Shuttle engineers believe the tools and techniques needed for spacewalking repair crews to access and patch areas of potentially catastrophic damage to an orbiter's heat-shield tiles should be in place by the end of the year, sources say, assuming upcoming tests go well.

Major challenges include development of reliable techniques for inspecting a shuttle for damage, gaining access to all possible damage sites and perfecting the tools and materials needed to actually patch over cracked, eroded or missing tiles. But so far, engineers have not identified any show-stoppers, at least for shuttle missions bound for the international space station where access is less of an issue.

But techniques for repairing damage to a shuttle's wing leading edge panels pose a much more difficult challenge and likely will take longer to develop. A breach in the leading edge of the shuttle Columbia's left wing led to the ship's destruction during re-entry Feb. 1, allowing a plume of super-heated air to burn its way into the wing's interior.

NASA has studied in-flight tile repair options in the past and is drawing on that background now to develop a workable repair technique for future flights. But no previous efforts were undertaken for the shuttle's carbon composite nose cap and wing leading edge panels, which are made of different material, feature complex curved surfaces and experience the most extreme temperatures during re-entry.

The panels play a critical role in the creation of a so-called boundary layer as the shuttle plows through the region of maximum heating. The boundary layer provides a natural insulating effect, limiting entry temperatures to "just" 3,000 degrees or so. Any repair option would have to not just plug a breach but also ensure the smooth airflow needed to set up an insulating boundary layer.

Analysis of a dozen or so possible RCC repair options and materials is underway, sources say, but testing is in its early stages and a final solution is far from clear.

Even in the case of tile repair options, engineers have not yet settled on what sort of caulk-like patch material is best suited for repairing broad areas of tile damage. Issues include the viscosity of the material, which astronauts must be able to apply and then spread or mold to some degree, and the time needed for any such material to cure, or "set up." Engineers currently are testing a silicon-based compound similar to one developed in the late 1970s as part of a tile repair technique that was never implemented.

Tests also will be required to ensure any such material can stand-up to worst-case re-entry temperatures and conditions.

The Columbia Accident Investigation Board is expected to recommend that NASA develop capabilities for on-orbit repair of tile and reinforced carbon carbon (RCC) leading edge panels. The board's final report is expected around the end of July. But NASA is not waiting for the CAIB report to begin developing a repair capability. A "tiger team" under the leadership of space station flight director Paul Hill at the Johnson Space Center in Houston has been in place for months.

But a request for an interview with Hill was turned down by Michael Kostelnik, deputy associate administrator for the shuttle and space station programs at NASA headquarters, on the grounds that any such discussion is "pre-decisional" and that no final decisions have been made. The real issue, however, appears to be a general reluctance on NASA's part to publicly address any topic the CAIB might discuss in its final report or in any interim recommendations that might be released between now and then.

Even so, a broad outline of NASA's on-orbit repair strategy has emerged in recent weeks that focuses on four general areas:

• Defining the critical damage size, i.e., the damage threshold that would trigger some sort of spacewalk inspection and/or repair attempt
  
  
• Techniques for inspecting a shuttle for damage
  
  
• The materials and tools needed to repair such damage
  
  
• The spacewalk access required to implement any such repairs

For the purposes of this discussion, it is assumed any inspections and repairs would be staged at the international space station. Only one non-station flight is currently on the books - a mission to service the Hubble Space Telescope - and it's not yet clear how inspections or repairs could be carried out in the absence of the space station's robot arm, multiple EVA anchor points and numerous external cameras. The Hubble flight, however, is several launchings down the road and engineers will have more time to develop viable "stand alone" repair techniques.

In the near term, shuttle missions to the space station will be launched in daylight to give engineers a better chance of spotting debris impacts that might damage an orbiter's thermal protection system (TPS). In addition, mission managers likely will require external fuel tank separation in daylight as well, to improve the odds of spotting any areas of foam shedding in orbit. Given those two requirements alone, and the orbital mechanics required to rendezvous with the space station, the number of possible launch days in a given month will be sharply reduced.

In any case, sources say, analysis to determine TPS damage criteria - what levels of damage would require repair - is not yet complete. But NASA hopes to have procedures in place to detect the smallest level of damage that could pose a threat to an orbiter and its crew.

It's not known what powerful spy satellites might be able to detect, but approaching shuttles will perform a pirouette of sorts 400 to 600 feet below the station to give lab crews a chance to photograph the orbiter's underside with telephoto lenses.

Tests indicate pre- and post-docking photography, by the crew and by cameras on the shuttle's robot arm and the station's mobile Canadarm2 spacecrane, should provide the coverage needed to spot any significant damage. But determining the depth of any tile damage - a critical factor - might not be possible without a spacewalk inspection or the development of some sort of laser scanner.

The materials and tools need to carry out a TPS repair in orbit are based on an existing "cure-in-place" ablator compound known as MA-25S that would be applied by a spacewalking astronaut using a sort of high-tech caulk gun. Tests are planned later this summer during flights aboard a NASA aircraft that provides brief periods of weightlessness. Sources say other tests are planned to determine vacuum cure times and temperature limits.

Different materials and application techniques are being developed for RCC damage but details are not yet available.

The most significant challenge, perhaps, is figuring out how to anchor a spacewalker in the weightlessness of orbit to apply the patch material and, depending on the situation, smooth it out or shape it without causing the station arm-work platform combination to flex too much.

Just gaining access will be difficult.

The currently favored scenario, known as option 1, calls for using the shuttle's robot arm to lock onto a grapple fixture on the space station and then, after docking latches are released, to properly position the orbiter. Spacewalkers then would ride the station's robot arm to the actual repair site.

An engineering analysis indicates the shuttle arm is strong enough to move the 120-ton space shuttle, but a telescoping boom of some sort may be needed to extend the reach of the Canadarm2 to all possible damage sites. A similar boom is being considered for use by the shuttle arm during non-station missions. Engineers are studying various ways to anchor such a boom at the work site to minimize flexing, possibly using small fixtures that would be glued to adjacent tiles.

Engineers initially considered the possibility of astronauts using small jetpacks, known as SAFERs, to reach possible damage sites for repair work, but that no longer appears feasible. The backpacks could, however, be used under certain conditions for initial inspections to determine the severity of any damage.