defined 45-day processing flow that would lead to a launch around May 11. But that is little more than a best-case hope at present and it assumes program managers decide it is safe to press ahead with the manual spraying required to fix the current tank.

NASA could opt to move Atlantis to the tank scheduled for use by the shuttle Endeavour on the next shuttle mission, STS-118, a tank that is scheduled to arrive at the Kennedy Space Center in early April. Switching to that tank, however, would delay the Atlantis launch until around June 21 at the earliest and delay Endeavour's flight to late August. Subsequent flights would be similarly affected.

Even if NASA ultimately moves Atlantis to Endeavour's tank, the current hail-damaged tank will still be repaired at the Kennedy Space Center. NASA managers have decided repair crews would have better access at Kennedy than at Lockheed Martin's Michoud Assembly Facility in New Orleans.

The long-term ramifications of a delay to mid May or late June are not yet clear. Shuttle managers believe they can eventually make up the lost time but with the shuttle program scheduled to end in 2010, "the clock is ticking," said one official. Falling behind now would make it more difficult to recover from problems down the road.

In the near term, the current launch window closes May 21 and reopens June 8, based on thermal issues related to the space station's orbit. The only other known deadline affecting STS-117 launch options involves the age of Atlantis's boosters. Officials say one or more segments will be five years old on July 8, the currently certified maximum age for booster segment propellant loads.