NASA officials said yesterday that they are satisfied they have reduced the possibility of debris damage to the space shuttle to an "acceptable risk," overcoming the most dangerous lingering obstacle to the orbiter's mid-July launch.
"We are ready to go fly," said Bill Parsons, the shuttle program manager, after a day-long meeting with engineers at the Kennedy Space Center to evaluate potential impacts should chunks of ice break away from the shuttle's external fuel tank during launch and smash into the orbiter's heat shielding.
"The recommendation is that we are in an acceptable risk posture, and I accepted that recommendation," Parsons said in a telephone news conference. Pending the results of a final readiness review next week, Parsons predicted a shuttle launch near the beginning of the launch window that opens July 13.
By successfully closing out the debris evaluation, NASA finally ended 21/2 years of study, testing and modifications to control the shedding of ice and foam insulation from the external tank during launch.
Damage from a 1.67-pound chunk of foam breached the heat shielding on the space shuttle Columbia, causing the orbiter to disintegrate on reentry Feb. 1, 2003. The shuttle fleet has been grounded ever since.
The launch of Discovery has been postponed twice this year for further testing of debris impacts, most recently so engineers could evaluate the potential for ice damage and add a heater to a fitting on the external tank to prevent ice formation. The tank at launch is loaded with 1.6 million gallons of liquid hydrogen and liquid oxygen at temperatures several hundred degrees below zero.
Despite their confidence in test results over six weeks, neither Parsons nor other NASA officials would put a numerical value on the chances of ice causing catastrophic damage to the orbiter's heat shielding.
"We just spent eight hours discussing results and six weeks generating them," said John Muratore, the shuttle systems engineering and integration manager. "It doesn't lend itself to a single number. You can't say, like in blackjack, 'If I have a 15, these are my odds of winning.' "
Instead, Muratore said, engineers first determined that they had reduced the risk by "an order of magnitude" simply by installing the heater on a bellows-like joint in a fuel line that carries liquid oxygen down the side of the external tank to the main engines.
They then evaluated the "relative risk" of ice and foam to the "reinforced carbon-carbon" that shields the shuttle's wing leading edges from the heat of reentry and the ceramic thermal tiles that pad the belly of the orbiter, Muratore said.
Ice damage to the tiles presents the greatest risk, he said, but it is still relatively low, and "our [computer] models for ice are very primitive." Unlike foam, a manufactured product with uniform, measurable properties, ice comes in almost limitless varieties.
Still, Muratore acknowledged that engineers at first mistakenly thought that debris impacts were the same whether they were ice or foam: "We've learned since that ice creates narrow, deep cavities in tile, while foam cavities are shallow and wide. Which is worse? It's not intuitive."