Discovery's shuttle crew got some encouraging news from the ground on Sunday: The orbiter's heat-shedding tiles, which looked like they had taken some hits from debris a few minutes into the Feb. 24 ascent, appear to have come through the events virtually unscathed.
That's the preliminary assessment, after engineers undertook an an exhaustive review of data from a painstaking inspection that the shuttle crew performed during their first full day orbiting, and after examining some 300 photos of the orbiter's underside take just before Discovery docked with the station.
Engineers found two tiles marred by debris strikes. The strikes scraped a bit of thin surface coating from each of the two tiles, but didn't compromise the tiles, according to LeRoy Cain, the space-shuttle program's deputy manager and chairman of the management team overseeing Discovery's final mission.
He acknowledges that while changes to the shuttle's external fuel tank following the Columbia tragedy in 2003 have significantly reduced the amount of foam and ice the tank sheds during ascent, "We can't make it zero."
The loss of Columbia and its seven-member crew during reentry was traced to debris striking areas of the thermal protection system relatively early during ascent. The hits occurred in regions where the heating from reentry is most intense.
Since then, engineers have focused their design changes on areas where shedding was most severe during a phase in the ascent where the atmosphere was still thick enough to turn freshly-freed debris into damaging projectiles.
Last Thursday, video cameras on the external fuel tank recorded four events where debris tumbled past Discovery. During one event, recorded some 3:51 seconds after launch, a small piece of debris appeared to strike the orbiter twice.
That timing was well past the point in Discovery's ascent when liberated foam would raise concerns about serious damage to tiles or the heat-resistant material on the wings' leading edges, mission managers have said.