HOUSTON — NASA appears to have licked the worst of its launch-debris problem, clearing the way for shuttle flights needed to finish construction of the International Space Station.
To the delight of rocket technicians, Discovery's latest launch was relatively free of the loose foam which led to the destruction of the shuttle Columbia and the death of its seven crew members. On-orbit photos and dramatic video from new cameras on the solid-rocket boosters and main fuel tank have shown this launch to be the cleanest in the shuttle program's history.
Agency officials and astronauts alike are encouraged by the progress. But NASA is cautious in declaring success.
Friday and Saturday, the orbiter Discovery undergoes one more round of inspections for damage to the tile heat shield on the underside of its wings before the shuttle reenters Earth's atmosphere during its planned return on Monday. This time, however, managers will focus on dings and dents from space junk or tiny meteoroids.
After a three-year, $1.3 billion effort to resolve the problem, changes to the external tank "seemed to be very successful" so far, says John Shannon, deputy program manager for the shuttle program. But, it's only "one data point," he adds.
"I'm fairly confident we will never finish redesigning the external tank," says Mr. Shannon. "Now that we are so sensitive to the foam, we're going to be in a state of continual improvement on the external tank" through the end of the program in 2010.
Prelaunch dissent from two top officials at NASA regarding the orbiter's readiness to fly suggests that the struggle with foam debris isn't over, notes John Logsdon, director of the Space Policy Institute at George Washington University in Washington and member of the Columbia Accident Investigation Board. Still, he says, "they've made impressive progress."
In February 2003, the shuttle Columbia broke up on reentry. The accident was traced to a suitcase-size chunk of foam that was jarred from the external tank and had smacked into Columbia's left wing soon after launch. The impact damaged several reinforced carbon panels that shield the wing's leading edge from the heat of reentry.
The damage allowed superheated air to eat its way through the wing during reentry. Additional foam losses during last year's first "return to flight" mission prompted yet another round of analyses and tank modifications.
Concerns center not just on the tank shedding foam, but on the size, mass, location, and time during the ascent when the foam comes loose, says John Chapman, manager of the external tank program at NASA's Marshall Space Flight Center in Huntsville, Ala. For example, during Discovery's climb through the atmosphere, the largest piece of foam the tank lost was about the size of a legal pad. It tipped the scales at less than an ounce, well within acceptable limits. It came off in three pieces, at an altitude where the air was too thin to push it to damaging speeds.
"We're pleased with the performance of the tank," Mr. Chapman says. But perhaps the most significant result was that the flight confirmed NASA's "understanding of the physics behind foam loss," which could benefit future efforts to minimize foam debris. Prior to the Columbia accident, those physics were poorly understood.
"We've been really impressed as a team with the way they were able to analyze the issues they needed to resolve," adds Ralph Roe, director of NASA's Engineering and Safety Center at the agency's facilities in Langley, Va. The center was established in response to a Columbia Accident Investigation Board recommendation and is charged with conducting rigorous safety and engineering studies for particular shuttle or space station systems, independent of their respective NASA facilities.
The work isn't over. Mr. Chapman notes that engineers are trying to first reduce, then eliminate, foam ice-frost ramps that surround a series of fixtures that run along the side of the tank.
Beyond the ice frost ramps, engineers are looking at ways to improve the way workers apply complex foam shapes to tank hardware. The odd nooks and crannies these shapes present can lead to voids in the foam that can increase the risk of shedding on launch.
The experience has left many in the agency with some hard-earned lessons: Make sure technical issues related to a launch decision get openly debated at the highest levels of the agency and with the new round of rockets and capsules being designed, don't place astronauts in the way of launch debris.
To which Chapman adds a third, rather topical lesson: Don't become so fixated on one problem that you ignore other potential disasters.